Dissertations / Theses on the topic 'Bulk alloys'

The usage of cobalt (Co) as binder phase material in cemented carbides has been questioned becauseof the potential health hazards associated with cobalt particle inhalation. Cobalt is used because ofits excellent adhesive and wetting properties, combined with adequate mechanical properties. Thepurpose of this work is to investigate the mechanical properties of Fe-Ni bulk alloys and WC-Cocemented carbides using Integrated Computational Materials Engineering (ICME) methods com-bined with FEM data. The report investigates the mechanical properties of several bulk alloys inthe Fe-Ni system as a function of void size and fraction. FEM indentation and FEM fracture datais interpolated and used to model the hardnessHand fracture toughnessKIc. A precipitationhardening model based on the Ashby-Orowan’s equation is implemented to predict the e↵ect on theyield strength from precipitated particles. A model for solid solution hardening is also implemented.Existing models are used to simulate the properties of WC-Co cemented carbides together with thesolid solution hardening model. Results show that the simulated properties of the Fe-Ni bulk alloysare comparable to those of cobalt. However, the results could not be confirmed due to a lack ofexperimental data. The properties of WC-Co cemented carbides are in reasonable agreement withexisting experimental data, with an average deviation of the hardness by 11.5% and of the fracturetoughness by 24.8%. The conclusions are that experimental data for di↵erent Fe-Ni bulk alloys isneeded to verify the presented models and that it is possible to accurately model the properties ofcemented carbides.
Anv¨andandet av kobolt (Co) som bindefas-material i h°ardmetall har blivit ifr°agasatt som en f¨oljdav av de potentiella h¨alsoriskerna associerade med inhalering av koboltpartiklar. Kobolt anv¨ands p°agrund av dess utm¨arkta vidh¨aftande och v¨atande egenskaper, kombinerat med tillr¨ackliga mekaniskaegenskaper. Syftet med detta arbete ¨ar att unders¨oka de mekaniska egenskaperna hos Fe-Ni bulklegeringarochWC-Co h°ardmetall genom att anv¨anda Integrated Computational Materials Engineering(ICME) metoder kombinerat med FEM-data. Rapporten unders¨oker de mekaniska egenskapernahos flera bulklegeringar i Fe-Ni systemet. FEM-indentering och FEM-fraktur data interpoleras ochanv¨ands f¨or att modellera h°ardheten H och brottsegheten KIc. En modell f¨or utskiljningsh¨ardningbaserad p°a Ashby-Orowans ekvation implementeras f¨or att f¨oruts¨aga e↵ekten p°a brottgr¨ansen av utskiljdapartiklar. ¨Aven en modell f¨or l¨osningsh¨ardning implementeras. Existerande modeller anv¨andsf¨or att simulera egenskaperna hos WC-Co h°ardmetall tillsammans med modellen f¨or l¨osningsh¨ardning.Resultaten visar att de simulerade egenskaperna hos Fe-Ni bulklegeringar ¨ar j¨amf¨orbara medde f¨or kobolt. Dock kan de inte bekr¨aftas p°a grund av avsaknad av experimentell data. Egenskapernahos WC-Co h°ardmetall st¨ammer rimligt ¨overens med existerande experimentell data, meden genomsnittlig avvikelse av h°ardheten med 11.5% och av brottsegheten med 24.8%. Slutsatserna¨ar att det beh¨ovs experimentell data f¨or Fe-Ni bulklegeringar f¨or att kunna verifiera modellernasnoggrannhet och att det ¨ar m¨ojligt att f¨oruts¨aga egenskaperna hos h°ardmetall.

In recent years, bulk amorphous alloys and nanocrystalline materials have been synthesized in a number of ferrous and non-ferrous based alloys systems, which have gained some applications due to their unique physico-chemical and mechanical properties. In the last decade, Zr-based alloys with a wide supercooled liquid region and excellent glass forming ability have been discovered. These systems have promising application fields due to their mechanical properties
high tensile strength, high fracture toughness, high corrosion resistance and good machinability. In this study, the aim is to model, synthesize and characterize the Zr-based bulk amorphous alloys. Initially, theoretical study on the basis of the semi-empirical rules well known in literature and the electronic theory of alloys in pseudopotential approximation has been provided in order to predict the potential impurity elements that would lead to an increase in the GFA of the selected Zr-Ni, Zr-Fe, Zr-Co and Zr-Al based binary systems. Furthermore, thermodynamic and structural parameters were calculated for mentioned binary and their ternary systems. According to the theoretical study, Zr67Ni33 binary system was selected and its multicomponent alloys were formed by adding its potential impurity elements
Mo, W and Al. Centrifugal casting method was used to produce alloy systems. Structural characterizations were performed by DSC, XRD, SEM and EDS methods. In the near-surface regions of Zr60Ni25Mo10W5 and Zr50Ni20Al15Mo10W5 alloys, amorphous structure has been observed. Experimental studies have shown that Zr-Ni based systems with impurity elements Mo, W and Al, not widely used in literature, might be good candidates for obtaining high GFA.

The aim of this study is to synthesize and characterize new bulk amorphous alloys in the Ni- based systems. Theoretical studies on the basis of semi-empirical rules and the electronic theory of alloys in pseudopotential approximation has been provided in order to predict the impurity elements that will lead to an increase in the glass forming ability of Ni-based alloy systems. Glass forming ability of ten different compositions of alloys of Ni-Nb, Ni-Fe, Ni-B, Ni-Hf and Ni-Cr was simulated by using FORTRAN programs based on pseudopotential theory. In addition to the binary alloys, ternary alloys, which were formed by addition of 1 at% of third element to these systems, were also simulated. Since ordering energy is an indicator of glass forming ability, theoretical studies allowed to predict the effect of various third elements on the formation of amorphous phase. Furthermore, ordering energies were also used to calculate other parameters important for glass forming ability. In the second part of the study, on the basis of theoretical results, a series of casting experiments were done. Different compositions of Ni-Nb, Ni-Nb-Sn and Ni-Nb-Al alloys were cast in the centrifugal casting machine. Alloys were melted in alumina crucibles and cast into the copper moulds. Characterizations of cast alloys were done by the use of Metallography, SEM, XRD and DSC. Fully amorphous Ni52Nb41Al7 alloy was synthesized in bulk form with 0.8 mm thickness.

The aim of the study was to investigate the crystallization kinetics and solidification behaviour of Fe60Co8Mo5Zr10W2B15 bulk glass forming alloy. The solidification behaviour in near-equilibrium and non-equilibrium cooling conditions was studied. The eutectic and peritectic reactions were found to exist in the solidification sequence of the alloy. The bulk metallic glass formation was achieved by using two methods: quenching from the liquid state and quenching from the semi-state. Scanning electron microscopy, x-ray diffraction and thermal analysis techniques were utilized in the characterization of the samples produced throughout the study. The choice of the starting material and the alloy preparation method was found to be effective in the amorphous phase formation. The critical cooling rate was calculated as 5.35 K/s by using the so-called Barandiaran and Colmenero method which was found to be comparable to the best glass former known to date. The isothermal crystallization kinetics of the alloy was studied at temperatures chosen in the supercooled liquid region and above the first crystallization temperature. The activation energies for glass transition and crystallization events were determined by using different analytical methods such as Kissinger and Ozawa methods. The magnetic properties of the alloy in the annealed, amorphous and as-cast states were characterized by using a vibrating sample magnetometer. The alloy was found to have soft magnetic properties in all states, however the annealed specimen was found to have less magnetic energy loss as compared to the others.

In this study molecular dynamics simulation program in NVT ensemble using Velocity Verlet integration was written in order to investigate the glass forming ability of two metallic systems. The Zn-Mg system, one of the frontiers of simple metal-metal metallic glasses and Fe-B, inquiring attention due to presence of many bulk glass forming alloy systems evolved from this binary with different alloying element additions. In addition to this, atomistic calculations on the basis of ordering were carried out for both Zn-Mg and Fe-B systems. Ordering energy values are calculated using electronic theory of alloys in pseudopotential approximation and elements which increase the ordering energy between atoms were determined. The elements which increase the ordering energy most were selected as candidate elements in order to design bulk amorphous alloy systems. In the experimental branch of the study centrifugal casting experiments were done in order to see the validity of atomistic calculations. Industrial low grade ferroboron was used as the master alloy and pure element additions were performed in order to constitute selected compositions. Fe62B21Mo5W2Zr6 alloy was successfully vitrified in bulk form using nearly conventional centrifugal casting processing. Specimens produced were characterized using SEM, XRD, and DSC in order to detect the amorphous structure and also the crystalline counterpart of the structure when the cooling rate is lower. Sequential peritectic and eutectic reaction pattern was found to be important for metallic glasses which can be vitrified in bulk forms with nearly conventional solidification methods.

Elemental W and Ni powders were mechanically alloyed in a SPEX mill with WC grinding media for durations ranging from 5 to 50 hours, then compacted samples were sintered in hydrogen to generate bulk tungsten heavy alloys with 2, 4 and 6 wt.% Ni. Evidence of a bimodal grain size distribution was seen in X-ray diffractograms of sintered samples and confirmed by scanning electron microscopy. Grain sizes in the small-grained regions ranged from 200–600 nm; those in the large-grained regions ranged from 1–2 µm. Furthermore, the volume fraction of the small-grained region increased linearly as milling time increased. A slice from a sintered sample was prepared for examination by TEM, in which particles 30–100 nm in diameter were regularly observed on the boundaries of the 200–600 nm grains. EDS point analysis showed that the particles are WC. Therefore it is concluded that heterogeneously distributed contamination from the grinding media is continually incorporated during mechanical alloying and, during sintering, inhibits grain growth through Zener pinning. Densities of sintered samples increased as milling time increased to a maximum of almost 96% of the theoretical value. Density increases with respect to milling time were initially great but diminished upon further milling. While the samples with 4 and 6 wt.% Ni both approached 96% of the theoretical density value by 50 hours of milling, densities in the samples with 2 wt.% Ni were considerably lower. Thus it appears that the Ni that becomes incorporated into the bcc W structure during mechanical alloying activates W diffusion during sintering, though there is a limit to the amount of Ni that the W structure can accommodate. This is evinced in W lattice parameter values from the as-milled powders; while the lattice parameter drops considerable from 2 to 4 wt.% Ni, the difference between 4 and 6 wt.% Ni is much smaller and the Ni content limit surely falls between the two values. Otherwise-equivalent samples with added WC powder were also produced, but did not increase the volume fraction of the small-grained region – probably because the particles remained large and were homogeneously distributed.
Ph. D.

Thesis advisor: Zhifeng Ren
SiGe alloys are the only proven thermoelectric materials in power generation devices operating above 600 °C and up to 1000 °C in heat conversion into electricity using a radioisotope as the heat source. In addition to radioisotope applications, SiGe thermoelectric materials have many other potential applications, for example, solar thermal to electricity energy conversion and waste heat recovery. However, traditional SiGe alloy material shows low ZT values of about 0.93 at 900 °C, thus, 8% is the highest device efficiency for commercial SiGe thermoelectric devices. Recently, many efforts have been made to enhance the dimensionless thermoelectric figure-of-merit (ZT) of SiGe alloys. Among them, the nano approach has been recognized as an effective mechanism to obtain thermoelectric materials with good performance. In this approach, dense bulk samples with random nanostructures with high interface densities are synthesized through ball milling and a direct current hot press, leading to an enhancement ZT through reduced phonon thermal conductivity. Such a practical technique produced samples of nanostructured p-type dense bulk bismuth antimony telluride with a peak ZT of 1.4 at 1000 °C from either alloy ingot or elemental chunks. However, the generality of this approach has not been demonstrated. Here, we applied the same technique in SiGe system in order to fabricate a nanostructured n-type SiGe alloy with enhanced thermoelectric properties. In this thesis, numerous nanostructured n-type SiGe alloy samples were successfully pressed. The structure of these nanostructured samples was investigated via XRD, EDS, and TEM. It has been confirmed that many nano grains exist in our nanostructured samples
Thesis (PhD) — Boston College, 2009
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

The effect of aluminium additions on the structure and morphology of the corrosion products formed on the surfaces of rapidly solidified Mg-Al alloy splats immersed at room temperature in a solution of 3%NaCl saturated with Mg(OH)2; has been studied under different analytical techniques. The adverse effect of contamination from copper particles during processing on the corrosion behaviour of the alloys is also highlighted. Aluminium additions were beneficial to the corrosive behaviour of the alloys with a marked improvement in their anti-corrosion resistance occurring in alloys containing more than 10 wt.% Al. This is attributed to the presence of aluminium ions in the prior oxide/hydroxide in the surface of the alloy. The thickness of the latter decreased with enrichment of aluminium ions and was 10-50nm for the Mg-16Al alloy splats as compared with 200nm for the Mg-3.5Al alloy splats. Hydromagnesite (3MgCO3. Mg(OH)2. 3H2O) formed as an overlayer on the surface of the alloy splats depending on the handling conditions. For the Mg-10Al and Mg-16Al alloy splats an admixture of a high temperature spinel (MgA12O4) in perlclase (MgO) and/or brucite (Mg(OH)2) was detected by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It Is proposed that in the corrosive environment the Al3+ ions on the surface compete successfully with the chlorine ions for the anodic sites on the surface and anchor the growth of the layered brucite structure by the formation of a compound belonging to the pyroaurite-sjogrenite group of compounds. Hydroxyl ions, water, chlorine ions and carbonate ions are incorporated in The interlayers of the layered brucite structure. The formation of a double hydroxide with an acicular morphology and a structure close to that of hydrotalcite-manasseite (Mg6A12(OH)16. CO3. 4H2O) has been supported by scanning transmission electron microscopy (STEM), XPS, XRD, multi-element mapping by electron probe microanalysis (EPMA) and Rutherford backscattering spectrometry (RBS) analyses on the corroded splats. A growth mechanism is proposed on the basis of the structural chemistry, surface morphology and crystal structure of the corrosion products. The implications of this work for the design of Mg base alloys with improved corrosion properties are also discussed. The selected alloying elements are in excellent agreement with those selected from other studies on the development of corrosion resistant Mg alloys.

Bulk metallic glasses and multi-principal element alloys represent relatively new classes of multi-component engineering materials designed for satisfying multiple functionalities simultaneously. Correlating the microstructure with mechanical behavior (at the microstructural length-scales) in these materials is key to understanding their performance. In this study, the structure evolution and nano-mechanical behavior of these two classes of materials was investigated with the objective of fundamental scientific understanding of their properties. The structure evolution, high temperature nano-mechanical behavior, and creep of two Zr-based alloys was studied: Zr41.2Ti13.8Cu12.5Ni10.0Be22 (Vitreloy1) and Zr52.5Ti5Cu17.9Ni14.6All0 (Vitreloy105). Devitrification was found to proceed via the formation of a metastable icosahedral phase with five-fold symmetry. The deformation mechanism changes from inhomogeneous or serrated flow to homogenous flow near 0.9Tg, where Tg is the glass transition temperature. The creep activation energy for Vitreloy1 and Vitreloy105 were 144 kJ/mol and 125 kJ/mol, respectively in the range of room temperature to 0.75Tg. The apparent activation energy increased drastically to 192 kJ/mol for Vitreloy1 and 215 kJ/mol for Vitreloy105 in the range of 0.9Tg to Tg, indicating a change in creep mechanism. Structure evolution in catalytic amorphous alloys, Pt57.5Cu14.7Ni5.3P22.5 and Pd43Cu27Ni10P20, was studied using 3D atom probe tomography and elemental segregation between different phases and the interface characteristics were identified. The structure evolution of three multi-principal element alloys were investigated namely CoCrNi, CoCrFeMnNi, and Al0.1CoCrFeNi. All three alloys formed a single-phase FCC structure in as-cast, cold worked and recrystallized state. No secondary phases precipitated after prolonged heat treatment or mechanical working. The multi-principal element alloys showed less strain gradient plasticity compared to pure metals like Ni during nano-indentation. This was attributed to the highly distorted lattice which resulted in lesser density of geometrically necessary dislocations (GNDs). Dislocation nucleation was studied by low load indentation along with the evaluation of activation volume and activation energy. This was done using a statistical approach of analyzing the "pop-in" load marking incipient plasticity. The strain rate sensitivity of nanocrystalline Al0.1CoCrFeNi alloy was determined by in situ compression of nano-pillars in a Pico-indenter. The nanocrystalline alloy demonstrated a yield strength of ~ 2.4 GPa, ten times greater than its coarse grained counterpart. The nanocrystalline alloy exhibited high strain rate sensitivity index of 0.043 and activation volume of 5b3 suggesting grain boundary dislocation nucleation.

In this study, the surface degradation behavior was studied for typical examples from bulk metallic glasses (BMGs), metallic glass composites (MGCs) and high entropy alloys (HEAs) alloy systems that are of scientific and commercial interest. The corrosion and wear behavior of two Zr-based bulk metallic glasses, Zr41.2Cu12.5Ni10Ti13.8Be22.5 and Zr57Cu15.4Ni12.6Al10Nb5, were evaluated in as-cast and thermally relaxed states. Significant improvement in corrosion rate, wear behavior, and friction coefficient was seen for both the alloys after thermal relaxation. Fully amorphous structure was retained with thermal relaxation below the glass transition temperature. This improvement in surface properties was explained by annihilation of free volume, the atomic scale defects in amorphous metals resulting from kinetic freezing. Recently developed MGCs, with in situ crystalline ductile phase, demonstrate a combination of mechanical properties and fracture behavior unseen in known structural metals. The composites showed higher wear rates but lower coefficient of friction compared to monolithic amorphous glasses. No tribolayer formation was seen for the composites in sharp contrast to that of the monolithic metallic glasses. Corrosion was evaluated by open circuit potential (OCP) analysis and potentiodynamic polarization. Site-specific corrosion behavior was studied by scanning vibration electrode technique (SVET) to identify formation of galvanic couples. Scanning kelvin probe microscope was used to map elecropositivity difference between the phases and linked to wear/corrosion behavior. Phases with higher elecropositivity were more susceptible to surface degradation. Wear and corrosion synergy in marine environment was evaluated for two high entropy alloys (HEAs), CoCrFeMnNi and Al0.1CoCrFeNi. Between the two alloys, Al0.1CoCrFeNi showed better wear resistance compared to CoCrFeMnNi in dry and marine conditions due to quicker passivation, a higher magnitude of polarization resistance and significantly larger pitting resistance.

The aim of this study is to form a theoretical model for simulation of glass forming ability of Fe &
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Based bulk amorphous alloys, to synthesize Fe &
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based multicomponent glassy alloys by using the predictions of the theoretical study, and to analyze the influence of crystallization and solidification kinetics on the microstructural features of this amorphous alloys. For this purpose, first, glass forming ability of Fe &
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(Mo, B, Cr, Nb, C) &
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X ( X = various alloying elements, selected from the periodic table) ternary alloy systems were simulated for twenty different alloy compositions by using the electronic theory of alloys in pseudopotential approximation and regular solution theory. Then, by using the results of the theoretical study, systematic casting experiments were performed by using centrifugal casting method. The alloying elements were melted with induction under argon atmosphere in alumina crucibles and casted into copper molds of different shapes. Characterization of the cast specimens were performed by using DSC, XRD, SEM, and optical microscopy. Comparison of equilibrium and nonequilibrium solidification structures of cast specimens were also performed so as to verify the existence of the amorphous phase. Good agreement of the results of experimental work, with the predictions of the theoretical study, and the related literature was obtained.

The aim of this study is to investigate the bulk glass forming ability (BGFA) of Ti-based alloy systems. These investigations were carried out in two main parts that are complementary to each other: theoretical and experimental. For theoretical studies, which are based on electronic theory of alloys in pseudopotential approximation, Ti-Zr, Ti-Co and Ti-Cu alloys were chosen as the binary systems. Alloying element additions were performed to each binary for the investigation of the BGFA of multicomponent Ti-based alloys. Among the three studied binary systems, Ti-Cu was found to exhibit better BGFA, and Mn, Al and Ni elements were found to be suitable for improving the BGFA of Ti-Cu binary alloy system. BGFA of Ti-Cu binary and Ti-Cu-(Mn, Al, Ni) multicomponent alloys were investigated with the experimental studies that were carried out with performing arc melting and centrifugal casting operations. The characterizations of these alloys were done with scanning electron microscopy, X-ray diffraction analysis and differential scanning calorimetry. Ti60Cu35Mn5, Ti60Cu35Al5 and Ti60Cu35Ni5 alloys were produced and characterized as examples for ternary systems. Among them, Ti60Cu35Mn5 system was found to have better indications regarding to BGFA. Therefore, it was chosen as the main composition and multicomponent alloys of Ti59Cu35Mn5Al1, Ti59Cu35Mn5Ni1 and Ti58Cu35Mn5Al1Ni1 were synthesized and characterized.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 96-100).
An accelerated sintering method called 'nano-phase separation sintering' is developed, with specific applicability to nanostructured tungsten alloys. Nanocrystalline tungsten alloys containing minority additions of chromium are produced by high-energy ball milling and then consolidated. Such alloys exhibit the onset of sintering at a very low temperature around 950 °C and a very rapid rate of densification. The mechanism of this accelerated sintering is established through understanding the role of nano-scale, solid second phase precipitation during the sintering cycle, as analyzed by thermomechanical analysis, electron microscopy and x-ray diffraction. In addition, control experiments are used to establish that the accelerated sintering is apparently accomplished from the combination of two features of the powders: (i) nanocrystallinity and (ii) alloy supersaturation. In addition to accelerating sintering, the incorporation of alloying elements and second phases are also beneficial for mitigating grain growth during a thermal cycle, so nanophase separation sintering is thus naturally appropriate to the production of fine-grained bulk materials. Sintered compacts achieved through nano-phase separation sintering display 10~30 times smaller grain sizes at comparable densities than those produced by conventional accelerated sintering methods such as solid-state activated sintering and liquid phase sintering. The thermodynamic features and conditions for nano-phase separation sintering are further explored based on the binary phase diagram in order to generalize the concept to other alloy systems. After presenting a series of proposed alloy design rules, the consolidation of chromium with an addition of nickel is accelerated. Prospects of the technique for the development of full density bulk products in more complex alloy systems are also discussed.
by Mansoo Park.
Ph. D.

Al93Fe3Cr2Nb2 (at.%) nanoquasicrystalline alloys have been shown to have the potential to push the applications of aluminium alloys to more elevated temperatures, by maintaining a high strength. They also have more thermally stable microstructures than previous nanoquasicrystalline alloys from similar systems (the most studied of which is Al93Fe3Cr2Ti2 (at.%)). Al93Fe3Cr2Nb2 (at.%) alloys have never previously been produced in samples on a scale larger than melt-spun ribbon. This study examines the production parameters of bulk nanoquasicrystalline Al-Fe-Cr-Nb alloys. Firstly an attempt was made to reduce the melting temperatures of thermally stable nanoquasicrystalline alloys through additional alloying. The melting processes of binary, ternary, quaternary and quinary nanoquasicrystalline alloys was analysed though DTA, with endothermic reactions up to 1034oC observed. Rapidly solidified Al-Fe-Cr-Nb alloys were then produced in kilogram quantities through gas atomisation at an industrial scale. The smallest atomised powder particles contained fine scale microstructures consisting of nano-scale quasicrystals embedded in an aluminium matrix. As the cooling rate of the powder particles decreased new phases, including the theta phase (Al13(Fe,Cr)2-4) and Al3Nb were produced. 0-25μm, 25-50μm and 50-75μm (diameter) size fractions of atomised powder were each consolidated through extrusion to produce nanoquasicrystalline Al-Fe-Cr-Nb bars. Composite bars of the nanoquasicrystalline alloy mixed with 10(vol.)% and 20(vol.)% pure aluminium were also produced. The consolidation of the nanoquasicrystalline atomised powders through extrusion led to precipitation of intermetallics including (Al13(Fe,Cr)2-4) and Al3Nb, particularly in the smallest powder size fractions with the most metastable microstructures. Finally the effects of the atomisation and extrusion conditions on the microstructure and its mechanical properties were studied. Improved strength, coupled with reduced ductility was observed with decreases in the elemental aluminium composition of the Al-Fe-Cr-Nb bars and the powder size fraction they were produced from. There was however improvements in toughness of the extruded composite bars, over the nanoquasicrystalline alloy bars.

Mg-V and Mg-Zr alloys with nominal compositions 1, 6, 17.5, 27 wt% V and 2, 8.6 and 10.6 wt% Zr respectively were produced by PVD. All deposits exhibited compositional inhomogeneity, columnar microstructures and a strong basal texture. The solid solubilities of V and Zr in Mg were extended approximately to 17 wt% V and 10 wt% respectively. Grain refinement occurred with increasing solute content. The solid solution break up temperature decreased as the V and Zr content in the alloys increased. Pure V precipitated when the extended solid solubility of was exceeded. Both c and a lattice parameters, as well as the c/a ratio decreased with increasing V content in the Mg-V alloys. The slight increase of the a-lattice parameter and the decrease of the c one led to a decrease of the c/a ratio with increasing Zr additions in the Mg-Zr alloys. The air-formed oxide on the surfaces of the Mg-V alloys consisted predominantly of hydromagnesite at the outermost surface with Mg(OH)2 in excess of MgO underneath. No evidence of V oxide in the surface film was found. Magnesium oxide was also found between the grains of the deposits. The air-formed oxide on the surfaces of the Mg-Zr alloys consisted of ZrO2, MgO and possibly Zr sub-oxide. The presence of the oxides beween the columnar grains gave rise to graded metal/oxide interfaces. The outermost surfaces of the Mg-Zr alloys were similar to the Mg-V ones. Analysis of changes of the Auger parameters of the Mg-V and Mg-Zr alloys was also undertaken in order to investigate the electronic changes that take place upon alloying Mg with V and Zr. Charge transfer between 0.09 and 0.11 electrons/atom from Mg to V as well as changes in the V d charge were calculated by measuring the Mg and V Auger parameters and using the charge transfer model of Thomas and Weightman. Electron transfer between 0.02 and 0.03 electrons/atom from Mg to Zr was also found to occur upon alloying Mg with Zr. The electron transfer has been related to changes in crystal structure. The Mg-V and Mg-Zr alloys were examined after immersion in 3 wt% NaCl solution for 5 and 15 minutes, 9 hours and 7 days. The dramatic increase in the corrosion rate of the Mg-V alloys was attributed to the precipitation of pure V. The unsatisfactory corrosion performance of the Mg-V alloys was attributed to the absence of compositional uniformity through the thickness of the Mg-V deposits and the low thermodynamic stability of the corrosion products in the saline environment. Hydromagnesite at the outermost surface and Mg(OH)2, MgO and V2O4 in the bulk of the corrosion layer were the corrosion products. MgH2 and areas enriched in metallic V within the bulk of the corrosion products were also detected. The low corrosion rates of the Mg-Zr alloys, the lowest ever reported for Mg alloys, were attributed to the nature of the corrosion products and particularly the Zr contribution. The corrosion products were enriched in Zr, and were non-porous and in many cases well adherent. X-ray and electron diffraction suggested the existence of only Mg(OH)2 and MgO in the corrosion products, indirectly implying the participation of zirconium oxide/hydroxide in an amorphous/nanocrystalline state. Surface analysis indicated that a Zr oxide coexisted with Mg(OH)2 and MgO below a magnesium carbonate overlayer and also suggested the existence of Zr hydrous oxide (hydroxide). The repetition of the substrate pattern, as well as the fact that Zr hydroxide was replaced with ZrO2 and Zr sub-oxide as the metal-oxide interface was approached, implied a corrosion mechanism involving inwards diffusion of the anionic species.

First-principles methods based on Density functional theory (DFT) are now adopted routinely to calculate the properties of materials. However, one of the biggest challenges of DFT is to describe the electronic behaviors of random alloys. One of the aims of this thesis is to study binary alloys, e.g. Ti-Al, Cu-Au, and multi-component alloys by using two models for chemically random structures: the special quasi-random structure (SQS) and coherent potential approximation (CPA). I investigate these approaches by focusing on the local lattice distortion (LLD) and the crystal symmetry effects. Within the SQS approach, the LLD effect can be modeled in a straightforward manner by relaxing the positions of atoms in the supercell. However, within this approach, it is difficult to model the random multi-components alloys due to the large size of the supercells. On the other hand, the CPA approach uses single-site approximation and thus it is not limited by the number of alloy components. But CPA suffers from the neglect of the local lattice relaxation effect, which in some systems and for some properties could be of significant importance. In my studies, the SQS and CPA approaches are combined with the pseudopotential method as implemented in the Vienna Ab-initio Simulation Package (VASP) and the Exact Muffin-Tin Orbitals (EMTO) methods, respectively. The mixing energies or formation enthalpies and elastic parameters of fcc Ti1-xAlx and Cu1-xAux (0 =< x =< 1) random solid solutions and high-entropy multicomponent TiZrVNb, TiZrNbMo and TiZrVNbMo alloys are calculated as a function of concentration. By comparing the results with and without local lattice relaxations, we find that the LLD effect is negligible for the elastic constants C11, C12, and C44. In general, the uncertainties in the elastic parameters associated with the symmetry lowering in supercell studies turn out to be superior to the differences between the two alloy techniques including the effect of LLD. However, the LLD effect on the mixing energies or formation enthalpies is significant and depends on the degree of size mismatch between alloy constituents. In the cases of random Cu-Au and high-entropy alloys, the formation enthalpies and mixing energies are significantly decreased when the LLD effect is considered. This finding sets the limitations of CPA for the mixing energies or formation enthalpies of alloys with large atomic size differences. The other goal of the thesis is to study the effect of exchange-correlation functionals on the formation energies of ordered alloys. For this investigation, we select the Cu-Au binary system which has for many years been in the focus of DFT and beyond DFT schemes. The Perdew-Burke-Ernzerhof (PBE) approximation to the exchange-correlation term in DFT is a mature approach and have been adopted routinely to investigate the properties of metallic alloys. In most cases, PBE provides theoretical results in good agreement with experiments. However, the ordered Cu-Au system turned out to be a special case where large deviations between the PBE predictions and observations occur. In this work, we make use of a recently developed exchange-correlation functional, the so-called quasi-nonuniform exchange-correlation approximation (QNA), to calculate the lattice constants and formation energies for ordered Cu-Au alloys as a function of composition. The calculations are performed using the EMTO method and verified by a full-potential method. We find that the QNA functional leads to an excellent agreement between theory and experiment. The PBE strongly overestimates the lattice constants for ordered Cu3Au, CuAu, CuAu3 compounds and also for the pure metals which are nicely corrected by the QNA approach. The errors in the formation energies of Cu3Au, CuAu, CuAu3 relative to the experimental data decrease from 38-45% obtained with PBE to 5-9% calculated for QNA. This excellent result demonstrates that one can reach superior accuracy within DFT for the formation energies and there is no need to go beyond DFT. Furthermore, it shows that error cancellation can be very effective for the formation energies as well and that the main DFT errors obtained at PBE or LDA levels originate from the core-valence overlap region, which is correctly captured by QNA due to its particular construction. Our findings are now extended to disordered alloys, which is briefly discussed already in one of my published papers.

Qc 20170630

Amorphous and bulk amorphous metallic alloys are an intriguing class of structural materials and possess a range of interesting properties, including near theoretical strength, high hardness, extremely low damping characteristics, excellent wear properties, high corrosion resistance, low shrinkage during cooling and almost perfect as-cast surfaces with good potential for forming and shaping. In this study, new Ti-based bulk amorphous alloys are tried to be modeled and synthesized. For that purpose, electronic theory of alloys in the pseudo potential approximation was used as a tool for understanding the theory lying beneath the bulk glass forming ability (BGFA). The results from this approach were evaluated both separately and together with the other theories supposed by our colleagues. Glass forming parameters of ordering energy, &
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HM, viscosity, mismatch entropy, Rc was calculated for various Ti-based binary and ternary and the change in these parameters in both cases was evaluated. The results of the theoretical calculations of glass forming parameters has shown good relation with the literature data that the predicted alloying elements, i.e. Mo, Hf, Zr, B, Fe, Sn, and Be, to increase GFA for Ti2Ni binary system were generally used in the production of Ti-based bulk amorphous alloys. In the second part of this thesis, new Ti-based compositions with high GFA were tried to be synthesized with light of these results and encouraging conclusions were drawn. The production of these alloys were made with centrifugal casting method which is relatively a new technique for producing such alloys and the characterization of these alloys were made with metallographic, X-ray and thermal means.

Bulk metallic glasses (BMGs) have received significant research interest due to their completely amorphous structure which results in unique structural and functional properties. Absence of grain boundaries and secondary phases in BMGs results in high corrosion resistance in many different environments. Understanding and tailoring the corrosion behavior can be significant for various structural applications in bulk form as well as coatings. In this study, the corrosion behavior of several Zr-based and Fe-Co based BMGs was evaluated to understand the effect of chemistry as well as quenched in free volume on corrosion behavior and mechanisms. Presence of Nb in Zr-based alloys was found to significantly improve corrosion resistance due to the formation of a stable passive oxide. Relaxed glasses showed lower rates compared to the as-cast alloys. This was attributed to lowering of chemical potential from the reduced fraction of free volume. Potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS) techniques helped in quantifying the corrosion rate and polarization resistance. The effect of alloy composition was quantified by extensive surface analysis using Raman spectroscopy, energy dispersive x-ray spectroscopy and auger spectroscopy. Pitting intensity was higher in the as-cast glasses than the relaxed glasses. The electrochemical behavior of a Zr-Ti-Cu-Ni-Be bulk metallic glass subjected to high strain processing was studied. High strain processing caused shear band formation and an increase in the free volume. Potentiodynamic polarization and EIS showed a strong correlation between the enthalpy of structural relaxation and corrosion rate and polarization resistance. Pitting was observed to preferentially occur on shear bands in the processed samples, while it was stochastic in unprocessed glass. The corrosion analysis of Co-Fe glasses showed an increase in corrosion current density when Fe content was increased from 0 to 7 at%. The corrosion resistance improved when Fe content was further increased to 15 at%. Similar trend was seen in EIS studies. The improved corrosion resistance at 15 at% Fe can be attributed to the large supercooled region that facilitates the formation of completely amorphous alloy, in contrast to lower Fe containing alloys, where short range ordering may deteriorate the corrosion resistance. Porous metallic glass structure was developed by electrochemical dealloying via cyclic voltammetry. Mechanical properties and changes in electrical conductivity were measured as a function of depth from surface by nano-indentation and nano electrical contact resistance technique. The nanoporous layer was found have hardness of 0.41 GPa and elastic modulus of nearly 22 GPa. The resistivity of the nanoporous layer continuously decreased when moving towards the substrate as the indentation depth increased which is attributed to the gradient in pore size.

The aim of this study, which was carried out in two main parts, is to investigate the glass forming ability of Fe-based systems. The first part involves the theoretical modeling to cover the requirement of a predictive model to identify the Fe-based alloy families that have high glass forming ability in the frame of atomistic and thermodynamic approach. The second part involves the experimental investigations to prove the results of the conducted theoretical modeling studies. For this purpose, in the first part, theoretical investigations were performed to identify the third alloying elements that will lead to an increase in the glass forming ability on the base of electronic theory of alloys in pseudopotential approximation for selected Fe- based systems, Fe - (B, Zr, Nb, C, W). In the experimental part, in the frame of the theoretical investigation results, one of the theoretically modeled binary system, and the third alloying elements that were predicted to lead an increase in the glass forming ability of the selected binary system, were determined. As a first step, designated compositions were synthesized by using low grade conventional Fe-B alloy as a raw material by using centrifugal casting technique and copper mold casting method. To compare the results, same compositions were also cast from the high purity elements by using the same technique and method. For the characterization of these cast specimens, DSC, XRD, SEM, EDS and metallographic examination techniques were used. Amorphous structure was successfully obtained in the thin sections of the wedge-cast samples for Fe-B-Nb and Fe-B-W ternary systems.

The subject of hard magnetic materials is important from the both practical as well as scientific point of view. Researches in this field are focused on new materials with strong enough hard magnetic properties but with lower rare earth content than for the classical Nd rich alloys. The presented PhD thesis refers to preparation, structural and magnetic properties of the Fe-Nb-B-RE type of bulk nanocrystalline alloys. As the preparation technology of the bulk alloys, the so-called vacuum suction casting was chosen. The chemical compositions of the examined alloys is originated from the Fe-Nb-B (NANOPERM) amorphous melt spun ribbons in which niobium, as an alloying addition, slows down crystallization of iron leading to some optimization of magnetic properties. The PhD thesis is focused on: i) magnetic interactions in multi-phase magnetic materials, ii) magnetism in TM-RE disordered structure, iii) influence of microstructure on selected physical properties and iv) numerical modeling and characterization of the nanomagnetic structures. From application point of view, especially important is a combination of chemical compositions and technology parameters (cooling rate, melting current) of the studied alloys, in order to improve hard magnetic characteristics and / or decrease the RE content without deterioration of their desired properties. The performed investigations consist of fabrication of about 80 different alloys characterized by several structural and magnetic measurement techniques like X-ray diffraction, Mössbauer spectroscopy, DSC, SEM, AFM / MFM, Kerr microscopy, magnetic balance as well as SQUID magnetometer. It was shown that the phase structure, microstructure and magnetic properties strongly depends on the chemical composition (the RE and Nb content) as well as technology parameters (the sample diameter and the melting current). The optimal parameters were established as: i) Tb as the RE element with the content of 10-12 at. %, ii) Nb content of 6-8 at. %, iii) sample diameter ranged from 0.5 to 1.5 mm and iv) melting current I = 35 A. The alloys reveal hard magnetic properties with a high and ultra-high coercivity depending on the niobium content. Particularly, for the field-annealed (Fe80Nb6B14)0.88Tb0.12 alloy, the coercive field measured at room temperature exceeds 7 T which is a unique feature in the case of bulks. The observed magnetic hardening effect is controlled by the niobium content in the combination with the specific solidification rate (during casting). The observed phase segregation leads to the formation of grain microstructure with the irregularly shaped dendrites separated by inter-dendritic regions. This structure is responsible for an additional shape as well as surface anisotropy and thereby it is a source of some ultra-hard magnetic objects. The carried out simulations proved the proposed micro-magnetic picture of the alloys and indicate a significant role of the ultra-hard magnetic objects in the magnetization processes. Generally, as was shown in the presented thesis, the examined alloys can be considered as high and ultra-high coercive materials with application potential in the fields of permanent magnets where increasing resistance to external magnetic field is required.

The structural, electronic, magnetic, magnetocaloric, and transport properties of doped Ni-Mn-(In, Sn) based Heusler alloys were studied using neutron diffraction, x-ray diffraction (XRD), differential scanning calorimetry (DSC), high field magnetization, specific heat, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and hydrostatic pressure measurements. The adiabatic temperature change (∆Tad) by a direct method and through thermomagnetic measurements in magnetic fields up to 14 T has been performed for these alloys. Also the mixed effect of pressure and magnetic field on the transition temperature of these alloys are discussed. In order to develop new magnetocaloric and multifunctional materials, the synthesis and characterization of Heusler alloys in reduced dimensions, i.e., ribbons and thin films has been performed. In addition, the structural, magnetic, and magnetocaloric properties of Ni-based binary alloys were investigated, including saturation magnetization and Curie temperature (TC) for the possible applications in self controlled magnetic hyperthermia applications.

Al-Mg alloys were infiltrated into porous alumina preforms at temperatures greater than 950°C where significant amount of nitride forms in the matrix. The present work aims to obtain a process window for growing A1N rich composites over uniform thicknesses so that bulk fabrication of these composites could be carried out. Initial experiments were carried out in a thermo-gravimetric analyser (TGA) to establish suitable conditions for growing useful thicknesses. Al- 2wt% Mg alloy, alumina preforms of particle size 53-63μm and N2 - 2% H2 (5ppm O2) were used for the present study based on previous work carried out in the fabrication of MMCs at low temperatures. Experiments carried out in the TGA indicate that oxygen in the system has to be gettered for the growth of nitride rich composites. Infiltration heights of about 8mm were obtained using an external getter (Al - 5wt%Mg) alloy in addition to the base alloy used for infiltration. The above process conditions were subsequently employed in a tube furnace to fabricate bulk composites and to study the effect of temperature on the volume fraction of aluminium nitride in the matrix. The volume fraction of nitride in the composite varied between 30 and 95 vol % with increase in process temperature from 950°C to 1075°C. Microstructures of these composites indicate that A1N starts to form on the particle surface and tends to grow outwards. The metal supplied through channels adjacent to the particle surface nitride until a point is reached when the composite growing from the adjacent particles meet each other and isolate the melt underneath from nitrogen thereby leading to a metal rich region underneath. Increase in temperature results in an increased nitridation rate resulting in reduced metal pocket size. Composites fabricated at 975°C had a minor leak at the O-rings, which seal the tube. This led to infiltration under conditions of varying oxygen partial pressure leading to different nitride fractions in the composite. The above fact was confirmed by conducting an experiment with commercial purity nitrogen, which has an oxygen content of about 5000ppm. The composite had an A1N content of about 30% whereas the composite fabricated with N2 -2%H2 (5ppm oxygen) showed a nitride content of 64%. This suggests that one can vary the nitride content in the composite by varying the oxygen content in the system at a particular process temperature. The hardness of the matrix increases with increase in process temperature from 3.5 ± 0.7 GPa at 975°C to about 9.8 ± 0.9 GPa at 1075°C. Porosity was observed in the composite processed at 1075°C. This increased porosity leads to decreased hardness though the nitride content in the composite has increased by 11%. The scatter in the data is attributed to variations in the microstructure as well as due to interference from underlying metal pockets or particles as well as due to porosity introduced in the composite at high processing temperatures.

The large elastic limit, the strength close to the theoretical limit, the excellent magnetic properties and good corrosion resistance of bulk metallic glasses (BMGs) make them promising for several applications such as micro-geared motor parts, pressure sensors, Coriolis flow meters, power inductors and coating materials. The main limitation of these materials is their reduced macroscopic ductility at room temperature, resulting from an inhomogeneous deformation concentrated in narrows shear bands. The poor ductility can be overcome by the incorporation of a ductile second phase in the glassy matrix to form composites, which exhibit a better balance between strength and ductility. Different types of BMG composites have been developed to date but considerable plastic strain during tensile or bending tests has been only obtained for composites with in-situ formation of the second phase during solidification. Among these in-situ formed composites, significant tensile ductility has been only observed for two types of alloys so far: TiZrBe-based and CuZr-based BMG composites. The former precipitate dendrites of the cubic β-(Ti,Zr) phase in the glass matrix, whereas the latter combine spherical precipitates of the cubic B2-CuZr shape memory phase within the glass. The CuZr-based BMG composites have certain advantages over the TiZrBe-based composites such as the absence of Be, which is a toxic element, and exhibit a strong work-hardening behaviour linked to the presence of the shape memory phase. This concept of “shape memory” BMG composites has been only applied to CuZr-based alloys so far. It is worth investigating if such a concept can be also used to enhance the plasticity of other BMGs. Additionally, the correlation between microstructure, phase formation and mechanical properties of these composites is still not fully understood, especially the role of the precipitates regarding shear band multiplication as well as the stress distribution in the glassy matrix, which should be significantly influenced by the precipitates. The aim of the present work is to develop a new family of shape memory bulk metallic glass composites in order to extend the concept initially developed for CuZr-based alloys. Their thermal and mechanical properties shall be correlated with the microstructure and phase formation in order to gain a deeper understanding of the fundamental deformation mechanisms and thermal behaviour. A candidate to form new shape memory BMG composites is the pseudo-binary TiCu-TiNi system because bulk glassy samples with a critical casting thickness of around 1 mm have been obtained in the compositional region where the cubic shape memory phase, B2-TiNi, precipitates. This phase undergoes a martensitic transformation to the orthorhombic B19-TiNi during cooling at around 325 K. The B2- and B19-TiNi exhibit an extensive deformation at room temperature up to 30% during tensile loading. Compositions in the Ti-Cu, Ti-Cu-Ni, Ti-Cu-Ni-Zr, Ti-Cu-Ni-Zr-(Si) and Ti-Cu-Ni-Co systems were selected based on literature data and on a recently proposed λ+Δh1/2 criterion, which considers the effect of atomic size mismatch between the elements and their electronic interaction. Samples were then produced by melt spinning (ribbons) and Cu-mould suction casting (rods and plates). The investigation started in the Ti-Cu system. A low glass-forming ability (GFA) was observed with formation of amorphous phase only in micrometer-thick ribbons and the results showed that the best glass former is located around Ti50Cu50. Considering that the GFA of the binary alloys can be further improved with additions of Ni, new Ti-Cu-Ni shape memory BMG composites were then developed in which the orthorhombic Ti(Ni,Cu) martensite precipitates in the glassy matrix. These alloys exhibit a high yield strength combined with large fracture strain and the precipitates show a reversible martensitic transformation from B19 to B2-type structure at a critical temperature around 320 K (during heating). The amorphous matrix stabilizes the high-temperature phase (B2 phase), which causes different transformation temperatures depending on whether the precipitates are partially or completely embedded in the glassy matrix. The deformation starts in the softer, crystalline phase, which generates a heterogeneous stress distribution in the glassy matrix and causes the formation of multiple shear bands. The precipitates also have the important function to block the fast movement of shear bands and hence retard fracture. However, the size of such composites is limited to 1 mm diameter rods because of their low GFA, which can be further improved by adding CuZr. New Ti-Cu-Ni-Zr composites with diameter ranging from 2 to 3 mm were developed, which consist mainly of spherical precipitates of the cubic B2-(Ti,Zr)(Cu,Ni) and the glassy phase. The interrelation between composite strength and volume fraction of B2 phase was analysed in detail, which follows the rule of mixture for values lower than 30 vol.% or the load-bearing model for higher values. The fracture strain is also affected by the volume fraction of the respective phases with a maximum observed around 30 vol.% of B2 phase, which agrees with the prediction given by the three-body element model. It was observed that the cubic B2 phase undergoes a martensitic transformation during deformation, resulting in a strong work hardening and a high fracture stress of these alloys. The GFA of the Ti-Cu -based alloys can be further increased by minor additions of Si. A maximum GFA is observed for additions of 1 and 0.5 at.% Si to binary Ti-Cu or quaternary Ti-Cu-Ni-Zr alloys, respectively. This optimum GFA results from the formation of a lower amount of highly stable Ti5Si3 precipitates, which act as nuclei for other crystalline phases, and the increased stability of the liquid and the supercooled liquid. The addition of Co has the opposite effect. It drastically decreases the GFA of Ti-Cu-Ni alloys and both the martensitic transformation temperature and their mechanical behaviour seem to correlate with the number and concentration of valence electrons of the B2 phase. The transformation temperature decreases by increasing the concentration of valence electrons. An excellent combination of high yield strength and large fracture strain occurs for Ti-Cu-Ni-Zr and Ti-Cu-Ni-Zr-Si alloys with a relatively low amount of CuZr, with a fracture strain in compression almost two times larger than the one usually observed for CuZr-based composites. For instance, the Ti45Cu39Ni11Zr5 alloy exhibit a yield strength of 1490±50 MPa combined with 23.7±0.5% of plastic strain. However, a reduced ductility was found for the CuZr-richer Ti-Cu-Ni-Zr compositions, which results from the precipitation of the brittle Cu2TiZr phase in the glassy matrix. The present study extends the concept of “shape memory BMG matrix composites” originally developed for CuZr-based alloys and delivers important insights into the correlation between phase formation and mechanical properties of this new family of high-strength TiCu-based alloys, which upon further optimization might be promising candidates for high-performance applications such as flow meters, sensors and micro- and mm-sized gears
Auf Grund der hohen Elastizitätsgrenze, Festigkeiten, die nahe an der theoretischen Grenze liegen, sehr guten magnetischen Eigenschaften, sowie einer guten Korrosionsbeständigkeit erscheint der Einsatz massiver metallischer Gläser (BMG) vielversprechend in zahlreichen Gebieten, wie z.B. in Mikro-Getriebemotorteilen, Coriolis-Massendurchflussmessern, Drucksensoren, Speicherdrosseln und als Beschichtungsmaterialien. Der Einsatz dieser Materialien wird jedoch hauptsächlich durch ihre begrenzte makroskopische Duktilität bei Raumtemperatur eingeschränkt. Diese resultiert aus einer inhomogenen Verformung, die in schmalen Scherbändern konzentriert ist. Die unzureichende Duktilität kann durch das Einbringen einer zweiten, duktilen Phase in die Glas-Matrix verbessert werden, so dass Komposite gebildet werden. Diese Komposite weisen in der Regel immer noch hohe Festigkeiten auf, lassen sich aber gleichzeitig deutlich besser plastisch verformen. Es wurden bereits verschiedene Arten von massiven metallischen Glas-Matrix-Kompositen entwickelt. Jedoch konnte die plastische Verformbarkeit in Zug- oder Biegeversuchen nur in den Materialien erhöht werden, in denen sich die zweite Phase bei der Erstarrung ausscheidet. Unter diesen in-situ Kompositen konnte eine signifikante Duktilität lediglich für zwei Legierungstypen beobachtet werden: massive metallische Gläser auf TiZrBe- und auf CuZr-Basis. Die Ausscheidungen der kubischen β-(Ti,Zr) Phase wachsen dendritenartig in die Glas-Matrix, wohingegen sich in letzterem Legierungstypen sphärische Ausscheidungen der Formgedächtnislegierung, B2-CuZr, im Glas bilden. CuZr-Basislegierungen haben dabei den großen Vorteil, dass sie kein Be enthalten, welches toxisch ist. Außerdem weisen diese Komposite auch dank der Formgedächtnisphase eine starke Kaltverfestigung auf. Das Konzept, massive metallische Formgedächtnis-Glas-Matrix-Komposite herzustellen, um die mechanischen Eigenschaften zu optimieren, wurde bisher nur auf CuZr-Basislegierungen angewandt. Es soll mittels dieser Arbeit nun erforscht werden, ob dieses Konzept auf andere massive metallische Gläser übertragbar ist. Des Weiteren ist der Zusammenhang zwischen Gefüge, Phasenbildung und mechanischen Eigenschaften der Komposite noch nicht vollständig verstanden, insbesondere die Rolle der Ausscheidungen in Bezug auf die Scherbandbildung und die Spannungsverteilung in der Glas-Matrix. Das Ziel der vorliegenden Arbeit ist die Entwicklung einer neuen Klasse massiver, metallischer Formgedächtnis-Glas-Matrix Komposite um das Konzept, welches ursprünglich für CuZr-Basislegierungen entwickelt wurde, zu erweitern. Die thermischen und mechanischen Eigenschaften sollen mit dem Gefüge und der Phasenbildung in Beziehung gesetzt werden, um so die fundamentalen Verformungsmechanismen und ihre Ursachen besser zu verstehen. Der Ausgangspunkt bei der Herstellung neuer massiver metallischer Formgedächtnis-Glas-Matrix Komposite ist das pseudobinäre TiCu-TiNi-System. In diesem System konnten massive Glasproben mit einem kritischen Gießdurchmesser von circa 1 mm hergestellt werden und zwar in dem Zusammensezungsbereich, in dem die kubische Formgedächtnisphase, B2-TiNi, gebildet wird. Während der Abkühlung findet in diesen Kompositen bei etwa 325 K eine martensitische Umwandlung der B2-Phase zur orthorhombischen B19-TiNi Phase statt. B2- und B19-TiNi weisen eine gute Verformbarkeit von bis zu 30% bei Raumtemperatur unter Zugbelastung auf. Die hier erzeugten Ti-Cu, Ti-Cu-Ni, Ti-Cu-Ni-Zr, Ti-Cu-Ni-Zr-(Si) und Ti-Cu-Ni-Co-Legierungen basieren auf Literaturangaben und Vorhersagen bezüglich der Glasbildungsfähigkeit in diesen Systemen mittels λ+Δh1/2-Kriterium, welches die Auswirkungen der Atomgrößenunterschiede der Elemente und deren elektronische Wechselwirkung einbezieht. Die Proben wurden im Schmelzspinnverfahren (Bänder) und mittels Saugguss in einer Cu-Kokille (Stäbe und Bleche) hergestellt. Die Weiter- und Neuentwicklung von Legierungen, beginnt mit dem Ti-Cu-System. Die Glasbildungsfähigkeit in diesem binären System ist nur gering, so dass lediglich mikrometerdicke amorphe Bänder hergestellt werden können. Die Ergebnisse zeigen, dass der beste Glasbildner eine Zusammensetzung von etwa Ti50Cu50 hat. Die Glasbildungsfähigkeit von binären Legierungen kann durch die Zugabe von Ni weiter verbessert werden. Dies führte innerhalb dieser Arbeit zur Entwicklung neuer Ti-Cu-Ni Formgedächtnis-Glas-Matrix Komposite, in welchen die orthorhombische Martensitphase in der Glas-Matrix ausgeschieden wird. Diese ternären Legierungen zeigen eine hohe Zugfestigkeit in Kombination mit einer hohen Bruchdehnung. Beim Überschreiten einer Temperatur von etwa 320 K vollziehen die Ausscheidungen eine reversible martensitische Umwandlung vom B19- zum B2-Strukturtyp. Durch die amorphe Matrix wird die Hochtemperaturphase (B2 Phase) stabilisiert. Dies verursacht unterschiedliche Umwandlungstemperaturen im Kompositmaterial, die davon abhängig sind, ob die Ausscheidungen nur teilweise oder vollständig in der Matrix eingebettet sind. Die Verformung beginnt in der weichen kristallinen Phase, welche eine heterogene Spannungsverteilung in der Glas-Matrix erzeugt und eine hohe Dichte an Scherbändern in der Matrix verursacht. Die Ausscheidungen haben zudem die Funktion, die Ausbreitung der Scherbänder zu blockieren und das Versagen des Materials zu verzögern. Die Größe der Komposite ist jedoch auf Grund der geringen Glasbildungsfähigkeit auf einen Stabdurchmesser von ca. 1 mm begrenzt. Dies kann mit dem Zulegieren von CuZr verbessert werden. Es wurden hier auf diese Weise neue Ti-Cu-Ni-Zr Komposite entwickelt, deren Durchmesser zwischen 2 und 3 mm liegt. Diese bestehen hauptsächlich aus sphärischen Ausscheidungen der kubischen B2-(Ti,Zr)(Cu,Ni)- und der Glasphase. Die wechselseitige Beziehung zwischen der Streckgrenze und dem Volumenanteil der B2-Phase wurde im Detail untersucht. Für kristalline Volumenanteile kleiner als 30 Vol.-% folgt die Streckgrenze der Mischungsregel und für größere Volumenanteile dem „lasttragenden Modell“ (load bearing model). Die Bruchdehnung wird ebenfalls vom Volumenanteil der Phasen beeinflusst und zeigt ein Maximum bei etwa 30 Vol.-% an B2-Phase. Dies stimmt mit der Vorhersage des „Drei-Element-Modells“ überein. Es wurde festgestellt dass die kubische B2-Phase während der Verformung eine martensitische Umwandlung durchführt, was die starke Kaltverfestigung und die hohen Bruchspannungen dieser Legierungen zur Folge hat. Die Glasbildungsfähigkeit von TiCu-Basislegierungen kann im Gegenzug weiterhin durch geringe Si-Zusätze gesteigert werden. Hierbei tritt jeweils ein Maximum bei Zusätzen von 1 und 0,5 at-% Si zu binären Ti-Cu- oder zu quarternären Ti-Cu-Ni-Zr-Legierung auf. Das Optimum der Glasbildungsfähigkeit ist das Ergebnis sowohl eines geringeren Anteils hochschmelzender Ti5Si3-Ausscheidungen, die als Keimbildner für andere kristalline Phasen dienen, als auch der erhöhten Stabilität der Schmelze sowie der unterkühlten Schmelze. Der Zusatz von Co wiederum hat einen gegenteiligen Effekt. Er vermindert die Glasbildungsfähigkeit von Ti-Cu-Ni-Legierungen drastisch. Zudem scheinen sowohl die martensitische Umwandlungstemperatur als auch das mechanische Verhalten mit der Zahl und Konzentration der Valenzelektronen der B2-Phase zu korrelieren. Die Umwandlungstemperatur sinkt mit steigender Valenzelektronenkonzentration. Eine ausgezeichnete Kombination von hoher Streckgrenze und Bruchdehnung tritt für die Legierungen Ti-Cu-Ni-Zr und Ti-Cu-Ni-Zr-Si mit einem relativ geringen CuZr-Anteil auf. Die Bruchdehnung unter Druck ist fast zweimal höher als es für CuZr-Basis-Komposite gewöhnlich beobachtet worden ist. Die Legierung Ti45Cu39Ni11Zr5 zeigt beispielsweise eine Streckgrenze von 1490±50 MPa in Kombination mit einer plastischen Dehnung von 23,7±0,5%. Für die CuZr-reicheren Ti-Cu-Ni-Zr Zusammensetzungen wurde jedoch eine geringere Duktilität festgestellt, was das Resultat spröder Cu2TiZr-Ausscheidungen in der Glas-Matrix ist. Die vorliegende Arbeit erweitert folglich das Konzept der „Formgedächtnis-Glas-Matrix Komposite“, welches bisher auf CuZr-basierte Legierungen beschränkt war und liefert wichtige Einblicke in die Beziehung zwischen Phasenbildung und mechanischen Eigenschaften der neuen Klasse hochfester TiCu-Basislegierungen, welche nach weiterer Optimierung vielversprechend sein könnten für Hochleistungsanwendungen wie Durchflussmesser, Sensoren und mikrometer- und mm-große Antriebe

Many modern technologies are enabled by the use of thin films and/or nanostructured composite materials. For example, many thermoelectric devices, solar cells, power electronics, thermal barrier coatings, and hard disk drives contain nanostructured materials where the thermal conductivity of the material is a critical parameter for the device performance. At the nanoscale, the mean free path and wavelength of heat carriers may become comparable to or smaller than the size of a nanostructured material and/or device. For nanostructured materials made from semiconductors and insulators, the additional phonon scattering mechanisms associated with the high density of interfaces and boundaries introduces additional resistances that can significantly change the thermal conductivity of the material as compared to a macroscale counterpart. Thus, better understanding and control of nanoscale heat conduction in solids is important scientifically and for the engineering applications mentioned above. In this dissertation, I discuss my work in two areas dealing with nanoscale thermal transport: (1) I describe my development and advancement of important thermal characterization tools for measurements of thermal and thermoelectric properties of a variety of materials from thin films to nanostructured bulk systems, and (2) I discuss my measurements on several materials systems done with these characterization tools. First, I describe the development, assembly, and modification of a time-domain thermoreflectance (TDTR) system that we use to measure the thermal conductivity and the interface thermal conductance of a variety of samples including nanocrystalline alloys of Ni-Fe and Co-P, bulk metallic glasses, and other thin films. Next, a unique thermoelectric measurement system was designed and assembled for measurements of electrical resistivity and thermopower of thermoelectric materials in the temperature range of 20 to 350 °C. Finally, a commercial Anter Flashline 3000 thermal diffusivity measurement system is used to measure the thermal diffusivitiy and heat capacity of bulk materials at high temperatures. With regards to the specific experiments, I examine the thermal conductivity and interface thermal conductance of two different types of nanocrystalline metallic alloys of nickel-iron and cobalt-phosphorus. I find that the thermal conductivity of the nanocrystalline alloys is reduced by a factor of approximately two from the thermal conductivity measured on metallic alloys with larger grain sizes. With subsequent molecular dynamics simulations performed by a collaborator, and my own electrical conductivity measurements, we determine that this strong reduction in thermal conductivity is the result of increased electron scattering at the grain boundaries, and that the phonon component of the thermal conductivity is largely unchanged by the grain boundaries. We also examine four complex bulk metallic glass (BMG) materials with compositions of Zr₅₀Cu₄₀Al₁₀, Cu_46.25Zr_44.25Al_7.5Er₂, Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂, and Ti_41.5Zr_2.5Hf₅Cu_42.5Ni_7.5Si₁. From these measurements, I find that the addition of even a small percentage of heavy atoms (i.e. Hf and Er) into complex disordered BMG structures can create a significant reduction in the phonon thermal conductivity of these materials. This work also indicates that the addition of these heavy atoms does not disrupt electron transport to the degree with which thermal transport is reduced.
Ph. D.

It is of great importance for a materials scientist both from fundamental and applicability aspects to have better understanding of solid-state phase transformations and its kinetics responsible for micro-/nano-structure development in alloys and corresponding physical and mechanical properties. Transformation kinetics can be analyzed by various experimental techniques such as thermal analysis, laborious electron microscopy combined with extensive image analysis or by measuring changes in electrical resistivity, specimen volume and relative intensities of diffraction lines caused by the phase transformation. Beyond these conventional techniques, this dissertation provides a novel magnetic monitoring approach to study the isothermal kinetics of phase transformations in multicomponent alloy systems involving measurable changes in overall magnetic moment as the transformation proceeds. This dissertation focuses on understanding the microstructural evolution, macro- and micro-alloying behavior, magnetic properties, thermal characteristics, mechanical properties and kinetics of solid-state transformations, i.e. nanoscale precipitation and nanocrystallization, in nickel aluminides and Fe-based bulk amorphous alloys. Microstructural characterization of alloys was done by X-ray diffraction, scanning electron microscopy and transmission electron microcopy techniques. Magnetic properties were analyzed by vibrating sample magnetometry whereas thermal characteristics were evaluated by differential scanning calorimetry. Mechanical properties of alloys were determined by microhardness measurements and compression tests. The influence of Fe macroalloying and 3d transition metal microalloying on the microstructure and properties of Ni-Al-Fe alloys were studied for as-cast and annealed states and it is shown that desired microstructure and related properties can be obtained by proper selection of the type and concentration of macro- or micro-alloying elements together with an appropriate annealing procedure. Thermomagnetic characterization reveals the nanoscale precipitation of a ferromagnetic second phase with annealing. In conjunction with saturation magnetization dependence on annealing, an optimum temperature is identified where nanoscale precipitates impart the highest extent of precipitation strengthening. The isothermal kinetics of ferromagnetic second phase precipitation reveals invariant Avrami exponents close to unity, indicating that nanoscale precipitation is governed by a diffusion-controlled growth process with decreasing growth rate, which closely resembles continuous precipitation kinetics. Appropriate annealing of the Fe-based bulk amorphous alloy precursor produced by suction casting demonstrated extremely fine microstructures containing uniformly distributed and densely dispersed nanocrystals inside a residual amorphous matrix. In order to have better understanding of nanocrystallization mechanisms, kinetic parameters were determined via isothermal magnetic monitoring and non-isothermal differential scanning calorimetry where excellent agreement was obtained in Avrami exponent and activation energy. Analyzing the local kinetics, the nanocrystalline phase was found to evolve through distinct transformation regimes during annealing which were discussed on the basis of transformation kinetics theory and microscopical investigations on each characteristic transformation regime.

The -TiAl based intermetallic alloys play an important role in structural applications, such as aerospace and automotive industries. The current work studied the novel development and microstructural evolution, along with their mechanical properties of the -TiAl based alloys viz. the binary (Ti--48Al), ternary (Ti--48Al-2Nb), quaternary (Ti--48Al-2Nb-0.7Cr) and quinary (Ti- -48Al-2Nb-0.7Cr-0.3Si). The alloys were fabricated employing both powder and ingot metallurgy routes. Consolidation of the alloys was achieved by vacuum arc melting. As-cast and thermally treated sections of sample ingots were characterized using optical microscopy (OM) and scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS), atomic force microscopy (AFM), Raman spectroscopy (RS), transmission electron microscopy (TEM), electron backscattered diffraction (EBSD). Thermal analysis, such as differential scanning calorimetry (DSC) and differential thermal analysis (DTA) were employed in order to give insight into phase transformations. Moreover, hardness, room temperature tensile testing with subsequent fractography, cyclic oxidation, nitriding treatment and incorporating first principle calculations with orientation relationships were some of the property behaviors studied in the dissertation. The key findings of the research were: 1) the addition of Nb and Cr to the -TiAl based alloys promoted grain refinement and induced phase transformation, 2) slight overall Al loss of the -TiAl based alloys was observed due to the compaction method prior to melting. Uni-axial cold pressing of the blended metal powders indicated that Al particles migrated to the surface in contact with the die facets, 3) of all the alloys studied, the quinary (Ti-48Al-2Nb-0.7Cr-0.3Si) alloy exhibited good mechanical properties, 4) phase transformation and microstructural evolution of an α-solidifying quinary alloy illustrated a homogeneous microstructure with Spheriodized/Widmanstätten laths, 5) it has been shown that the formation of /α2/Ti5Si3 in the Ti-48Al-2Nb-0.7Cr-0.3Si intermetallic alloy followed the Blackburn orientation relationship of both the as-cast (-solidifying) and heat-treated (α-solidifying) phases, 6) surface cladding in a N/Ar atmosphere indicated that the oxidation properties of the ternary and quinary showed improved significance compared to the binary and quaternary alloys and, 7) Sn and Mn, were some of the doping elements added to the quinary alloy excluding the Cr due to its inducement of the brittle β-phase which exhibits low mechanical properties. As a result, -TiAl based Ti- 48Al-2Nb-0.3Si, Ti-48Al-2Nb-1Sn-0.3Si, and Ti-48Al-2Nb-1Sn-0.7Mn-0.3Si intermetallic alloys were developed and investigated.
Thesis (PhD)--University of Pretoria, 2019.
Materials Science and Metallurgical Engineering
PhD
Unrestricted

In this thesis, we pursue a simulation-based approach whereby microstructure-sensitive finite element simulations are performed within a statistical perspective to examine the VHCF life variability and assess the surface initiation probability. The methodology introduced in this thesis lends itself as a cost-effective platform for development of microstructure-property relations to support design of new or modified alloys, or to more accurately predict the properties of existing alloys.

Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β.

Ag is a widely used electrical contact material due to its excellent electrical properties. The problems with Ag are that it is soft and has poor tribological properties (high friction and wear in Ag/Ag sliding contacts). For smart grid applications, friction and wear became increasingly important issues to be improved, due to much higher sliding frequency in the harsh operation environment. The aim of this thesis is to explore several different concepts to improve the properties of Ag electrical contacts for smart grid applications. Bulk Ag-X (X=Al, Sn In) alloys were synthesized by melting of metals. An important result was that the presence of a hcp phase in the alloys significantly reduced friction coefficients and wear rates compared to Ag. This was explained by a sliding-induced reorientation of easy-shearing planes in the hexagonal structure. The Ag-In system showed the best combination of properties for potential use in future contact applications.  This thesis has also demonstrated the strength of a combinatorial approach as a high-throughput method to rapidly screen Ag-based alloy coatings. It was also used for a rapid identification of optimal deposition parameters for reactive sputtering of a complex AgFeO2 oxide with narrow synthesis window. A new and rapid process was developed to grow low frictional AgI coatings and a novel designed microstructure of nanoporous Ag filled with AgI (n-porous Ag/AgI) using a solution chemical method was also explored. The AgI coatings exhibited low friction coefficient and acceptable contact resistance. However, under very harsh conditions, their lifetime is too short. The initial tribotests showed high friction coefficient of the n-porous Ag/AgI coating, indicating an issue regarding its mechanical integrity. The use of graphene as a solid lubricant in sliding electrical contacts was investigated as well. The results show that graphene is an excellent solid lubricant in Ag-based contacts. Furthermore, the lubricating effect was found to be dependent on chemical composition of the counter surface. As an alternative lubricant, graphene oxide is cheaper and easier to produce. Preliminary tests with graphene oxide showed a similar frictional behavior as graphene suggesting a potential use of this material as lubricant in Ag contacts.

The mechanical alloying (MA) process and hot isostatic pressing (HIP) were used to synthesize bulk amorphous Ni-Ti alloy as an alternative to the traditional methods of casting multi-component metallic alloys. Samples milled cryogenically for 10 hours provided a homogeneous lamella structure with spacing of 30-110 nm. X-ray diffraction and transmission electron microscopy studies indicated that there were alloying and amorphous phase within the layers of the MA powder prior to annealing or HIPing. The amount of amorphous phase increased with time when the milled powder was annealed at a constant temperature and with temperature when annealing time was held constant. The microhardness of the powder correspondingly increased with the amount of amorphous formed in the powders. The HIPing of the MA powder produced a close to 100% amorphous compact with some dispersion of nanocrystals in the amorphous matrix. However, densification was not achieved.
Master of Science

Due to lightweight and high specific strength, the research and development of magnesium-based alloys has been widely expanded. New magnesium alloys and novel processing technology also have been developed to satisfy the need for applications in the automotive, communications and aerospace industries. In particular, ultrafine grain sized (UFG) and even nanostructured (NS) magnesium alloys fabricated via severe plastic deformation have attracted most of researchers’ attention because of impressive mechanical properties compared to micro sized Mg alloys. With a UFG, NS or a mixed microstructure, exceptional high strength with good ductility could be achieved. A combination of cryomilling and spark plasma sintering (SPS) was employed in this project to fabricate a NS magnesium alloy. Nanocrystalline (NC) AZ31 powders were produced by cryomilling. A minimum average grain size of approximately 26 nm was obtained when the cryomilling time was extended to 6 hours or longer. Cold welding played a dominant role in the early stage of cryomilling, while fracture took place in the late stage and surpassed cold welding. The former increased while the latter decreased the particle size. The highest hardness of around 160 HV was obtained after 6 hours cryomilling. This cryomilled NC powder showed excellent thermal stability during annealing at elevated temperatures. There were two separate growth stages with a transition point around 400 °C. More specifically, between 350 and 400 °C, NC Mg grains stabilized around 32 nm, even after 1h heating. At 450 °C, the nano grains grew to 37 nm in the first 5 minutes and grew quickly to approximately 60 nm after 15 minutes. Nevertheless, the average grain size was still less than 100 nm even after 60 minutes annealing at 450 °C. Bulk nanostructured (NS) Mg AZ31 alloy was produced by spark plasma. The bulk samples consolidated at 400 °C with an average grain size of 45 nm showed exceptional average true compressive yield strength of 408.7 MPa and true ultimate compressive strength of 504.0 MPa. These values were superior to published results for most of conventional Mg alloys. Higher sintering temperature (425-450 °C) improved compressive strain at the sacrifice of strength, while samples consolidated at 350 °C displayed brittle behavior with low strength. However, true compressive strains of these four samples were all less than 0.06 at true ultimate compressive strength. To enhance the ductility of bulk NS Mg AZ31 alloy in this study, a facile strategy, in situ powder casting during SPS, was introduced. Different amounts of eutectic Mg-Zn alloy powders with low melting temperature approximately 350 °C were mixed with cryomilled powder. During SPS at 400 °C, the low melting temperature eutectic alloy particles melted, and flowed along cryomilled powder particle boundaries and partly dissolved into the Mg matrix. The compressive strain was improved by in situ powder casting during SPS without loss of strength, especially when 20 % (wt. %) of eutectic alloy powder was added. Compared to samples sintered by pure cryomilled powder, its compressive strain was extended from 3.6% to 6.6%. The reason for this was in situ powder casting can simultaneously significantly remove the artifacts such as porosity, enhance the inter-particle bounding between nanostructured Mg particles and introduce very small dense precipitates into bulk NS Mg alloy.

博士
國立清華大學
材料科學工程學系
95
This work focused on the development and study of Fe-based soft magnetic bulk metallic glasses (BMGs) and a strategy to produce bulk nanocrystalline alloys (BNCAs). Ternary Fe-based bulk metallic glasses were for the first time in the world developed. The BMG research contains two parts: (1) Ternary Fe-R-B (R= Sc, Y, Dy, Ho and Er). (2) Quaternary, Fe-(Co or Ni)-Y-B and Fe-Y-(Nb or Ta)-B BMG systems. Thermal properties, glass forming ability and magnetic properties were investigated. Ternary Fe-based BMGs represented by the formulae FeaMbBc are based on two simple selection rules: (1) M is an element with atomic radius at least 130% that of Fe; (2) M possesses an eutectic point with Fe and the M-Fe eutectic is at the Fe-rich end. The M elements, Sc, Y, Dy, Ho and Er fulfill the two rules exhibit BMG capability at the wide composition range, in atomic %, 3 < b < 10, 18< c < 27, whereas a+b+c = 100. It is much remarkable that bulk amorphous state is achievable with only 3 elements (conventional ones 4 to 7 elements). The ternary BMGs thus developed are characteristic of high saturation magnetization 1.2 to 1.56 T, low coercivity less than 40 A/m, and high electrical resistivity, larger than 200 microphm-cm. Among the explored ternary BMGs, Fe-Y-B alloys show the highest saturation magnetization 1.56 T. The properties of subsequently modified Fe-Y-B by Co, Ni and other transition metal (Nb and Ta) were also investigated. It shows a wide composition range retaining the BMG capability while replacing Fe by Co or Ni revealing a great advantage in modifying the magnetic properties to suit various industrial applications. The partial replacement of Y by Nb or Ta greatly improves the GFA and also retains the soft magnetic properties. The reduction of Y content to decrease the high chemical reactivity to improve industrial production is achieved. The Fe-Y-Nb-B and Fe-Y-Ta-B BMG exhibits extreme high compressive strength above 4000 MPa. New bulk nanocrystalline alloys were successfully achieved in Fe-Y-Nb-Cu-B, Fe-Si-B-Nb and Cu-Zr-Al alloy systems according to proposed “crystallization-and stop” model including (1) there is at least one principal element (PE) that dominates the crystallization temperature (Tx) and the Tx increases steeply with PE concentration, (2) the PE is barely soluble in the primary crystallites so that they pile up around the crystallizing nano-grains hence the Tx is manifestly increased locally, (3) once the increased Tx is higher than the raised sample temperature (due to heat of crystallization), the crystallization will be stopped to maintain a nano-grain structure, and (4) a nucleation agent is much helpful to enhance nucleation frequency hence reduce the resultant nano-sizes. The development of this model unveils a simpler and more practical way to design an alloy which can achieve bulk nanocrystallization.

碩士
國立臺灣海洋大學
光電科學研究所
102
Magnetostrictive materials have potential applications to MEMS and other sensing devices, because of their capabilities to convert magnetic energy into mechanical energy, and vice versa. This study focuses on a series of Fe81-xMnXGa19(x=0、3、8、13、18、23) bulks. First of all, deployed the weight of Fe、Mn、Ga, fuse the Fe、Mn、Ga by ARC and made into ingots, then use the WEDM cut into bulks. Structural property was investigated by X-ray diffraction (XRD) and magnetic property by vibration sample magnetometer (VSM). Magnetoelastic and magnetostriction were studied by using strain gauges. In the result of VSM measurement show that coercive force (i.e. HC) is increasing with x increasing. In other words, magnetic dipoles are harder to be flipped ever by reversing H with more and more Manganese. The VSM measurement also show the saturation magnetization (i.e. Ms) is decreasing with x increasing, when x=18 it chnage from ferromagnetic to paramagnetic. In the pat of measured magnetostriction, we used the train-gauge method. As the result, the magnetostriction of the Fe81Ga19 bulk is the largest, and decreasing with x increasing. In order to improve results, we use the furnace to anneal the sample made by 850℃ for 3 hours. Then measure those sample by VSM、SEM-EDS、XRD, compare the result of as cast sample and anneal sample.

碩士
國立臺灣科技大學
機械工程系
102
Magnetostrictive materials have potential applications to MEMS and other sensing devices, because of their capabilities to convert magnetic energy into mechanical energy, and vice versa. This study focuses on a series of Fe81-XNiXGa19(X =0、4、11、17、22、26) bulk. We examined the bulk components by SEM-EDS. Structural property was investigated by X-ray diffraction (XRD) and magnetic property by vibration sample magnetometer (VSM). Magnetostriction were studied by using strain gauges. Mechanical properties were using by Resonant Frequency &; Damping Analyser (RFDA) at Magnetic field measured. After analyzing via the above-listed instrument, the Fe81-XNiXGa19(X=0,4,11,17,22,26) bulks grow with the <110>&;<220> texture. About resistivity, samples with composition X=4 have larger resistivity. About magnetization, the measured saturation magnetization decrease with increases in Ni for Fe81-xNixGa19 alloys. As to the magnetostrictive part, the magnetostriction value has been decrease when the Nickel increases. In the mechanical properties part, the Young's modulus and shear modulus increases with the magnetic field becomes larger, the comparative hardness found in 0% Ni and 26% Ni has pretty good mechanical properties.

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The viscosity of the [...] bulk metallic glass forming alloy in both the supercooled liquid and the equilibrium molten state was measured. Parallel plate rheometry and three-point beam-bending were used to measure the viscosity as a function of temperature in the supercooled liquid, and the technique of capillary flow was used in the molten state. The high thermal stability above the glass transition of this bulk metallic glass former with respect to crystallization allows measuring viscosities 120 K into the supercooled liquid region. Viscosity in the range from [...] to [...] poise has been measured using parallel plate rheometry, a region of viscosities that has not been previously accessible for supercooled metallic melts. The measurements were carried out with different heating rates between 0.0167 K/s and 1.167 K/s as well as isothermally. Using three-point beam bending, viscosity in the range from [...] to [...] poise has also been measured. These two methods, which involve completely different geometries for the measurement of flow, yielded consistent values for viscosity where their applicable regions overlap. The viscosity of the supercooled liquid of this bulk glass former, above the glass transition temperature, can be fit by a Vogel-Fulcher relation which exhibits a small Vogel-Fulcher temperature relative to the glass transition temperature. The values of viscosity measured by capillary flow above the liquidus temperature agree with the extrapolation of the Vogel-Fulcher relation to these temperatures. This bulk metallic glass former exhibits strong glass behavior, similar to silicate glasses. The relatively high viscosity in the supercooled liquid and smaller Gibbs free energy difference compared to the crystal both favor bulk glass formation. This glass forming ability is directly related to the fragility index and the relaxation kinetics as measured by viscosity, as well as the critical cooling rate. Knowledge of the viscosity as a function of temperature is essential for shaping and molding, and has led to possible extended commercial applications of these novel materials which exhibit unique mechanical properties.

碩士
國立清華大學
材料科學工程學系
95
The purpose of this study was to explore binary and ternary Cu-based bulk metallic glasses (BMG), which were divided into three systems: Cu-Hf-Al, Cu-Y-(M), Cu-Ti-M alloys, where M is an additive element. First, high Cu-content binary Cu-Hf BMG modified with Al, (Cu65Hf35)100-xAlx (x= 1, 2, 3, 4, 6) were studied. With only x= 2, the glass-forming ability (GFA) is greatly improved to form bulk glassy rod up to at least 3 mm in diameter. The microstructure consists of inhomogeneous amorphous phases. The Cu-Y binary system was chosen to study the GFA on compositions around the eutectic points and their modification. The microstructure of as-cast Cu-Y rods contained amorphous and crystalline mixture phases. According to the XRD results the best glass forming alloy is Cu78Y22. The further addition of Ag improves the GFA. The composition (Cu78Y22)97Ag3 was very close to form fully 1 mm BMG rod. Finally, we tried to develop the ternary BMGs based on high Cu content Cu-Ti BMG. The best result is the Cu50Ti40Zr10 glassy rod with the diameter at least 1 mm. The Cu-Y-M ternary alloys are brittle and poor electrochemical resistance due to their structure that contained amorphous and crystalline phases. Fully amorphous Cu-Hf-Al system showed the best mechanical properties with a compressive fracture strength up to 2340 MPa and a plastic strain up to about 1 % due to the inhomogeneous amorphous phases. Cu50Ti40Zr10 alloy performed low corrosion current density, 10-5 A/cm2, in 1 N H2SO4 + 0.01 N NaCl solution showed excellent electrochemical property, and its compressive fracture strength is up to about 2000 MPa.

碩士
國立臺灣海洋大學
光電科學研究所
99
The magnetostrictive materials are potential for MEMS and device application, because of their capability to convert magnetic energy to mechanical energy. The thesis reports the magnetostriction of Fe81-xCoxGa19(x=0,7,11,19,23) bulks by arc-melting in annealing treatment. We examined the bulk components by SEM-EDS. The crystalline structure was analyzed by XRD. After analyzing via the above-listed instrument, we know that the Fe81-xCoxGa19(x=0,7,11,19,23) bulks grow with the &lt;110> texture, we can know the structure of Fe81-xCoxGa19 bulks is body center cubic. About magnetization, the measured coercivity(HC) increases and saturation magnetization decrease with increases in Co for Fe81-xCoxGa19 alloys. As to the magnetostrictive part, the magnetostriction value has been decrease when the cobalt increases. In different conditions, 850°C SQW have more magnetostriction value than others. In other research, the magnetostriction value of the Fe81-XCoXGa19 thin films has been decreases. when the cobalt increases, but in this study didn't. Because of Fe81-xCoxGa19(x=0,7,11,19,23) bulks grow with the &lt;200>and&lt;211> texture more than thin films.

The present thesis deals with Nb-Si alloy composites in both bulk and multilayer forms. The work has been divided into two parts. First part (chapter 4-6) deals with Nb based silicides binary and ternary alloys with alloying additions like Ga and Al. These alloys are synthesized by vacuum arc melting and suction casting (non-equilibrium processing techniques). The studies on intermetallic coatings of Nb-Si alloys and Nb/Si multilayer synthesized by pulsed laser deposition technique have been presented in the second part (chapter7-8). Nb-Si alloys are one of the candidate materials for the advanced structural and microelectronic applications. There are few issues with these materials like poor oxidation resistance, low fracture toughness and brittleness which need to be solved. Microstructure plays a crucial role to control these properties. The main focus of this work is to understand the process of phase transformation and thereby control the microstructure in both bulk alloys and thin films. We have also investigated in a limited manner mechanical and environmental properties of bulk alloys. This thesis is subdivided into nine chapters. After a brief introduction in the first chapter, a brief overview on Nb-Si phase diagram and literature reviews on Nb-Si based alloys are presented with emphasis on the current work in the second chapter. Literature reviews on the phase formations sequence and stability in Nb-Si alloys thin films and Nb/Si multilayers are also discussed in the same chapter. In the third chapter different experimental techniques, processing parameters and characterization tools like XRD, SEM, TEM etc. are briefly discussed. Special emphasis is given on two non-equilibrium techniques: laser deposition technique to deposit the thin film/multilayer and vacuum suction casting to produce the 3 mm diameter rods of different Nb-Si alloys. The fourth chapter discusses the microstructural aspects of Nb-Si alloys prepared by suction casting and its mechanical behavior. The samples have the compositions hypoeutectic (Nb-10at.%Si and Nb-14at.%Si), eutectic (Nb-18.7at.%Si) and hypereutectic (Nb-22at.% Si and Nb-25at.% Si). SEM microstructural analyses of all the samples clearly show the enhancement in the volume fraction of eutectic and decease in the eutectic spacings in microstructure due to large undercooling. Rod eutectic is observed in most of places with irregular eutectic a few places in all samples. First check of phases has been done by XRD in all samples. Phase confirmation using TEM showed the eutectic between Nbss and Nb3Si phases in all samples. The primary phase for hypoeutectic alloys is Nbss (dendritic structure), Nb3Si phase for eutectic composition and β-Nb5Si3 phase for hypereutectic alloys. Compositional analysis using EDS and EPMA also supported the above results. No signature of eutectoid reaction (Nb3Si→Nb+α-Nb5Si3) is observed. Mechanical properties like hardness, strength, ductility and indentation fracture toughness have been determined for above mention alloy compositions. SEM micrographs showed that silicides fractured by cleavage and Nb phase in a ductile manner during the compression tests carried out at room temperature. We attempt to explain how the above mention mechanical properties change with alloy compositions and processing. Chapter five deals with the effect of Ga addition on the microstructure and mechanical properties of the Nb-Si alloy. The composition selected for this study is Nb-20.2at.%Si-2.7at.%Ga. The results of ternary alloy have been compared with the binary alloy composition Nb-18.7at.%Si. Phase analysis has been carried out using TEM and XRD. Ga addition has suppressed the formation of Nb3Si phase and promoted the formation of β-Nb5Si3 phase. Ga addition also established the eutectic between Nbss and β-Nb5Si3, which is a metastable eutectic. Ga added ternary alloy, on suction casting, yields ultrafine eutectic with nanometer length scale (50-100nm). From the compression tests, it is concluded that the combination of ultrafine eutectic (Nbss-β-Nb5Si3) and primary β-Nb5Si3 in ternary alloy results in a high compressive strength ~2.8±0.1 GPa with 4.3% plasticity. In contrast binary alloy under identical conditions shows the compressive strength ~1.35±0.1 GPa and 0.2% plasticity. Ga addition also enhances the indentation fracture toughness from 9.2±0.05 MPa√m (binary) to 24.11±0.5 MPa√m (ternary). Composite hardness values of the ternary and binary alloys are 1064±20 Hv and 1031±20 Hv respectively. Chapter six deals with Al added Nb-Si ternary alloy. Here we have discussed microstructural and mechanical properties like in chapter 5 along with oxidation behavior for the alloy composition Nb-12.7at.%Si-9at.%Al. SEM micrograph shows the presence of primary dendrites structure with ultra fine lamellar eutectic (50-100nm). Detailed TEM studies confirm the Nbss as primary phase present in form of dendrites. These dendrites contain the plate shape precipitates of δ-Nb11Si4 (body centered orthorhombic structure) phase in Nb matrix (primary dendrites). Eutectic phases are Nbss and β-Nb5Si3. The analysis of the results indicates that Al addition promote the formation of β-Nb5Si3 phase in the eutectic. The results of this ternary composition were also compared with the binary alloy composition Nb-18.7at.%Si. Compression tests have been carried out at room and elevated temperatures to measure the strength of the material. Al added ternary alloy yields the compressive strength value 1.6±0.01 GPa whereas binary alloy yields the compressive strength value 1.1±0.01 GPa. Enhancement in indentation fractured toughness is observed in Al added ternary alloy (20.4±0.5MPa√m) compare to binary alloy (9.2±0.05 MPa√m). Thermal analysis by TGA and DTA were used to see the oxidation behavior of Al added ternary alloy. Chapter seven deals with the deposition characteristics and the TEM studies on the laser deposited Nb-Si thin films. Films were deposited on the NaCl crystals and Si single crystal substrates. The compositions chosen in this case are Nb-25at.%Si, Nb-37.5at.%Si and Nb-66.7at.%Si. These compositions correspond to the equilibrium intermetallic compounds Nb3Si, Nb5Si3 and NbSi2 respectively. In this chapter we have briefly discussed the microstructural and phase evolutions in the intermetallic coatings. The smooth films quenched from the vapor and/or plasma state show amorphous structure. The sequence of crystallization was studied by hot stage TEM experiments as well as by cross sectional TEM in the films deposited at the elevated temperatures (600oC and 700oC) on Si substrates. During the hot stage experiment, crystallization is observed in Nb-25at.%Si film around 850oC with nucleation of metastable cubic Nb3Si phase. Occasionally metastable hexagonal Nb3Si3 phase has also been observed (close to Si substrate) along with cubic Nb3Si phase in the films at elevated temperatures. For Nb-37.5at.%Si film, crystallization is observed at 800oC with the nucleation of grains of metastable hexagonal Nb5Si3 phase. Cross-sectional TEM shows the presence of hexagonal Nb5Si3 phase along with few grains of NbSi3 (equilibrium) phase in the films deposited at elevated temperatures. Hot stage experiment of Nb-66.3at.%Si film showed the onset of crystallization much earlier at 400oC and complete crystallization at 600oC. This crystallization leads to the nucleation of grains of NbSi2 phase. Films of this composition deposited at elevated temperatures showed the presence of NbSi2 and metastable hexagonal Nb5Si3 phases (occasionally). The laser ablated films, besides the film matrix also contain the micron and submicron sized spherical droplets of different sizes. These droplets travel at very high velocities and impinge on the substrate resulting in a very high rate of heat transfer during solidification from liquid state. Therefore in this work we have also studied the microstructural evolution in the droplets for each composition. The phases observed in the droplets embedded in the matrix of Nb-25 at% Si alloy film are the bcc Nb and the cubic Nb3Si (metastable phase). The droplets in the matrix of Nb-37.5 at% Si alloy showed the bcc Nb and tetragonal β-Nb5Si3 phases. The phases observed in the droplets of in the Nb-66.3at.%Si alloy are the bcc Nb, tetragonal β-Nb5Si3 and the hexagonal NbSi2 (metastable phase). Chapter eight describes the synthesis and microstructural characterization using TEM of Nb/Si multilayers. The aim of this work is to check the stability and phase formation sequence in Nb/Si multilayer. Nb/Si multilayers were first annealed at different time intervals at 600oC and at different temperatures (for 2 hours) and then characterized by the cross-sectional transmission electron microscopy. As-deposited Nb layer is crystalline while Si layer is amorphous. Microstructural and compositional evidences suggest the intermixing between the Nb and Si layers at the interfaces. Nb/Si multilayer annealed at 600oC for 1 hour, NbSi2 was identified as the first crystalline nucleating phase. However amorphous silicide layers were also observed between Nb and NbSi2 layers. Metastable hexagonal Nb5Si3 was identified as the next crystalline phase that nucleated from the amorphous silicide layers at the interfaces of Nb and NbSi2 layers. Occasionally few grains of cubic Nb3Si phase were also observed after 8 hours of annealing at 600oC. In the chapter we have compared the results to the other reported works in Nb-Si bulk diffusion couples and also thin film couples. The final chapter summarizes the major conclusions of the present work and scope of future work.

Zirconium alloys are susceptible to engineering problems associated with the uptake of hydrogen throughout their design lifetime in nuclear reactors. Understanding of hydrogen embrittlement associated with the precipitation of brittle hydride phases and a sub-critical crack growth mechanism known as Delayed Hydride Cracking (DHC) is required to provide the engineering justifications for safe reactor operation. The nature of bulk zirconium hydrides at low concentrations (< 100 wt. ppm) is subject to several contradictory descriptions in the literature associated with the stability and metastability of γ-phase zirconium hydride. Due to the differing volume expansions (12-17%) and crystallography between γ and δ hydride phases, it is suggested that the matrix yield strength may have an effect on the phase stability. The present work indicated that although yield strength can shift the phase stability, other factors such as microstructure and phase distribution can be as or more important. This suggests that small material differences are the reason for the literature discrepancies. DHC is characterised by the repeated precipitation, growth, fracture of brittle hydride phases and subsequent crack arrest in the ductile metal. DHC growth is associated primarily the ability of hydrogen to diffuse under a stress induced chemical potential towards a stress raiser. Knowledge of the factors controlling DHC are paramount in being able to appropriately describe DHC for engineering purposes. Most studies characterise DHC upon cooling to the test temperature. DHC upon heating has not been extensively studied and the mechanism by which it occurs is somewhat controversial in the literature. This work shows that previous thermo-mechanical processing of hydrided zirconium can have a significant effect on the dissolution behaviour of the bulk hydride upon heating. DHC tests with γ-quenched, furnace cooled-δ and reoriented bulk hydrides upon heating and DHC upon cooling suggest that the amount of hydrogen in solution is the primary factor controlling the occurrence of DHC and consistent with the postulation that the stress induced chemical potential is the driving force for DHC.
Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-01-24 06:14:14.152