Dissertations / Theses on the topic 'Zr-based alloys'

In the course of this PhD thesis metastable Cu50Zr50-xTix (0≤ x ≤ 10) and (Cu0.5Zr0.5)100-xAlx (5 ≤ x ≤ 8) alloys were prepared and characterised in terms of phase formation, thermal behaviour, crystallisation kinetics and most importantly in terms of mechanical properties. The addition of Al clearly enhances the glass-forming ability although it does not affect the phase formation. This means that the Cu-Zr-Al system follows the characteristics of the binary Cu-Zr phase diagram, at least for Al additions up to 8 at.%. Conversely, the presence of at least 6 at.% Ti changes the crystallisation sequence of Cu50Zr50-xTix metallic glasses and a metastable C15 CuZrTi Laves phase (Fd-3m) precipitates prior to the equilibrium phases, Cu10Zr7 and CuZr2. A structurally related phase, i.e. the “big cube” phase (Cu4(Zr,Ti)2O, Fd-3m), crystallises in a first step when a significant amount of oxygen, on the order of several thousands of mass-ppm (parts per million), is added. Both phases, the C15 Laves as well as the big cube phase, contain pronounced icosahedral coordination and their formation might be related to an icosahedral-like short-range order of the as-cast glass. However, when the metallic glasses obey the phase formation as established in the binary Cu-Zr phase diagram, the short-range order seems to more closely resemble the coordination of the high-temperature equilibrium phase, B2 CuZr. During the tensile deformation of (Cu0.5Zr0.5)100-xAlx bulk metallic glasses where B2 CuZr nanocrystals precipitate polymorphically in the bulk and some of them undergo twinning, which is due to the shape memory effect inherent in B2 CuZr. Qualitatively, this unique deformation process can be understood in the framework of the potential energy landscape (PEL) model. The shear stress, applied by mechanically loading the material, softens the shear modulus, thus biasing structural rearrangements towards the more stable, crystalline state. One major prerequisite in this process is believed to be a B2-like short-range order of the glass in the as-cast state, which could account for the polymorphic precipitation of the B2 nanocrystals at a comparatively small amount of shear. Diffraction experiments using high-energy X-rays suggest that there might be a correlation between the B2 phase and the glass structure on a length-scale less than 4 Å. Additional corroboration for this finding comes from the fact that the interatomic distances of a Cu50Zr47.5Ti2.5 metallic glass are reduced by cold-rolling. Instead of experiencing shear-induced dilation, the atoms become more closely packed, indicating that the metallic glass is driven towards the more densely packed state associated with the more stable, crystalline state. It is noteworthy, that two Cu-Zr intermetallic compounds were identified to be plastically deformable. Cubic B2 CuZr undergoes a deformation-induced martensitic phase transformation to monoclinic B19’and B33 structures, resulting in transformation-induced plasticity (TRIP effect). On the other hand, tetragonal CuZr2 can also be deformed in compression up to a strain of 15%, yet, exhibiting a dislocation-borne deformation mechanism. The shear-induced nanocrystallisation and twinning seem to be competitive phenomena regarding shear band generation and propagation, which is why very few shear offsets, due to shear banding, can be observed at the surface of the bulk metallic glasses tested in quasistatic tension. The average distance between the crystalline precipitates is on the order of the typical shear band thickness (10 - 50 nm) meaning that an efficient interaction between nanocrystals and shear bands becomes feasible. Macroscopically, these microscopic processes reflect as an appreciable plastic strain combined with work hardening. When the same CuZr-based BMGs are tested in tension at room temperature and at high strain rate (10-2 s-1) there seems to be a “strain rate sensitivity”, which could be related to a crossover of the experimental time-scale and the time-scale of the intrinsic deformation processes (nanocrystallisation, twinning, shear band generation and propagation). However, further work is required to investigate the reasons for the varying slope in the elastic regime. As B2 CuZr is the phase, that competes with vitrification, it precipitates in a glassy matrix if the cooling rate is not sufficient to freeze the structure of the liquid completely. The pronounced work hardening and the plasticity of the B2 phase, which are a result of the deformation-induced martensitic transformation, leave their footprints in the stress-strain curves of these bulk metallic glass matrix composites. The behaviour of the yield strength as a function of the crystalline volume fraction can be captured by the rule of mixtures at low crystalline volume fractions and by the load bearing model at high crystalline volume fractions. In between both of these regions there is a transition caused by percolation (impingement) of the B2 crystals. Furthermore, the fracture strain can be modelled as a function of the crystalline volume fraction by a three-microstructural-element body and the results imply that the interface between B2 crystals and glassy matrix determines the plastic strain of the composites. The combination of shape memory crystals and a glassy matrix leads to a material with a markedly high yield strength and an enhanced plastic strain. In the CuZr-based metastable alloys investigated, there is an intimate relationship between the microstructure and the mechanical properties. The insights gained here should prove useful regarding the optimisation of the mechanical properties of bulk metallic glasses and bulk metallic glass composites.

Die Motivation zur Untersuchung ternärer und quaternären CuZr-Legierungen bestand in der Annahme, dass die Zugabe von Kobalt den Stabilitätsbereich von B2 CuZr bis zur Raumtemperatur erweitert und Aluminium einen signifikanten Effekt auf die Glasbildungsfähigkeit des CuZr-Systems hat. Die vorliegende Dissertation befasst sich mit der Herstellung und Charakterisierung von Cu50-xCoxZr50 (0 ≤ x ≤ 20) und Cu50-xCoxZr45Al5- (x = 2, 5, 10 und 20) Legierungen. Hierbei wurden die Phasenbildung, die thermische Stabilität, die Mikrostruktur, die Martensitbildung und die mechanischen Eigenschaften der Legierungen untersucht. Die Abhängigkeit der Phasenbildung von der Erstarrungsrate und der thermodynamischen Stabilität von Cu-Co-Zr-Legierungen zeigte, dass die Zugabe von Kobalt die Glasbildungsfähigkeit von Cu-Co-Zr-Legierungen absenkt und die stabilen kristallinen Produkte des Systems von Cu10Zr7 + CuZr2 zu (Cu,Co)Zr Phase mit einer B2 Struktur verändert. Die Ergebnisse weisen darauf hin, dass bei den schmelzgesponnene Bänder mit wenigstens 5 Atom-% Co das Glas direkt in B2 (Cu,Co)Zr kristallisiert, während Massivproben mit Co-Gehalten zwischen 0 ≤ x < 5 die monokline (Cu,Co)Zr Phase und Cu10Zr7 sowie CuZr2 beinhalten, wobei für x ≥ 10 die B2 (Cu,Co)Zr Phase bei Raumtemperatur im Gleichgewicht ist. Des Weiteren werden mit steigendem Co-Gehalt die Martensitumwandlungstemperaturen zu niedrigeren Werten verschoben. Die Phasenbildung im ternären System wird im pseudo-binären (Cu,Co)Zr-Phasendiagramm zusammengefasst, welches die Entwicklung neuer Formgedächtnislegierungen sowie metallischer Glas-Komposite bei Zugabe des Glasbildungselementes Aluminium vereinfacht. In den Vierstofflegierungen erhöht Al die Glasübergangs- und Kristallisationstemperaturen und verbessert dadurch die Glasbildungsfähigkeit des Systems. Die röntgenographische Analyse zeigte, dass die Kristallisationsprodukte der schmelzgesponnenen Bänder variieren: von Cu10Zr7 + CuZr2 + AlCu2Zr zu (Cu,Co)Zr + AlCu2Zr, wenn Co ≤ 5 und Co ≥ 10. Die Herstellung von Massivproben mit unterschiedlichen Durchmessern führte zu einem vollständig amorphen Gefüge, einem metallischen Glas-Komposit oder einem vollständig kristallinen Gefüge. Für Co ≤ 5 tritt neben (Cu,Co)Zr und AlCu2Zr ebenfalls Cu10Zr7 auf. Mittels Rasterelektronen (REM)- und Transmissionselektronenmikroskopie (TEM) erfolgte die Analyse des Einflusses von Al- und Co-Zugaben auf die Mikrostruktur von CuZr-Legierungen. Für die Cu-Co-Zr-Al-Legierungen sowie Cu30Co20Zr45Al5 (ø = 4 mm) und Cu45Co5Zr45Al5 (ø = 2 mm) wurden mikrostrukturelle Untersuchungen mittels TEM durchgeführt. Nachfolgend wurde die Heterogenität der Mikrostruktur in der Cu40Co10Zr45Al5 (ø = 2 mm) untersucht. Der Einfluss von Co auf die mechanischen Eigenschaften von rascherstarrten Cu50-xCoxZr50 (x = 2, 5, 10 und 20 Atom-%) Legierungen zeigt, dass das Verformungsverhalten der Stäbe unter Druckbeanspruchung stark von der Mikrostruktur der (Cu,Co)Zr Phase und somit von der Legierungszusammensetzung abhängt. Kobalt beeinflusst die Bruchfestigkeit der Gussproben. Weiterhin zeigen Proben mit martensitischem Gefüge eine Kaltverfestigung. Neben der Kaltverfestigung zeigen die Legierungen mit hohem Co-Gehalt eine verformungsinduzierte Martensitumwandlung und weisen zwei Streckgrenzen auf. Für die Vierstofflegierungen wurde der Einfluss der Kühlrate und der chemischen Zusammensetzung auf die mechanischen Eigenschaften untersucht. Für Cu48Co2Zr45Al5 (ø = 1.5, 2, 3 und 4 mm) und Cu45Co5Zr45Al5 (ø = 3 mm) wurde der Einfluss der Kühlrate und der Heterogenität diskutiert. Die Ergebnisse zeigen, dass die mechanischen Eigenschaften der Cu50-xCoxZr45Al5-Legierungen stark von der Makrostruktur und dem Volumenanteil der amorphen und kristallinen Phase abhängen. Die verformungsinduzierte Martensitumwandlung in Cu40Co10Zr50- und Cu40Co10Zr45Al5-Gussstäben wurde mittels hochenergetischer Röntgenstrahlung durchgeführt. Die In-situ- Druckversuche erfolgten weg- und kraftkontrolliert. Das makroskopische und mikroskopische Spannung-Dehnungs-Verhalten sowie die Phasenumwandlungskinetik wurden dabei betrachtet. Die relativen Veränderungen der vollständig integrierten Intensität der ausgewählten B2- und Martensitreflexe, die auf die Veränderungen der Volumenanteile der entsprechenden Phasen unter Verformung hinweisen, wurden als Phasenumwandlungsvolumen M/M+B2 beschrieben
The fact that the presence of Co extends the stability range of B2 CuZr to room temperature, together with the significant effect of Al on improving the glass forming ability of the CuZr system was the motivation to investigate the ternary and quaternary CuZr alloys with the aim of synthesizing BMG composites containing B2 (Cu,Co)Zr crystals. This PhD thesis deals with preparation and characterization of Cu50-xCoxZr50 (0 ≤ x ≤ 20) and Cu50-xCoxZr45Al5 (x = 2, 5, 10 and 20) alloys. The phase formation, thermal stability, microstructure, martensitic transformation and mechanical properties of these alloys were investigated. The dependence of phase formation on solidification rate and the thermodynamically stability of Cu-Co-Zr alloys reveals that the addition of Co decreases the glass forming ability (GFA) of the Cu-Co-Zr alloys and changes the stable crystalline products of the system from Cu10Zr7 + CuZr2 to (Cu,Co)Zr phase with a B2 structure. The results indicate that for the melt-spun ribbons with at least 5 % Co, the glass crystallizes directly into B2 (Cu,Co)Zr, while in the case of bulk specimens, compositions with 0 ≤ x < 5 of Co contain the monoclinic (Cu,Co)Zr phase and Cu10Zr7 and CuZr2, whereas, for x ≥ 10, the B2 (Cu,Co)Zr phase is the equilibrium phase at room temperature. Furthermore, increasing the cobalt content decreases the martensitic transformation temperatures to lower temperatures. The phase formation in the ternary system is summarized in a pseudo-binary (Cu,Co)Zr phase diagram, that helps for designing new shape memory alloys, as well as bulk metallic glass composites with the addition of glass former elements. In the quaternary alloys, Al increases the glass transition and crystallization temperatures and hence improves the GFA of the system. The X-ray analysis illustrates that for the melt-spun ribbons, the crystallization products vary from Cu10Zr7 + CuZr2 + AlCu2Zr to (Cu,Co)Zr + AlCu2Zr when Co ≤ 5 and Co ≥ 10, respectively. Depending on the cooling rates, the bulk samples represent a fully amorphous structure or BMG composites or a fully crystalline structure. For Co ≤ 5, beside (Cu,Co)Zr and AlCu2Zr, Cu10Zr7 exists as well. Scanning (SEM) and transmission (TEM) electron microscopy investigations were done to investigate the effect of Al and Co addition to the microstructure of CuZr alloys. In the case of Cu-Co-Zr-Al alloys, Cu30Co20Zr45Al5 (ɸ = 4 mm) and Cu45Co5Zr45Al5 (ɸ = 2 mm) compositions were selected for the microstructure verification using TEM. Later, the heterogeneity of the microstructure in Cu40Co10Zr45Al5 (ɸ = 2 mm) alloy was considered. The effect of Co on the mechanical properties of rapidly solidified Cu50-xCoxZr50 (x = 2, 5, 10 and 20 at.%) alloys depict that the deformation behavior of the rods under compressive loading strongly depends on the microstructure, and as a results, on the alloy composition. Cobalt affects the fracture strength of the as-cast samples; and deformation is accompanied with two yield stresses for high Co-content alloys, which undergo deformation-induced martensitic transformation. Instead samples with a martensitic structure show a work-hardening behavior. For quaternary alloys, the effects of cooling rate and chemical composition on mechanical properties of the alloys were investigated. Cu48Co2Zr45Al5 (ɸ= 1.5, 2, 3 and 4 mm) and Cu45Co5Zr45Al5 (ɸ = 3 mm) compositions were selected to discuss the effect of cooling rate and heterogeneity, respectively. The results depict that the mechanical properties of Cu50-xCoxZr45Al5 alloys strongly depend on the microstructure and the volume fraction of the amorphous and crystalline phases. The deformation-induced martensitic transformation of Cu40Co10Zr50 and Cu40Co10Zr45Al5 as-cast rods, was studied by means of high-energy X-rays. The in situ compression measurements were performed in track control and load control modes. The macroscopic and microscopic stress-strain behavior, as well as the phase transformation kinetics were considered. The relative changes in the fully integrated intensity of the selected B2 and martensite peaks, which indicate the changes in volume fraction of the corresponding phases under deformation, was described as phase transformation volume, M/M+B2

Dans cette thèse, nous démontrons que des nanostructures originales peuvent être obtenues en travaillant autour de la transition cristallin/amorphe dans des films minces déposés par pulvérisation cathodique. En particulier, nous étudions deux systèmes, Zr-Mo et Zr-W, dans lesquels une telle transition se produit. Dans ce système, lorsque la teneur en Mo est réduite, une transition structurale d’une solution solide nanocristalline de Zr dans le réseau bbc de Mo à une structure amorphe peut être obtenue autour de 60 at % de Mo. Les films obtenus présentent une dureté H élevée, un faible module de Young E et, par conséquent, un ratio H/E élevé par rapport à celui de Zr et Mo. Par ailleurs, nous démontrons qu'une auto-séparation des phases nanocristalline et amorphe se produit à une composition spécifique. La géométrie particulière dans laquelle la phase nanocristalline se développe en concurrence avec la phase amorphe est exploitée pour contrôler la morphologie de surface et, par conséquence, la réflectance par l’intermédiaire de l’épaisseur. Un modèle a été développé pour décrire la cinétique de la croissance compétitive entre les phases nanocristalline et amorphe. De plus, cela permet de construire un diagramme de phase épaisseur-composition qui montre que la croissance compétitive nanocristalline/amorphe est facilement dissimulée expérimentalement. Finalement, nous démontrons que des grains monocristallins massifs de taille latérale supérieure à 1 µm peuvent être obtenus en travaillant à basse pression d’Ar si la composition des films se rapproche du bord de la transition amorphe. Nos résultats suggèrent que les phénomènes observés pour les systèmes Zr-Mo et Zr-W peuvent être étendus à d'autres systèmes
In this thesis, we demonstrate that original nanostructures can be obtained by working around the crystalline-to-amorphous transition in sputter-deposited thin films. In particular, we study two systems, Zr-Mo and Zr-W, in which such transition occurs. By decreasing the Mo content in the Zr-Mo system, a structural transition from a nanocrystalline solid solution of Zr in the bcc lattice of Mo to an amorphous structure can be achieved around 60 at% Mo. The films obtained present high hardness H, low Young's modulus E and, consequently, high H/E ratio compared with bulk Zr and Mo. Furthermore, we demonstrate that a self-separation of the nanocrystalline and the amorphous phases occurs at the composition intermediate to those necessary to form single-phased amorphous and nanocrystalline films. The particular geometry in which the nanocrystalline phase grows in competition with the amorphous phase is exploited to achieve a thickness-controlled surface morphology which allows to tune the film reflectance. A model was developed to describe the kinetics of the competitive growth between the nanocrystalline and the amorphous phases. Furthermore, it allows to construct a thickness-composition phase diagram evidencing that the nanocrystalline/amorphous competitive growth is easily hidden experimentally. Finally, we demonstrate that massive monocrystalline grains with lateral size larger than 1 µm can be obtained by working at low Ar pressure if the composition of the films approaches to the edge of the amorphous transition. Our results suggest that the phenomena reported here for Zr-Mo and Zr-W can be extended to other systems

In this work was shown the high stability of the single-phase structural state of high entropy alloy of Ti-V-Zr-Nb-Hf system in a vacuum-arc method of obtaining of coatings based on it. In the process of deposition single-phase high entropy coatings with bcc-lattice which characterizes the cast state are formed in vacuum, and upon obtaining in a nitrogen atmosphere single-phase nitride superhard coatings based on fcc-metal lattice are formed. Such a stability of structure of multi-element alloy to high temperature evaporation and deposition from high-energy plasma flows allows to use the techniques developed for simple substitution phases in the analysis of their structural-stress state. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/34808

碩士
國立高雄應用科技大學
機械與精密工程研究所
96
In this research, the workpiece is Zr-based amorphous alloy (Zr38.5Cu15.25Ni9.75Ti16.5Be20) was prepared from die casting machine, carried on the electrical discharge machining. During EDM process, I take the copper as the electrode in the different electric discharge mediums (electric spark machining oil EP1, deionized water and argon). The EDM parameter of the different peak current, the pulse duration and the positive/negative polarity explore the EDM Characteristics of the workpiece materials removal rate, electrode wear rate, surface hardness, surface topography and roughness, recasted layer thickness and its crystallization. The results shows when the discharge current increase, the material electrodes removal rate and electrode wear rate will increase. In the negative polarity process, the material removal rate and electrode wear rate under using the electric spark machining oil are the most; in the positive polarity process, the material removal rate of the deionized water is the best, but the electrode wear rate is still more. Both positive and negative polarity process, under using the electric spark machining oil, the greatest effect factors of surface roughness is the discharge current. The surface roughness after EDM in deionized water is the worst, and in gas is the best. In addition, all of the EDM’s surfaces have pores, craters and the recasted layers. In the same EDM conditions, only the negative polarity process under using the EP1 oil and gas will not produce surface cracks. In the polarity process, using the EP1 oil, deionized water and gas can be produce the thick recasted layers and then by the XRD analysis, we can know the crystallization phase are zirconium carbide, (ZrC) phase, zirconium oxide (ZrO2) phase and amorphous phase respectively, and by the TEM analysis, the surface shows a nanometer grain and some amorphous structure. For the base metal hardness is Hv 518. After EDM process, the hardness of the recasted layer are affected by crystallization and the thickness of recasted layer .The hardness of recasted layer under using the deionized water is the best (Hv 1592), and using the oil is the second (Hv 601).

The first metallic glass was discovered by Pol Duwez and his colleagues at Caltech in 1960. Shortly after that, Turnbull pointed out that a high ratio of melting temperature over glass transition temperature favors glass formation based on classical nucleation theory. Using Turnbull's theory as a guideline, many alloy systems were found forming glasses. As a new state of metallic alloys, metallic glass is of scientific interest by itself as well as attractive for practical applications due to unique properties. However, since the critical cooling rates for glass formation of conventional alloys are relatively high, at least one dimension of the metallic glass is limited to a few tenths of milimeter. This severely limited the application of metallic glasses.

Recently, several multicomponent alloy systems were discovered that form bulk metallic glasses with the smallest dimension of several mm up to several cm. Two of these alloy systems are the Ti-Zr-Cu-Ni and the Zr-Ti-(Nb)-Cu-Ni-Al alloys, which were discovered during this thesis research.

In this thesis homogeneous and heterogeneous nucleation theories and their influence on glass formation are reviewed. Experimental methods for metallic glass processing are introduced. The formation of Ti-Zr-Cu-Ni and Zr-Ti-(Nb)-Cu-Ni-Al bulk metallic alloys are reported. The glass transition, crystallization behavior, and mechanical properties of these glassy alloys are presented. The optimized combinations of four or five elements in these alloys leads to very deep eutetic multicomponent alloys, in which nucleation of crystals upon undercooling becomes increasingly improbable. Thermodynamic, kinetic and topological factors are discussed that contribute the exceptional glass forming ability of these alloys.

The very high stability of the undercooled liquids of bulk glass forming alloys makes it possible to perform studies on undercooled liquids in a time and temperature window that was not accessible thus far. In this thesis the time temperature transformation diagrams were measured for the crystallization of undercooled Zr-Ti-(Nb)-Cu-Ni-Al liquids. It has been discovered that the crystallization of the undercooled liquid is caused by heterogeneous nucleation induced by oxide particles, even when the oxygen concentration is as low as 250 atomic ppm. Overheating the melts above the liquidus temperature of the oxide particle results in maximum undercooling. Strong oxygen concentration dependence of the overheating, undercooling, and TTT diagrams was found. These results suggest that impurities play a key role in the crystallization of undercooled liquid. This first systematic study of impurity effect on crystallization of undercooled liquid will have many important implications on making bulk glasses and any study of undercooled liquids.

To summarize this thesis, important factors which affect the crystallization are revealed and discussed. Strategies on how to find new bulk metallic glasses are suggested.

碩士
義守大學
材料科學與工程學系
106
Amorphous alloys have excellent mechanical properties, but it lack of ability of plastic deformation and dutility at room temperature. In order to improve these disadvantage, there are many methods that can improve the properties have been developed. For example, thermal treatments, adding of elements, rolling and so on. Zr53Cu30Ni9Al8, this bulk metallic glasses has good glass forming ability, its γm is about 0.739 and wide supercooled liquid region, its ΔTx is about 80K. In this study, for research the working ability of bulk metallic glasses at room temperature, the Cu sheets have been chosen to place at the upside and downside of Zr53Cu30Ni9Al8 bulk metallic glasses to carry out the roll bonding to disperse stress distribution and achieve the goal of average deformation, make bulk metallic glasses and Cu sheet become a composite material. The thin metal coating slows down shear band dynamics and retards its attainment to a critical unstable state. It not only is beneficial to working of bulk metallic glasses but also can keep the thermal properties of bulk metallic glasses, that is different from warm rolling. This study also through Zr53Cu30Ni9Al8 bulk metallic glasses roll bonding with Cu sheet to investigate how the process of roll bonding influence the effet of roll bonding, like surface treatments, the rolling direct of roll bonding, the reduction rate of roll bonding and the thickness of bulk metallic glasses.

Bulk-metallic glasses have established a formidable presence in the scientific community in recent years, due to a number of properties that are uncharacteristic of metallically-bonded materials. One of the fundamental challenges facing researchers in this field is to develop new and improved processing methods with the ultimate goal of facilitating a large-scale industrial integration of the materials. The research described herein is directed toward the pursuit of developing and improving upon the current state-of-the art in the science of bulk-metallic glass processing. A number of research and development projects were undertaken in this pursuit. First, the technology to process bulk-metallic glasses at the University of Tennessee was developed and successfully implemented. Second, bulk-metallic glasses were produced using aerodynamic levitation, which showed an improvement over the accessible cooling rates achievable employing other containerless-processing methods. Third, erbium was found to be a superior dopant to other rare-earth elements to neutralize the oxygen in a Zr-based glass-forming alloy. The alloy was found to form a glass in the presence of up to 16,000-atomic-ppm oxygen by microalloying with Er, with a relatively minor effect on the thermal and mechanical integrity of the materials. Fourth, metastable intermetallic phases were identified in as-cast VIT-105 alloy materials that contained oxygen, using diffraction. The diffraction study included the whole pattern fitting of diffraction from crystalline species in a BMG, an analytical approach that, if existing at all in the literature, is quite rare. Furthermore, this study included a novel approach to fitting diffraction from the glass. Fifth, oxygen-stabilized analogues to intermetallic phases were found in the superheated-liquid state. The presence of Er was found to inhibit surface reoxidation, revealing its mechanism for the neutralization of oxygen. The results were used to propose a model for heterogeneous nucleation and the so-called "overheating threshold" in the alloy.

Die Motivation zur Untersuchung ternärer und quaternären CuZr-Legierungen bestand in der Annahme, dass die Zugabe von Kobalt den Stabilitätsbereich von B2 CuZr bis zur Raumtemperatur erweitert und Aluminium einen signifikanten Effekt auf die Glasbildungsfähigkeit des CuZr-Systems hat. Die vorliegende Dissertation befasst sich mit der Herstellung und Charakterisierung von Cu50-xCoxZr50 (0 ≤ x ≤ 20) und Cu50-xCoxZr45Al5- (x = 2, 5, 10 und 20) Legierungen. Hierbei wurden die Phasenbildung, die thermische Stabilität, die Mikrostruktur, die Martensitbildung und die mechanischen Eigenschaften der Legierungen untersucht. Die Abhängigkeit der Phasenbildung von der Erstarrungsrate und der thermodynamischen Stabilität von Cu-Co-Zr-Legierungen zeigte, dass die Zugabe von Kobalt die Glasbildungsfähigkeit von Cu-Co-Zr-Legierungen absenkt und die stabilen kristallinen Produkte des Systems von Cu10Zr7 + CuZr2 zu (Cu,Co)Zr Phase mit einer B2 Struktur verändert. Die Ergebnisse weisen darauf hin, dass bei den schmelzgesponnene Bänder mit wenigstens 5 Atom-% Co das Glas direkt in B2 (Cu,Co)Zr kristallisiert, während Massivproben mit Co-Gehalten zwischen 0 ≤ x < 5 die monokline (Cu,Co)Zr Phase und Cu10Zr7 sowie CuZr2 beinhalten, wobei für x ≥ 10 die B2 (Cu,Co)Zr Phase bei Raumtemperatur im Gleichgewicht ist. Des Weiteren werden mit steigendem Co-Gehalt die Martensitumwandlungstemperaturen zu niedrigeren Werten verschoben. Die Phasenbildung im ternären System wird im pseudo-binären (Cu,Co)Zr-Phasendiagramm zusammengefasst, welches die Entwicklung neuer Formgedächtnislegierungen sowie metallischer Glas-Komposite bei Zugabe des Glasbildungselementes Aluminium vereinfacht. In den Vierstofflegierungen erhöht Al die Glasübergangs- und Kristallisationstemperaturen und verbessert dadurch die Glasbildungsfähigkeit des Systems. Die röntgenographische Analyse zeigte, dass die Kristallisationsprodukte der schmelzgesponnenen Bänder variieren: von Cu10Zr7 + CuZr2 + AlCu2Zr zu (Cu,Co)Zr + AlCu2Zr, wenn Co ≤ 5 und Co ≥ 10. Die Herstellung von Massivproben mit unterschiedlichen Durchmessern führte zu einem vollständig amorphen Gefüge, einem metallischen Glas-Komposit oder einem vollständig kristallinen Gefüge. Für Co ≤ 5 tritt neben (Cu,Co)Zr und AlCu2Zr ebenfalls Cu10Zr7 auf. Mittels Rasterelektronen (REM)- und Transmissionselektronenmikroskopie (TEM) erfolgte die Analyse des Einflusses von Al- und Co-Zugaben auf die Mikrostruktur von CuZr-Legierungen. Für die Cu-Co-Zr-Al-Legierungen sowie Cu30Co20Zr45Al5 (ø = 4 mm) und Cu45Co5Zr45Al5 (ø = 2 mm) wurden mikrostrukturelle Untersuchungen mittels TEM durchgeführt. Nachfolgend wurde die Heterogenität der Mikrostruktur in der Cu40Co10Zr45Al5 (ø = 2 mm) untersucht. Der Einfluss von Co auf die mechanischen Eigenschaften von rascherstarrten Cu50-xCoxZr50 (x = 2, 5, 10 und 20 Atom-%) Legierungen zeigt, dass das Verformungsverhalten der Stäbe unter Druckbeanspruchung stark von der Mikrostruktur der (Cu,Co)Zr Phase und somit von der Legierungszusammensetzung abhängt. Kobalt beeinflusst die Bruchfestigkeit der Gussproben. Weiterhin zeigen Proben mit martensitischem Gefüge eine Kaltverfestigung. Neben der Kaltverfestigung zeigen die Legierungen mit hohem Co-Gehalt eine verformungsinduzierte Martensitumwandlung und weisen zwei Streckgrenzen auf. Für die Vierstofflegierungen wurde der Einfluss der Kühlrate und der chemischen Zusammensetzung auf die mechanischen Eigenschaften untersucht. Für Cu48Co2Zr45Al5 (ø = 1.5, 2, 3 und 4 mm) und Cu45Co5Zr45Al5 (ø = 3 mm) wurde der Einfluss der Kühlrate und der Heterogenität diskutiert. Die Ergebnisse zeigen, dass die mechanischen Eigenschaften der Cu50-xCoxZr45Al5-Legierungen stark von der Makrostruktur und dem Volumenanteil der amorphen und kristallinen Phase abhängen. Die verformungsinduzierte Martensitumwandlung in Cu40Co10Zr50- und Cu40Co10Zr45Al5-Gussstäben wurde mittels hochenergetischer Röntgenstrahlung durchgeführt. Die In-situ- Druckversuche erfolgten weg- und kraftkontrolliert. Das makroskopische und mikroskopische Spannung-Dehnungs-Verhalten sowie die Phasenumwandlungskinetik wurden dabei betrachtet. Die relativen Veränderungen der vollständig integrierten Intensität der ausgewählten B2- und Martensitreflexe, die auf die Veränderungen der Volumenanteile der entsprechenden Phasen unter Verformung hinweisen, wurden als Phasenumwandlungsvolumen M/M+B2 beschrieben.
The fact that the presence of Co extends the stability range of B2 CuZr to room temperature, together with the significant effect of Al on improving the glass forming ability of the CuZr system was the motivation to investigate the ternary and quaternary CuZr alloys with the aim of synthesizing BMG composites containing B2 (Cu,Co)Zr crystals. This PhD thesis deals with preparation and characterization of Cu50-xCoxZr50 (0 ≤ x ≤ 20) and Cu50-xCoxZr45Al5 (x = 2, 5, 10 and 20) alloys. The phase formation, thermal stability, microstructure, martensitic transformation and mechanical properties of these alloys were investigated. The dependence of phase formation on solidification rate and the thermodynamically stability of Cu-Co-Zr alloys reveals that the addition of Co decreases the glass forming ability (GFA) of the Cu-Co-Zr alloys and changes the stable crystalline products of the system from Cu10Zr7 + CuZr2 to (Cu,Co)Zr phase with a B2 structure. The results indicate that for the melt-spun ribbons with at least 5 % Co, the glass crystallizes directly into B2 (Cu,Co)Zr, while in the case of bulk specimens, compositions with 0 ≤ x < 5 of Co contain the monoclinic (Cu,Co)Zr phase and Cu10Zr7 and CuZr2, whereas, for x ≥ 10, the B2 (Cu,Co)Zr phase is the equilibrium phase at room temperature. Furthermore, increasing the cobalt content decreases the martensitic transformation temperatures to lower temperatures. The phase formation in the ternary system is summarized in a pseudo-binary (Cu,Co)Zr phase diagram, that helps for designing new shape memory alloys, as well as bulk metallic glass composites with the addition of glass former elements. In the quaternary alloys, Al increases the glass transition and crystallization temperatures and hence improves the GFA of the system. The X-ray analysis illustrates that for the melt-spun ribbons, the crystallization products vary from Cu10Zr7 + CuZr2 + AlCu2Zr to (Cu,Co)Zr + AlCu2Zr when Co ≤ 5 and Co ≥ 10, respectively. Depending on the cooling rates, the bulk samples represent a fully amorphous structure or BMG composites or a fully crystalline structure. For Co ≤ 5, beside (Cu,Co)Zr and AlCu2Zr, Cu10Zr7 exists as well. Scanning (SEM) and transmission (TEM) electron microscopy investigations were done to investigate the effect of Al and Co addition to the microstructure of CuZr alloys. In the case of Cu-Co-Zr-Al alloys, Cu30Co20Zr45Al5 (ɸ = 4 mm) and Cu45Co5Zr45Al5 (ɸ = 2 mm) compositions were selected for the microstructure verification using TEM. Later, the heterogeneity of the microstructure in Cu40Co10Zr45Al5 (ɸ = 2 mm) alloy was considered. The effect of Co on the mechanical properties of rapidly solidified Cu50-xCoxZr50 (x = 2, 5, 10 and 20 at.%) alloys depict that the deformation behavior of the rods under compressive loading strongly depends on the microstructure, and as a results, on the alloy composition. Cobalt affects the fracture strength of the as-cast samples; and deformation is accompanied with two yield stresses for high Co-content alloys, which undergo deformation-induced martensitic transformation. Instead samples with a martensitic structure show a work-hardening behavior. For quaternary alloys, the effects of cooling rate and chemical composition on mechanical properties of the alloys were investigated. Cu48Co2Zr45Al5 (ɸ= 1.5, 2, 3 and 4 mm) and Cu45Co5Zr45Al5 (ɸ = 3 mm) compositions were selected to discuss the effect of cooling rate and heterogeneity, respectively. The results depict that the mechanical properties of Cu50-xCoxZr45Al5 alloys strongly depend on the microstructure and the volume fraction of the amorphous and crystalline phases. The deformation-induced martensitic transformation of Cu40Co10Zr50 and Cu40Co10Zr45Al5 as-cast rods, was studied by means of high-energy X-rays. The in situ compression measurements were performed in track control and load control modes. The macroscopic and microscopic stress-strain behavior, as well as the phase transformation kinetics were considered. The relative changes in the fully integrated intensity of the selected B2 and martensite peaks, which indicate the changes in volume fraction of the corresponding phases under deformation, was described as phase transformation volume, M/M+B2.

The various influences on glass-forming ability and crystallization in a series of alloy compositions ranging along a tie-line from Zr41.2Ti13.8Cu12.5Ni10Be22.5 to Zr46.75Ti8.25Cu7.5Ni10Be27.5 are studied using multiple techniques. Both the kinetic and thermodynamic aspects of crystallization are examined experimentally and used to explain why, in this series, thermal stability is greatest in those alloys with the poorest glass-forming ability. Overall, the investigations reveal that a single criterion like thermal stability or high viscosity is an insufficient indicator of the glass-forming ability in certain complex bulk glass-forming systems. Instead, in these complex systems, multiple criteria must be combined to get a complete picture.

The equilibrium viscosity of Zr41.2Ti13.8Cu12.5Ni10Be22.5 and three other alloys was determined for temperatures near the glass transition using three-point beam bending. The results of these measurements indicate that the alloys in this system are moderately strong liquids, and relaxation behavior is directly influenced by a chemical decomposition process which occurs in the supercooled liquid.

This decomposition process also appears to affect the crystallization behavior in the same series of alloys. Critical cooling rates were determined by observing crystallization upon cooling from the molten state with different rates, and these were compared to thermal analysis performed under constant heating conditions. The latter measurements show that the supercooled liquid region is largest for compositions midway between the two endpoints; in contrast, the critical cooling rate increases continuously from 1.4 K/s to 28 K/s for each successive alloy in this series.

In order to examine the crystallization process more closely, low temperature time-temperature-transformation diagrams for the alloy series were measured during isothermal annealing of initially amorphous specimens. For all investigated alloys, a primary quasicrystalline phase forms at a rate which varies substantially with alloy composition. Describing the complicated influences on crystallization required not only the previous thermal analysis but also constant heating measurements and x-ray diffraction data obtained after various states of annealing.

碩士
國立臺灣大學
材料科學與工程學研究所
93
Abstract Ren-Jay Hieh This study focused on the damping capacity of TiNiCu ternary shape memory alloys and of Zr-based amorphous alloys, Zr55Al10Ni5Cu30. The damping values, tanδ, at transient peaks of Ti50Ni35Cu15 and Ti51Ni41Cu8 are both over 0.05 at cooling rate of 3℃/min with frequency 1Hz; after keeping the peak temperature for 30mins and above, the tanδ values of Ti50Ni35Cu15 and Ti51Ni41Cu8 are both descends over 82%. On the other hand, the tanδ values at relaxation peaks don’t drops dramatically after keeping peak temperature, however, they ascends with increasing amplitude. Comparing to TiNiCu shape memory alloys, Zr-based amorphous alloys have lower damping capacity but higher strength, However, its tanδ values increase with larger amplitude. Hydrogenation improves the damping capacity of Zr-based amorphous alloys with thinner thickness remarkably, the tanδ value at cooling rate of 3℃/min with frequency 1Hz exceeds 0.01 after hydrogenation under 100mA/mm2 current density for 2 hrs, but hydrogenation can’t improve the damping capacity of Zr-based amorphous alloys with higher thickness as well, it might due to the difference of H atoms diffusing into specimen result from the different thickness.

博士
國立臺灣大學
材料科學與工程學研究所
87
Five AB2-type Ti-Zr based alloys with a C14 crystal structure have been studied in this work. For a TiCr2 alloy system, substitution of half of Ti by Zr (A atom) and half of Cr by Mn (B atom) significantly increased the hydrogen storage capacity at room temperature. Partially substituted of Mn and Cr by Ni increased the plateau pressure of pressure-composition-isotherm (PCI) curve, helped releasing the absorbed hydrogen in the dehydriding step. However, it also decreased the rate of hydrogen absorption. In addition, the presence of Ni inside the alloy increased the discharge capacity (from 17mAh/g to 80mAh/g) and reduced the internal resistance of metal hydride electrode. The partial substitution of Cr by V significantly increased the discharge capacity of the metal hydride electrode (80mAh/g vs. 300mAh/g) and decreased the plateau pressure of PCI curve at room temperature. It is believed that this improvement was due to the dissolution of V in the electrolyte and the synergistic effect of Ni and V inside the electrode. Since V is good in absorbing hydrogen and Ni helps releasing the absorbed hydrogen, an alloy composed of these two elements in lattice might gain substantial improvement on its performance. For a Ni-free alloy, a process of Ni encapsulation on the alloy surface not only enhanced the discharge capacity but also increased the activation rate. A Ni-P encapsulated Ti0.5Zr0.5Mn1Cr1 alloy showed a discharge capacity of 175 mAh/g, which was nine times better than the bare alloy (17 mAh/g). However, this improvement was deteriorated by a heat treatment at 750oC for 62 hours. This was especially true for the alloy coated with a Ni-P layer. This damaging effect was resulted from Ni migration into the alloy matrix during heat treatment. SEM and EPMA results indicated that a complicated diffusion layer was formed around the particle surface, which inhibited charge/discharge reactions. The thickness of this diffusion layer increased with the temperature of heat treatment, and was more significant for the Ni layer coated in an acidic solution. The crystal structure of the diffusion layer was different from C14. In comparison with the alloy powder produced by cast/hydriding-dehydriding (cast/HDH) method, the atomized powder had a high degree of homogeneity. However, the results of Auger depth profile and TEM/EDS showed that there was an oxide layer on powder surface, together with a structure of partially amorphous phase coexisted with C14 Laves phase. The gas/solid hydrogenation capability of the atomized powder was impaired by the surface oxide and the amorphous phase, although there was still 70% discharge capacity remaining. This problem was solved by crushing the atomized powder, so that new surfaces were created for hydrogen absorption. An annealing heat treatment of the atomized powder at 850oC precipitated a Ni-rich phase, which effectively improved its discharge capacity. The hydride/dehydride and charge/discharge reactions were basically different in kinetics. In gas/solid hydrogenation the lattice volume and the heat of formation (DH) played important roles, while in charge/discharge reaction the charge transfer capability was also an important requirement. The barrier for hydrogen diffusion (such as oxide and amorphous phase) was found to be a major concern in the gas/solid hydrogenation, while the ability of charge transfer was the most important factor in charge/discharge reaction. The prerequisites for these two reactions of a hydrogen storage alloy are suitable composition and crystal structure. However, according to the results of this study, a high capacity in gas/solid hydrogenation does not guarantee a good discharge capacity. On the contrary, except for the case of surface passivation, alloys with a high discharge capacity always possess a good performance in gas/solid hydrogenation.

碩士
義守大學
材料科學與工程學系
103
Bulk metallic glasses (BMGs) have attracted attention due to their high strength and outstanding thermal properties. However, the low workability at room temperature of BMGs limited their applications. Different methods have been developed to solve the problem of poor room temperature ductility. For example, the methods of the minor addition of element, thermal treatments, and hot rolling are used to improve the mechanical properties of BMGs by the formation of phase transformation. It has been reported that the method of the plasticity improvement by rolling process is a possible way to increase the room temperature plasticity. Zr53Cu30Ni9Al8 bulk metallic glasses has high glass forming ability (γm is about 0.752) and wide supercooled liquid region (ΔTx is about 80K) are the base alloys in this study. Room temperature rolling process was used to study the mechanical properties and the workability of Zr53Cu30Ni9Al8 bulk metallic glasses with various rolling strain. The results revealed that the brittle Zr53Cu30Ni9Al8 BMGs was plastically deformed at room temperature by the introducing of rolling process before the compression test. Room temperature compressive fracture strength and strain increased form 2032MPa and 4.6% (the base alloy) to 2266MPa and 20.1% (cold-rolled specimen with 5% rolling strain). The great enhancement of plastic strain can be attributed to the formation of pre-introduced shear bands (called pre-shear bands and secondary pre-shear bands) after the pre-rolling process. Dense dispersion of pre-introduced shear bands impeded the propagation of primary shear bands during the compression test and made the formation of multiple shear bands, indicating a typical ductile fracture behavior and the improved of plasticity of alloys with increasing rolling strains (up to 5%).

碩士
國立中央大學
機械工程學系
101
In recent years, due to the rapid development of industry, the endurance of materials becomes a major issue on each kinds of industrial product. Therefore, the evaluation of crack resistance ability by three-point bending test is applied as the reference of structure design. Zr-based bulk amorphous alloys (BAA) possess good engineering properties and its glass forming ability can be adjusted via minor-alloying of silicon element. Therefore, these two Zr53Cu30Ni9Al8 and Zr48Cu36Al8Ag8 Zr-based amorphous alloys are selected for this study. Both bulk amorphous alloys (BAA) plates were fabricated by tilt-casting and suction-casting. Then these bulk amorphous alloys (BAA) plates were followed ASTM E399 to machine into the three-point bending specimens for investigate their fracture toughness. Experimental results revealed that tilt-casting (Zr53Cu30Ni9Al8) specimen possess the best fracture toughness among all specimens in this study and reaches value about 120 MPa√m. On the other hand, the calculate fracture energy and Poisson’s ratio show same trend to prove tilt-casting Zr-53 owns the best mechanical properties.

In the present study alloy powders with nominal composition of 65.0Zr–20.0Fe–10.0Cu– 5.0Ti (alloy A), 64.0Zr–20.0Fe–10.0Cu–5.0Ti–1.0(Y2O3) (alloy B), 55.0Zr–20.0Fe– 20.0Cu–5.0Ti (alloy C) and 54.0Zr–20.0Fe–20.0Cu–5.0Ti–1.0(Y2O3) (alloy D) (in wt. %) were synthesized by mechanical alloying (MA) followed by conventional sintering at 1050C temperature. Phase evolution and microstructure characterization during milling and after consolidation were characterized by using X- ray diffraction (XRD) and scanning electron microscope (SEM) analysis. An individual element goes into solid solution with increase in milling time and after 10h of milling complete solid solution was observed in all the alloy powders. The XRD analysis revealed that an increase in lattice strain and decrease in crystallite size with increase milling time. The final average particle size of mechanical alloyed powders at 10 hours of milling was in the range of 50-55 nm. The bulk density and hardness of sintered products were found to increases with the presence of dispersion of Y2O3 in Zr matrix. Alloy D sintered at 1050°C exhibits maximum hardness of 2.775 GPa which was 1.5 to 2 times higher than that of the commercially available Zr alloy. The wear tests indicate similar trends as that of hardness and abrasion was the predominant wear mechanism

碩士
國立臺灣大學
材料科學與工程學研究所
88
The effect of sintering on the charging/discharging of an AB2-type hydrogen storage alloy (Ti0.5Zr0.5V0.2Mn0.7Cr0.5Ni0.6) has been investigated. The reasoning behind the use of sintering is that no binder additives are needed in the process, thus the negative electrodes of Ni-MH batteries so prepared have a lower electrical resistance compared to that of pasted electrodes, making them suitable for high-rate charging/discharging applications. The alloy in this study was prepared by arc melting, and pulverized by repeating hydrogen absorption-desorption cycles. The powder was then sintered at different temperatures (400℃, 600℃ and 800℃) under argon for one hour. The capacity of the electrodes was found to improve when higher sintering temperatures were used. The electrodes sintered at 600℃ and 800℃have similar capacities, i.e. 231 mAh/g and 227 mAh/g, respectively. However, the electrodes sintered at 800℃ have a better high-rate dischargeability compared to that sintered at 600℃: its capacity remained at the 100 mAh/g level when a discharge current of 240 mA/g was used. It is believed that an increase in sintering temperature enhanced the bridging of the powder, and the electric conductivity was improved, resulting in a better high-rate dischargeability. In this study an etching treatment in HNO3＋HF solution was found to have no effect on the capacity of the sintered electrode. Another objective of this work has been to study the effect of different electroless nickel coating on the charging /discharging behavior of the sintered electrodes. Two types of electroless coatings, namely Ni-P coating, and duplex coating consisting of Ni-P and pure Ni layers, have been preformed on the powder before sintering. When sintered at 400℃, both types of coatings improved the discharge capacity as compared with that of powder without Ni-coating. The improvement in discharge capacity can be attributed to the existence of a Ni-rich surface, which acted as a catalysts for hydrogen adsorption and desorption. For electrodes sintered at 600℃, the one with a duplex coating has the maximum capacity of 295 mAh/g, the largest obtained in this study. When the sintering temperature was raised to 800℃, the capacity of electrodes with Ni-coating deteriorated to 180 mAh/g for duplex coated electrodes, and to 50 mAh/g for Ni-P coated electrodes. This was because of the nickel diffusion into the bulk, which reduced its catalytic effect on the surface. It has also been suggested in other''s work that a high nickel concentration in the powder may also be detrimental to the charging/discharging of the electrodes. When sintered at 600℃, the duplex coating improved the high-rate dischargeability of electrodes. It is suggested that the pure nickel layer on the powder surface can be bonded together effectively with the 600℃ sintering, thus increasing the electrical conductivity of the electrode. The strong bonding of the nickel on the powder''s surface also prevented the electrode from pulverization, hence a higher rate dischargeability can be achieved.

The present work aims at developing a new class of high temperature alloys based on ordered intermetallic compound that forms coherently with the matrix during solid state transformation. The chosen intermetallics have L12 ordered structure, which is a derivative of fcc unit cell. Most popular example of this fcc derivative is Ni3Al that is critical in developing high strength at high temperatures (~900°C) in commercially successful Ni based superalloys. Similar ordered structures form either in stable or metastable form can act as a main strengthening constituent in Al and Co matrices. For example Al3Sc, Al3Zr, Al3Hf can be dispersed in fcc Al matrix that are stable at temperatures ~ 400°C due to very low diffusivity of transition metals (Sc, Zr, Hf etc.) in the matrix. However, due to low solid solubility of these transition metals, the obtained volume fraction of these precipitates in the matrix is not sufficient to provide adequate room temperature strength. In fcc Co matrix, stable Co3Ti phase with L12 ordered structure forms with cuboidal morphology. However, besides having lower melting point, the precipitates have large misfit that lowers thermal stability at high temperatures. Recently, addition of Al and W with a proper ratio in Co is reported to lead the formation of metastable Co3(Al,W) L12 ordered phase in fcc α-Co matrix. This provides significant strength at high temperatures (~ 900°C). The main drawback for these alloys is their high densities (9.6 to 10.5 gm.cm-3) due to the requirement of compulsory addition of W (~ 15 to 25 wt%) for stabilising the ordered phase. In the present work, these problems are overcome leading to the development of new class of Al and Co alloys. The thesis is organized in three parts. In the first part, the principles of strengthening that can be optimized to develop newer high temperature high strength alloys are reviewed. The ordered L12 structure, which is the mainstay of the current effort of new alloy development, is elaborated. In the second part we present the results of our effort to the development a new class of high strength high temperature Al alloys. A new approach has been adopted to get a microstructure that contains both high temperature stable and room temperature strengthening precipitates. This has been illustrated by two Al rich compositions, Al-2Cu-0.1Nb-0.15Zr and Al-2Cu-0.1Hf-0.15Zr (at% unless stated otherwise). Addition of Nb/Zr or Hf/Zr in Al alloys leads to the formation of high temperature stable L12 ordered spherical coherent precipitates in the fcc Al matrix. Cu addition gives room temperature strengthening θ’ and θ” precipitates. The arc melted alloys were chill cast (suction cast) in the form of 3 mm rods followed by a novel three stage heat treatment process, as shown below. In the case of Al-2Cu-0.1Nb-0.15Zr alloy, the chill cast structure consists of Cu rich phase at the boundaries along the α-Al dendrites while Zr and Nb partition inside the α-Al dendrites. Aging at 400°C leads to an increase in the hardness of the cast alloy due to the precipitation of coherent L12 ordered Al3(Zr,Nb) spherical precipitates (~5nm) in the α-Al dendrites. Zr strongly partitions to the L12 ordered precipitate relative to the matrix. Nb exhibits weak partitioning in the precipitate. Further solutionising was optimized at 535°C for 30 minutes such that the segregation of Cu in the chill cast samples can be eliminated. The WDS mapping shows that Cu dissolved uniformly in the α-matrix while the Zr/Nb enriched α-Al dendrites are still present. The L12 ordered precipitates are mostly found in these Zr/Nb enriched dendrites formed during solidification. The precipitates sizes are finer (~5 nm) in dendrites and larger in the interdendritic region. The Nb partitioning increases in the ordered L12 precipitates relative to the matrix after solutionising. On aging at 190°C, fine θ” precipitates nucleate on prior Al3(Zr,Nb) precipitates present in α-Al dendrites while the interdendritic regions contain coarser θ’ nucleated on larger size L12 precipitates. The θ”/θ’ are much finer and higher in number density for the quaternary alloy compared to binary Al-2Cu alloy subjected to conventional heat treatment. The quaternary alloy show higher peak hardness of 1500 ± 8 MPa after 5 hours of aging at 190°C compared to binary Al-2Cu alloy with peak hardness of 1260 ± 11 MPa.

The objective of this thesis is to study the fracture behavior of bulk metallic glasses. For this purpose, detailed finite element investigation of the mode I and mixed mode (I and II) stationary crack tip fields under plane strain, small scale yielding conditions is carried out. An implicit backward Euler finite element implementation of the Anand and Su constitutive model [Anand, L. and Su, C., 2005, J. Mech. Phys. Solids 53, 1362] is used in the simulations. The effects of internal friction (μ), strain softening, Poisson's ratio (ν) and elastic mode mixity (Me) on the near-tip stress and deformation fields are examined. The results show that under mode I loading, a higher μ leads to a larger normalized plastic zone size and higher plastic strain level near the notch tip, but causes a substantial decrease in the opening stress. The brittle crack trajectories and shear band patterns around the notch are also simulated. An increase in ν reduces the extent of plastic zone and plastic strain levels in front of the notch tip. The results from mixed mode simulations show that increase in the mode II component of loading dramatically increases the maximum plastic zone extent, lowers the stresses and significantly enhances the plastic strain levels near the notch tip. Higher μ causes the peak magnitudes of tensile tangential stress to decrease. The implications of the above results on the fracture response of bulk metallic glasses are discussed. The possible variations of fracture toughness with mode mixity predicted by employing two simple fracture criteria are examined. Finally, mixed mode (I and II) fracture experiments on a Zr-based bulk metallic glass are performed. It is found that the fracture toughness increases with Me and Jc under mode I is higher than that under mode II loading by a factor of 4. The operative failure mechanism and fracture process zone size are discerned based on observations of incipient crack growth and fractographs. Lastly, a fracture criterion is proposed which predicts the experimentally observed variation of fracture toughness with mode mixity.

The objective of this thesis is to study the fracture behavior of bulk metallic glasses. For this purpose, detailed finite element investigation of the mode I and mixed mode (I and II) stationary crack tip fields under plane strain, small scale yielding conditions is carried out. An implicit backward Euler finite element implementation of the Anand and Su constitutive model [Anand, L. and Su, C., 2005, J. Mech. Phys. Solids 53, 1362] is used in the simulations. The effects of internal friction (μ), strain softening, Poisson's ratio (ν) and elastic mode mixity (Me) on the near-tip stress and deformation fields are examined. The results show that under mode I loading, a higher μ leads to a larger normalized plastic zone size and higher plastic strain level near the notch tip, but causes a substantial decrease in the opening stress. The brittle crack trajectories and shear band patterns around the notch are also simulated. An increase in ν reduces the extent of plastic zone and plastic strain levels in front of the notch tip. The results from mixed mode simulations show that increase in the mode II component of loading dramatically increases the maximum plastic zone extent, lowers the stresses and significantly enhances the plastic strain levels near the notch tip. Higher μ causes the peak magnitudes of tensile tangential stress to decrease. The implications of the above results on the fracture response of bulk metallic glasses are discussed. The possible variations of fracture toughness with mode mixity predicted by employing two simple fracture criteria are examined. Finally, mixed mode (I and II) fracture experiments on a Zr-based bulk metallic glass are performed. It is found that the fracture toughness increases with Me and Jc under mode I is higher than that under mode II loading by a factor of 4. The operative failure mechanism and fracture process zone size are discerned based on observations of incipient crack growth and fractographs. Lastly, a fracture criterion is proposed which predicts the experimentally observed variation of fracture toughness with mode mixity.

碩士
國立中央大學
機械工程研究所
99
We proposed with the Zr-based and Zr-Cu based metallic glass thin film (MGTF) as promising coating for aluminum alloy fatigue property enhancement. According to the four-point-bending fatigue results, 7075-T6 aluminum alloy with a 200-nm-thick Zr-based MGTF improved its fatigue life cycle 22 times at a stress level of 250 MPa than the bare one. And the other fatigue life cycle of Zr-Cu based MGTF is further improved 44 times which ups to 107 cycles. The improvements of MGTF coating samples in fatigue limit were 235 MPa (56.7 % increase) and 250 MPa (66.7 % increase) for Zr-based and Zr-Cu based glass-forming film, respectively, and 150 MPa for uncoated sample. The films actually restrict the surface offsets and cracks propagating during the fatigue test. Zr-Cu based glass-forming film have better fatigue resistance than Zr-based MGTF, the fatigue life had improved by more than 2 times under a stress of 250 MPa, due to higher hardness and strength, better plasticity, thus it exhibits better improvement in fatigue property. A 50-nm-thick Titanium buffer layer between the film and the substrate was reported adhesion enhancement. The superior mechanical properties of MGTF, such as high strength and good bending ductility, coupled with good adhesion between the film and the substrate as well as the reduced surface roughness, and high compressive residual stress of the metallic film yield the fatigue property improvement of aluminum alloy. Thus demonstrating MGTF as promising coating materials for improving the fatigue properties of materials, and further applied to aerospace, automobile industry and bicycle manufacturing etc.

碩士
國立成功大學
材料科學及工程學系碩博士班
92
We applied plasma-sprayed technique of high cooling rate to obtain the coating with the property of the quite quick coagulation speed. Experimental results showed that we can’t get fully amorphous coating, the coating layer contains around 80% fraction of amorphous. Because of insufficient data for the microstructure, coagulation defects and particle erosion resistance of the Zr-based high entropy alloy coating, this study was to quantitate the non-crystallinity rate of the coating layer. Moreover, the correlation between bonding strength, microstructural defects and particle erosion characteristic of the coating was also investigated. Although the coating layer contains around 80% fraction of amorphous, there still exists two crystalline phases, ZrO2 and Zr(Cr0.25Fe0.75)2, according to the XRD. In addition, SEM/BEI result the coating with quite amount of the coagulation defects arising with increasing coating thickness. The result of particle erosion experiment indicated that the increase in the coating thickness. This phenomenon is closely related to the increasing in spraying defects content of the coating, especially at high angle erosion. The material erosion behavior is the mainly wear mechanism of brittle cracks. At the low angle erosion, the coagulation defect effect reduced, therefore the particle erosion resistance obviously surpasses the high angle erosion sample.

碩士
國立臺灣大學
材料科學與工程學研究所
88
The objective of this research has been to study the self—discharge properties of the Mm based and Ti-Zr based hydrogen storage alloy electrodes as negative electrodes of Ni-MH batteries. Another purpose of this study has been to study the effect of electro less nickel coating on the Mm based electrodes. The alloy powder in this study was prepared by arc melting, and pulverized by mechanical crushing. The self -discharge testing this study was divided into two parts, namely continuous model and step model. The variation of capacity retention with storage time and storage temperature was measured in continuous self-discharge test. The correlation between reversible self-discharge, irreversible self-discharge and used time was investigated in step self- discharge test. It is believed that the reversible self-discharge loss was caused by the difference between the plateau pressure of negative electrodes and hydrogen partial pressure in the cell. The oxidation of negative electrodes and leaching of some elements from the hydrogen storage alloy increased the irreversible self-discharge loss. The capacity retention of the original AB5 alloy electrodes storage for 16 days at 25℃ decayed to 55% and to 22% for 7 days storage at 60℃. However, the capacity retention of the Ni coated AB5 alloy electrodes storage for 16 days at 25℃ improved to 63% and to 31% for 7 days storage at 60℃. The raise of capacity retention of Ni -MHX battery was attributed to the Ni coating layer served as a diffusion barrier for the hydrogen atom. In step self-discharge test, the reversible capacity loss of the original AB5 alloy electrodes increased from 40mAh/g to 75mAh/g after 15 charging discharging cycles at 25℃. However, the one of the original AB2 alloy electrodes maintained at 30mAh/g after 15 charging discharging cycles at 25℃. It’s revealed that the mechanism of the reversible self—discharge between the Mm based and Ti-Zr based alloy electrodes is different. The reversible capacity loss can be kept to 25 mAh /g after 11 charging discharging cycles at 25℃ by adding 2wt.% Al in electrolyte. The same results can be obtained for the Ni coated alloy electrodes without Al additives. It’s suggested that the Al can be retained in the alloy powder by nickel coating treatment and adding Al in electrolyte, thus the reversible capacity loss can be kept to a constant value in our experiment.

碩士
國立臺灣海洋大學
材料工程研究所
100
The purpose of this study is to investigate the corrosion behavior of Zr-based amorphous alloy and its crystalline state in 3.5wt% NaCl, 1N H2SO4, 1M NaOH and 1M NH4OH aqueous solutions at room temperature. For Zr41.2Cu12.5Ni10Ti13.8Be22.5 in 3.5wt% NaCl aqueous solutions, both amorphous and crystalline state show susceptibility to pitting corrosion, and the corrosion resistance of amorphous state is better than crystalline state. For Zr41.2Cu12.5Ni10Ti13.8Be22.5 in 1N H2SO4 aqueous solution, the corrosion resistance of its crystalline state is much better than amorphous state, due to its passive film formation. In 1M NaOH and 1M NH4OH aqueous solutions, the corrosion resistance of their crystalline states are superior to their amorphous states. The result by TEM micrograph shows that the nanocrystalline states of Zr-based amorphous alloys can be obtained after annealing heat treatment.

碩士
國立中央大學
材料科學與工程研究所
103
Due to the high specific ratios of strength to weight of 7075 aluminum alloy, it is widely used in light weight sport equipment, automobile bodies, and aircraft frames. However, the 7075 aluminum alloy has poor fatigue property in comparison to steel. In this study, different thicknesses (namely 200, 300, 400, and 500 nm) of Zr-based ((Zr53Cu30Ni9Al8)99.5Si0.5) and 200 nm Ti-based (Ti40Zr10Cu36Nb7Co7) metallic glass thin films (MGTF) were successfully coated on the 7075 aluminum alloy substrates by DC-sputtering method. Then the fatigue properties of these coated and as-polished samples were tested following the standard of ASTM-C1161-02c. The results revealed that the fatigue life and fatigue strength show an opposite trend with increasing the thickness of MGTF due to the changes of surface roughness and residual stress. The sample coated with 200-nm-thick Zr-based MGTF is found to have the best fatigue properties than others thickness-coated samples. Moreover, the fatigue life of 7075-T6 aluminum alloy can be improved about 26.3 and 7.9 times than the bare one at the high stress level of 250 MPa by coating with 200 nm-thick Zr-based and Ti-based MGTF. In parallel, the improvements of Zr-based and Ti-based MGTF-coated samples in fatigue strength were 235 MPa (56.7 %) and 205 MPa (43.3 %) compared with as-polished sample (150 MPa). Based on the TEM examination, the formation of offsets and cracks from the surface of specimen was revealed to be effectively restricted by both MGTF coatings. This is attributed to the high strength, good flexibility, and strong adhesion of both MGTF coatings, which can provide effective compressive stress to suppress the slip band protruding. In summary, Zr-based MGTF coating is believed to be a promising coating material for improving 7075-T6 aluminum fatigue properties.