Dissertations / Theses on the topic 'Superalloys High entropy alloys'
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Slone, Connor. "Influence of composition and processing on the mechanical response of multi-principal element alloys containing Ni, Cr, and Co." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555522223986934.
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Jensen, Jacob K. "Characterization of a High Strength, Refractory High Entropy Alloy, AlMo0.5NbTa0.5TiZr." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492175560975813.
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Huang, Shuo. "Theoretical Investigations of High-Entropy Alloys." Licentiate thesis, KTH, Tillämpad materialfysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-218162.
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High-entropy alloys (HEAs) are composed of multi-principal elements with equal or near-equal concentrations, which open up a vast compositional space for alloy design. Based on first-principle theory, we focus on the fundamental characteristics of the reported HEAs, as well as on the optimization and prediction of alternative HEAs with promising technological applications. The ab initio calculations presented in the thesis confirm and predict the relatively structural stability of different HEAs, and discuss the composition and temperature-induced phase transformations. The elastic behavior of several HEAs are evaluated through the single-crystal and polycrystalline elastic moduli by making use of a series of phenomenological models. The competition between dislocation full slip, twinning, and martensitic transformation during plastic deformation of HEAs with face-centered cubic phase are analyzed by studying the generalized stacking fault energy. The magnetic moments and magnetic exchange interactions for selected HEAs are calculated, and then applied in the Heisenberg Hamiltonian model in connection with Monte-Carlo simulations to get further insight into the magnetic characteristics including Curie point. The Debye-Grüneisen model is used to estimate the temperature variation of the thermal expansion coefficient. This work provides specific theoretical points of view to try to understand the intrinsic physical mechanisms behind the observed complex behavior in multi-component systems, and reveals some opportunities for designing and optimizing the properties of materialsQC 20171127
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Gwalani, Bharat. "Developing Precipitation Hardenable High Entropy Alloys." Thesis, University of North Texas, 2017. https://digital.library.unt.edu/ark:/67531/metadc1011755/.
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High entropy alloys (HEAs) is a concept wherein alloys are constructed with five or more elements mixed in equal proportions; these are also known as multi-principle elements (MPEs) or complex concentrated alloys (CCAs). This PhD thesis dissertation presents research conducted to develop precipitation-hardenable high entropy alloys using a much-studied fcc-based equi-atomic quaternary alloy (CoCrFeNi). Minor additions of aluminium make the alloy amenable for precipitating ordered intermetallic phases in an fcc matrix. Aluminum also affects grain growth kinetics and Hall-Petch hardenability.
The use of a combinatorial approach for assessing composition-microstructure-property relationships in high entropy alloys, or more broadly in complex concentrated alloys; using laser deposited compositionally graded AlxCrCuFeNi2 (0 < x < 1.5) complex concentrated alloys as a candidate system. The composition gradient has been achieved from CrCuFeNi2 to Al1.5CrCuFeNi2 over a length of ~25 mm, deposited using the laser engineered net shaping process from a blend of elemental powders. With increasing Al content, there was a gradual change from an fcc-based microstructure (including the ordered L12 phase) to a bcc-based microstructure (including the ordered B2 phase), accompanied with a progressive increase in microhardness. Based on this combinatorial assessment, two promising fcc-based precipitation strengthened systems have been identified; Al0.3CuCrFeNi2 and Al0.3CoCrFeNi, and both compositions were subsequently thermo-mechanically processed via conventional techniques. The phase stability and mechanical properties of these alloys have been investigated and will be presented. Additionally, the activation energy for grain growth as a function of Al content in these complex alloys has also been investigated.
Change in fcc grain growth kinetic was studied as a function of aluminum; the apparent activation energy for grain growth increases by about three times going from Al0.1CoCrFeNi (3% Al (at%)) to Al0.3CoCrFeNi. (7% Al (at%)). Furthermore, Al addition leads to the precipitation of highly refined ordered L12 (γ′) and B2 precipitates in Al0.3CoCrFeNi. A detailed investigation of precipitation of the ordered phases in Al0.3CoCrFeNi and their thermal stability is done using atom probe tomography (APT), transmission electron microscopy (TEM) and Synchrotron X-ray in situ and ex situ analyses.
The alloy strengthened via grain boundary strengthening following the Hall-Petch relationship offers a large increment of strength with small variation in grain size. Tensile strength of the Al0.3CoFeNi is increased by 50% on precipitation fine-scale γ′ precipitates. Furthermore, precipitation of bcc based ordered phase B2 in Al0.3CoCrFeNi can further strengthen the alloy. Fine-tuning the microstructure by thermo-mechanical treatments achieved a wide range of mechanical properties in the same alloy. The Al0.3CoCrFeNi HEA exhibited ultimate tensile strength (UTS) of ~250 MPa and ductility of ~65%; a UTS of ~1100 MPa and ductility of ~30%; and a UTS of 1850 MPa and a ductility of 5% after various thermo-mechanical treatments. Grain sizes, precipitates type and size scales manipulated in the alloy result in different strength ductility combinations. Henceforth, the alloy presents a fertile ground for development by grain boundary strengthening and precipitation strengthening, and offers very high activation energy of grain growth aptly suitable for high-temperature applications.
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Shittu, Jibril. "Tribo-Corrosion of High Entropy Alloys." Thesis, University of North Texas, 2020. https://digital.library.unt.edu/ark:/67531/metadc1752392/.
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In this dissertation, tribo-corrosion behavior of several single-phase and multi-phase high entropy alloys were investigated. Tribo-corrosion of body centered cubic MoNbTaTiZr high entropy alloy in simulated physiological environment showed very low friction coefficient (~ 0.04), low wear rate (~ 10-8 mm3/Nm), body-temperature assisted passivation, and excellent biocompatibility with respect to stem cells and bone forming osteoblast cells. Tribo-corrosion resistance was evaluated for additively manufactured face centered cubic CoCrFeMnNi high entropy alloy in simulated marine environment. The additively manufactured alloy was found to be significantly better than its as-cast counterpart which was attributed to the refined microstructure and homogeneous elemental distribution. Additively manufactured CoCrFeMnNi showed lower wear rate, regenerative passivation, less wear volume loss, and nobler corrosion potential during tribo-corrosion test compared to its as-cast equivalent. Furthermore, in the elevated temperature (100 °C) tribo-corrosion environment, AlCoCrFeNi2.1 eutectic high entropy alloy showed excellent microstructural stability and pitting resistance with an order of magnitude lower wear volume loss compared to duplex stainless steel. The knowledge gained from tribo-corrosion response and stress-corrosion susceptibility of high entropy alloys was used in the development of bio-electrochemical sensors to sense implant degradation. The results obtained herewith support the promise of high entropy alloys in outperforming currently used structural alloys in the harsh tribo-corrosion environment.
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Asensio, Dominguez Laura. "Combinatorial high throughput synthesis of high entropy alloys." Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/16722/.
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This PhD thesis is a part of the Accelerated Metallurgy (AccMet) project funded under the Seventh Framework Programme. AccMet’s aims consists on the delivery of an integrated pilot-scale facility for the combinatorial synthesis and testing of those unexplored material. The contribution of this thesis to AccMet has been expanded in 3 years while focused in the understanding and development of a methodology suitable for the combinatorial synthesis of novel materials, and particularly of High Entropy Alloys (HEAs). These novel materials are composed of multiple elements at near equiatomic levels with the capacity of forming simple crystalline phases such as bcc and fcc instead of the expected intermetallic compounds as well as their excellent combination of structural and functional properties compared to the traditional materials. A mathematical technique known as Principal Component Analysis has been used here to identify patterns within a set of metallic systems forming a wide range of crystalline structures. This technique would not only speed up the compositional design stage but also contribute to the development of a virtual library containing all the explored systems. Mercury Centre has been an important key during the synthesis of HEAs where Spark Plasma Sintering (SPS) and Electron Beam Melting (EBM) have been successfully applied for the development of the thesis. The final combination of the design stage, production and characterisation of HEAs developed in this thesis would result in an advances technique suitable not only for the synthesis of novel HEAs, but also for the discovery of other unexplored systems.
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Cunliffe, Andrew. "Origin of properties in high entropy alloys." Thesis, University of Sheffield, 2018. http://etheses.whiterose.ac.uk/22395/.
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The novel class of alloys known as high entropy alloys (HEAs) present two fundamental problems; 1) prediction of their properties and reaction to alloying adjustments, 2) prediction of compositions capable of forming the random solid solution with simple crystal structure that appears to be key to their behaviour. Here DFT is applied to model the electronic structure of HEAs based on the CoCrFeNi pseudo base metal. This approach explains a number of properties such as preferred crystal structure and allows fundamental properties such as elastic moduli to be calculated accurately. The stability of HEAs is discussed and compared to that of bulk metallic glasses and a composition is produced which is capable of forming both a glassy and high entropy solid solution phase. A simple thermodynamic model is proposed to allow likely HEA solid solution forming compositions to be identified. This modelling approach using both DFT and thermodynamics is used to assess two potential high entropy alloys based on light metals. The approach shows that the electronic structure of HEAs may be used to predict their properties and therefore their behaviour is due to a free electron structure, it also suggests that the most important consideration in their stability as solid solution alloys is a lack of strong covalent interactions, ie a close to zero entropy of mixing.
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Soni, Vishal. "Phase Transformations in Refractory High Entropy Alloys." Thesis, University of North Texas, 2019. https://digital.library.unt.edu/ark:/67531/metadc1538735/.
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High entropy alloys (HEAs) based on refractory elements have shown a great potential for high temperature structural applications. In particular, the ones containing Al, exhibits a microstructure similar to the γ-γ' in Ni-based superalloys. While these alloys exhibit impressive strengths at room temperature (RT) and at elevated temperatures, the continuous B2 matrix in these alloys is likely to be responsible for their brittle behavior at RT. Phase stability of five such alloys are studied by thermo-mechanical treatments and characterization techniques using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Two of these alloys showed an inverted microstructure, where the disordered BCC phase becomes continuous, and therefore, they were characterized in detail using SEM, TEM, atom probe tomography (APT) and synchrotron x-ray diffraction experiments. The phenomenon of phase inversion lead to a better combination of strength and ductility as compared to the non-inverted microstructure.To enhance the stability of B2 intermetallic phase which provides the strength when present in a BCC matrix, multicomponent B2 phase compositions stable at 1000°C in some of the above studied alloys, were melted separately. The aim was to establish a single phase B2 at 1000°C and understand the mechanical behavior of these single-phase multicomponent B2 intermetallic alloys. These alloys exhibited a ductile behavior under compression and retained ~1 GPa yield strength at temperature up to 600°C. The ductile nature of these alloys is attributed to the change in bonding nature form directional to metallic bonding, possibly resulting from a significantly high configurational entropy compared to binary or ternary stoichiometric B2 compounds.
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Stasiak, Tomasz. "High Entropy Alloys with improved mechanical properties." Thesis, Lille 1, 2020. http://www.theses.fr/2020LIL1R050.
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Les Alliages à Haute Entropie (AHEs ou HEAs en anglais) sont un nouveau type d'alliages multi-élémentaires. Ils contiennent au moins cinq éléments de teneur comprise entre 5 et 35 at %. L'entropie de configuration élevée, qui est une raison du nom de cette famille d'alliages, ainsi que d'autres paramètres, tels que l'enthalpie de mélange, la différence de taille atomique, la différence d'électronégativité ou la concentration d'électrons de valence, stabilisent une solution solide plutôt que des composés intermétalliques. L'attention de la communauté scientifique a été attirée par les propriétés prometteuses et les microstructures intéressantes des HEAs.Dans ce travail, une nouvelle famille de HEAs Al-Cr-Fe-Mn-Mo a été étudiée. Les analyses microstructurales et chimiques ont été menées par DRX, spectrométrie Mössbauer, MEB, MET, EDX, EBSD. Dans un premier temps, des calculs basés sur une approche paramétrique ont été réalisés pour optimiser la composition chimique. Les compositions sélectionnées ont été préparées par mécanosynthèse dans différents types broyeurs. Les conditions optimisées garantissant une homogénéité chimique maximale de la poudre et une faible contamination par les matériaux des billes et des jarres ont été déterminées. Deux phases cubique centrée (cc) se forment pendant la mécanosynthèse avec les paramètres de maille 3,13 Å (cc#1) et 2,93 Å (cc#2). Le traitement thermique de la poudre entraîne plusieurs transformations de phase (la formation de la phase χ). Le recuit à 950 °C/1 h favorise l'augmentation de la fraction volumique de la phase cc#2, tandis que les cc#1 et χ disparaissent. Néanmoins, de petites fractions de carbures et d'oxydes ont été trouvées.Les échantillons massifs ont été fabriqués par frittage à chaud des poudres mécanosynthétisées. Les conditions de consolidation ont été évaluées et optimisées pour favoriser la formation de la phase cc et réduire la formation de carbures et d'oxydes résultant de la contamination. Les échantillons massifs optimisés présentent une phase majoritaire cubique centrée (> 95 % volumique) avec un paramètre de maille de 2,92 Å et une très petite quantité de carbures (M6C, M23C6) et d'oxydes (Al2O3). La phase cc est stable après recuit à 950 °C pendant 10 h. De plus, l'alliage présente une dureté très élevée jusqu'à 950 HV2N. Les essais de compression de l'échantillon massif optimisé, entre la température ambiante et 800 °C, révèlent des propriétés prometteuses, en particulier entre 600 et 700 °C. L'alliage présente un comportement fragile entre la température ambiante et 400 °C. Cependant, l'alliage commence à démontrer un certain degré de plasticité à 500 °C. À 600 °C, la limite d'élasticité est de 1022 MPa, tandis que la déformation à la rupture est d'environ 22 %High Entropy Alloys (HEAs) are a new type of multicomponent alloys. They contain at least five elements with the content of each between 5 and 35 at. %. The high configuration entropy, which is the source of the name of the whole family of alloys, together with other parameters, such as mixing enthalpy, atomic size difference, electronegativity difference, or valence electron concentration, stabilize a solid solution instead of complex intermetallic compounds. Promising properties and interesting microstructures focus the attention of the scientific community to HEAs.In this work, the novel Al-Cr-Fe-Mn-Mo high entropy alloy family was studied. The microstructural and chemical analyses were performed by XRD, Mössbauer spectrometry, SEM, TEM, EDX, EBSD. In the first stage, parametric approach calculations were carried out to optimize the chemical composition of the alloy. The selected compositions were prepared by mechanical alloying in different devices. The optimized conditions that ensure maximum chemical homogeneity of powder and the small contamination from balls and vial materials were chosen. In most of the powders, two bcc phases form during mechanical alloying with the lattice parameters about 3.13 Å (bcc#1) and 2.93 Å (bcc#2). The heat treatment of powder results in several phase transformations (e.g., the formation of the χ phase). The annealing at 950 °C for 1 h promotes the significant increase of volume fraction of the bcc#2 phase, while the bcc#1 and χ disappear. Nevertheless, small fractions of carbides and oxides were found. The bulk samples were fabricated by hot press sintering of the optimized mechanically alloyed powders. The conditions of consolidation were evaluated and optimized to promote the formation of the bcc phase and reduce the formation of carbides and oxides resulting from the contamination during mechanical alloying and sintering. The optimized bulk samples present a major disordered body-centered cubic phase (> 95 % of volume fraction) with a lattice parameter of 2.92 Å and a very small fraction of carbides (M6C, M23C6) and oxides (Al2O3). The bcc phase is stable after annealing at 950 °C for 10 h. Moreover, the alloy presents very high hardness up to 950 HV2N. The compression tests of the optimized bulk sample from room temperature to 800 °C reveal promising properties, especially between 600 and 700 °C. The alloy shows brittle behavior between room temperature and 400 °C. However, the alloy starts to demonstrate some degree of plasticity at 500 °C. At 600 °C, the yield strength is 1022 MPa, while strain to failure is about 22 %
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Alagarsamy, Karthik. "Application of High Entropy Alloys in Stent Implants." Thesis, University of North Texas, 2017. https://digital.library.unt.edu/ark:/67531/metadc984159/.
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High entropy alloys (HEAs) are alloys with five or more principal elements. Due to these distinct concept of alloying, the HEA exhibits unique and superior properties. The outstanding properties of HEA includes higher strength/hardness, superior wear resistance, high temperature stability, higher fatigue life, good corrosion and oxidation resistance. Such characteristics of HEA has been significant interest leading to researches on these emerging field. Even though many works are done to understand the characteristic of these HEAs, very few works are made on how the HEAs can be applied for commercial uses. This work discusses the application of High entropy alloys in biomedical applications. The coronary heart disease, the leading cause of death in the United States kills more than 350,000 persons/year and it costs $108.9 billion for the nation each year in spite of significant advancements in medical care and public awareness. A cardiovascular disease affects heart or blood vessels (arteries, veins and capillaries) or both by blocking the blood flow. As a surgical interventions, stent implants are deployed to cure or ameliorate the disease. However, the high failure rate of stents has lead researchers to give special attention towards analyzing stent structure, materials and characteristics. Many works related to alternate material and/or design are carried out in recent time. This paper discusses the feasibility of CoCrFeNiMn and Al0.1CoCrFeNi HEAs in stent implant application. This work is based on the speculation that CoCrFeNiMn and Al0.1CoCrFeNi HEAs are biocompatible material. These HEAs are characterized to determine the microstructure and mechanical properties. Computational modeling and analysis were carried out on stent implant by applying CoCrFeNiMn and Al0.1CoCrFeNi HEAs as material to understand the structural behavior.
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Swanson, Orion John. "Corrosion of High-Entropy Alloys in Chloride Solutions." The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1532709505615889.
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Martin, Alexander Charles. "Initial Weldability of High Entropy Alloys for High Temperature Applications." The Ohio State University, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=osu1555496040477991.
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Gondhalekar, Akash Avinash. "Design and Development of Light Weight High Entropy Alloys." Thesis, Tekniska Högskolan, Högskolan i Jönköping, JTH, Material och tillverkning, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:hj:diva-45551.
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The main aim of this thesis was to design and develop new Aluminium based compositionally complex alloys (CCAs) using the high entropy alloy (HEA) concept, and to understand their evolution of microstructures during casting and also after the secondary process which is heat-treatment, and finally to evaluate their subsequent mechanical properties. Prior to the development of alloys, a computational technique ThermoCalc was used which helped in understanding the phase formation in various results. Use of thermodynamic physical parameters for predicting the stability of single-phase fields was done to assess their validity in predicting the compositional regions of the alloys developed. The first alloy developed is Al73.6Mg18Ni1.5Ti1.9Zr1Zn4 in at% (NiTiZrZn) CCA. The microstructure consists of the FCC as a primary phase with ~49% volume fraction along with β-AlMg and intermetallic (IM) phases including Al3Ni, Al3Ti, and Al3Zr. After casting, the microstructure showed some presence of eutectic structures. The Al3Ti, and Al3Zr IM phases seemed to precipitate early which led to less homogenization of Ti and Zr, causing deviation in the amount of these elements in the matrix. Further, the CCA was heat-treated at 375 oC for 24hrs and 48hrs and the evolution of microstructure along with its hardness and phase transformation characterisation was investigated. The second developed alloy was quaternary Al65.65Mg21.39Ag10.02Ni2.94 in at% (AgNi) CCA. In the as-cast state, the main phase (matrix) was FCC with ~64 % volume fraction along with BCC, β-AlMg and Al3Ni IM phases. There was a good level homogenization of all elements in the alloy. They were further heat- treated at 400 oC for 24 hrs and 48 hrs and were studied for any change in microstructure along with its hardness and thermal stability. This CCA had the highest hardness value from all developed CCAs. Lastly, in order to check how Ni affects the microstructure and properties of (AgNi) CCA, a ternary Al67.2Mg22.09Ag10.7 in at% (Ag) CCA was developed. The composition was kept such that it is exactly 97% by excluding the Ni. During the development of this alloy, the cast was cooled in two ways first being the normal cooled just like other CCAs and second being a fast cooling method. Both of these alloys consists of the FCC phase as a primary phase with 72% volume fraction along with BCC and β-AlMg. Both of them were also heat treated at 400 oC for 24 hrs and 48 hrs to evaluate any changes in microstructure and also to assess its hardness and thermal stability.
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Hernandez-Negrete, Ofelia. "Study of intermetallic alloys for high temperature applications 'beyond the Ni superalloys'." Thesis, University of Sheffield, 2016. http://etheses.whiterose.ac.uk/16186/.
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Mignanelli, Paul Michael. "The design of new nickel-base superalloys with high niobium contents." Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.709110.
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Akbari, Azin. "COMBINATORIAL SCREENING APPROACH IN DEVELOPING NON-EQUIATOMIC HIGH ENTROPY ALLOYS." UKnowledge, 2018. https://uknowledge.uky.edu/cme_etds/87.
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High entropy alloys (HEA) are a relatively new group of alloys first introduced in 2004. They usually contain 5 to 6 different principle elements. Each of these elements comprise 5-35 at. % of the chemical composition of the alloy. There is a growing interest in the research community about the development of these alloys as well as their engineering applications. Some HEAs have interesting properties that have made them well suited for higher temperature applications, particularly refractory uses, while some have been shown to maintain their mechanical properties even at cryogenic temperatures.
Initially, the HEA research was focused on developing alloys with equiatomic compositions as it was believed that the single phase HEA would only form at such composition ratios. However, further research have found multiple HEAs with non-equiatomic chemical compositions. A major question that needs to be answered at this point is how to identify these non-equiatomic single phase alloy systems. Unlike the conventional alloys, the HEAs do not have a base element as a solvent, which complicates the identification of new alloy systems via conventional development techniques. To find a potential HEA, alloy development techniques of both exploratory and computational natures are being conducted within the community. Even though multiple HEAs have been successfully identified and fabricated by these techniques, in most cases they require extensive experimental data and are relatively time consuming and expensive. This study proposes a thin film combinatorial approach as a more efficient experimental method in developing new HEA alloy systems.
In order to study HEA systems with different crystal structures, nominal HEA compositions were selected, including: CoFeMnNiCu in order to achieve face centered cubic (FCC) HEA, OsRuWMoRe to obtain hexagonal closed packed (HCP) and VNbMoTaW in an attempt to form a body centered cubic (BCC) crystal structure. Thin film samples were fabricated by simultaneous magnetron sputtering of the elements onto silicon wafer substrates. The arrangement of the sputtering targets yielded a chemical composition gradient in the films which ultimately resulted in the formation of various phases. Some of these phases exhibited the desired single-phase HEA, albeit with non-equiatomic chemical compositions. Bulk samples of the identified HEA compositions were prepared by arc melting mixtures of the metals. Microstructure of both thin film samples and bulk samples were characterized via scanning electron microscopy (SEM), focused ion beam (FIB) and energy dispersive x-ray spectroscopy (EDX). The crystal structures of the samples were studied by X-ray diffraction (XRD) and electron backscattered diffraction (EBSD) technique. Applying nano-indentation technique, the mechanical properties of some of the samples were screened over the composition gradient as well.
By applying this combinatorial thin film approach, single-phase FCC, HCP and BCC HEAs were detected and successfully produced in bulk form. Additionally, screening the properties of the compositionally gradient thin films, as well as their chemical composition and crystal structure, provided a thorough understanding of the phase space. This experimental approach proved to be more efficient in identifying new alloy systems than conventional exploratory development methods.
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Sun, Xun. "Ab initio Investigation of Al-doped CrMnFeCoNi High-Entropy Alloys." Licentiate thesis, KTH, Materialvetenskap, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-251330.
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High-entropy alloys (HEAs) represent a special group of solid solutions containing five or more principal elements. The new design strategy has attracted extensive attention from the materials science community. The design and development of HEAs with desired properties have become an important subject in materials science and technology. For understanding the basic properties of HEAs, here we investigate the magnetic properties, Curie temperatures, electronic structures, phase stabilities, and elastic properties of paramagnetic (PM) body-centered cubic (bcc) and face-centered cubic (fcc) AlxCrMnFeCoNi (0 ≤ x ≤ 5, in molar fraction) HEAs using the first-principles exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) for dealing with the chemical and magnetic disorder. Whenever possible, we compare the theoretical predictions to the available experimental data in order to verify our methodology. In addition, we make use of the previous theoretical investigations carried out on AlxCrFeCoNi HEAs to reveal and understand the role of Mn in the present HEAs. The theoretical lattice constants are found to increase with increasing x, which is in good agreement with the available experimental data. The magnetic transition temperature for the bcc structure strongly decreases with x, whereas that for the fcc structure shows a weak composition dependence. Within their own stability fields, both structures are predicted to be PM at ambient conditions. Upon Al addition, the crystal structure changes from fcc to bcc with a broad two-phase field region, in line with the observations. Bain path calculations suggest that within the duplex region both phases are dynamically stable. Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli of the present HEAs. The elastic parameters exhibit complex composition dependences, although the predicted lattice constants increase monotonously with Al addition. The elastic anisotropy is unusually high for both phases. The brittle/ductile transitions formulated in terms of Cauchy pressure and Pugh ratio become consistent only when the strong elastic anisotropy is accounted for. The negative Cauchy pressure of CrMnFeCoNi is found to be due to the relatively low bulk modulus and C12 elastic constant, which in turn are consistent with the relatively low cohesive energy. Our findings in combination with the experimental data suggest anomalous metallic character for the present HEAs system. The work and results presented in this thesis give a good background to go further and study the plasticity of AlxCrMnFeCoNi type of HEAs as a function of chemistry and temperature. This is a very challenging task and only a very careful pre-study concerning the phase stability, magnetism and elasticity can provide enough information to turn my plan regarding ab initio description of the thermo-plastic deformation mechanisms in AlxCrMnFeCoNi HEAs into a successful research.
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Kushnerov, O. I. "MD simulation of AlCoCuFeNi high-entropy alloy nanoparticle." Thesis, Sumy State University, 2016. http://essuir.sumdu.edu.ua/handle/123456789/45791.
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High entropy alloys (HEA) are metallic compounds containing from 5 to 13 metallic elements in equiatomic ratios. In HEAs, because of the high mixing entropy, formation of brittle intermetallic phases is usually avoided and simple solid solutions are rather stabilized (BCC and/or FCC). This study used molecular dynamics (MD) package LAMMPS to simulate the AlCoCuFeNi nanoparticle (NP) crystallization.
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Talekar, Anjali S. "Oxidation behavior of Ni-base superalloys and high strength low alloy (HSLA) steels at elevated temperatures." abstract and full text PDF (UNR users only), 2008. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3316371.
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Ayyagari, Venkata A. "Surface Degradation Behavior of Bulk Metallic Glasses and High Entropy Alloys." Thesis, University of North Texas, 2017. https://digital.library.unt.edu/ark:/67531/metadc1062863/.
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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.
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Tian, Fuyang. "Ab initio atomistic simulation of metals and multicomponent alloys." Doctoral thesis, KTH, Tillämpad materialfysik, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-133237.
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Ab initio theory provides a powerful tool to understand and predict the behavior of materials. This thesis contains both of these aspects. First we use ab initio alloy theory to investigate a new kind of complex alloy (high-entropy alloy). Second we introduce a novel potential (interlayer potential), which can be extracted from ab inito total energy calculations using the Chen-Möbius inversion method. High-entropy alloys (HEAs) are composed of four or more metallic elements with nearly equimolar composition. In spite of the large number of components, most of the HEAs have a simple solid-solution phase rather than forming complex intermetallic structures. Extensive experiments have reported the unique microstructures and special properties of HEAs. Single-phase HEAs may be divided into three types, i.e. the 3d-HEAs adopting the face centered cubic (fcc) phase, the refractory-HEAs with a body centered cubic (bcc) phase, and the HEAs with the duplex fcc-bcc structure. We employ the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) to investigate the electronic structure, the equilibrium volume and the elastic properties of these three-type HEAs. First we compare the CPA with the super cell technique (SC) to assess the performance of the EMTO-CPA method. As typical fcc 3d-HEAs, we consider the CuNiCoFeCrTix systems in the paramagnetic state. Starting from the calculated electronic structure, we give an explanation for the observed magnetic states. Furthermore, we provide a theoretical prediction for the elastic parameters and polycrystalline elastic moduli for CuNiCoFeCrTix (x= 0.0−0.5, 1.0) and NiCoTeCrTi. A detailed comparison between the theoretical results and the available experimental data demonstrates that ab initio theory can properly describe the fundamental properties of this important class of engineering alloys. Refractory-HEAs are composed of Ti, Zr, Hf, V, Nb, Ta, Mo, and W. These HEAs have a simple bcc structure. Taking the TiZrNbMoVx and TiZrVNb HEAs as examples, we provide a detailed investigation of the effect of alloying elements on the elastic parameters and the elastic isotropy. Our results indicate that vanadium enhances the anisotropy and ductility of TiZrNbMoVx. As an application of the present theoretical database, we verify the often quoted correlation between the valence charge concentration (VEC) and the micro-mechanical properties in the case of multi-component alloys. Furthermore, we predict that the present HEAs become elastically isotropic for VEC ≃ 4.72. With increase of the aluminum content, phase transformations (fcc→(fcc+bcc)→bcc) occur in NiCoFeCrAlx HEAs. Our ab initio results predict that at room temperature the paramagnetic NiCoFeCrAlx HEAs adopt the fcc structure for x ≤ 0.60 and the bcc structure for x ≥ 1.23, with an fcc-bcc duplex region in between the two pure phases. The calculated single- and polycrystal elastic parameters exhibit strong composition and crystal structure dependence. Based on the present theoretical findings, it is concluded that alloys around the equimolar NiCoFeCrAl composition have superior mechanical performance as compared to the single-phase regions. Many modern materials and material systems are layered. The properties related to layers are connected to interactions between atomic layers. We introduce the interlayer potential (ILP), a novel model potential which fully describes the interaction between layers. The ILPs are different from the usual interatomic potentials which present inter- action between atoms. We use the Chen-Möbius inversion method to extract the ILPs from ab initio total energy calculations. The so obtained ILPs can be employed to investigate several physical parameters connected with the particular set of atomic layers, e.g. surface energy, stacking fault energy, elastic parameters, etc. As an application, we adopt the supercell method and the axial interaction model in connection with the ILPs to calculate the stacking fault energy along the fcc ⟨111⟩ direction, including the intrinsic stacking fault energy, extrinsic stacking fault energy and twin stacking fault energy as well as the interactions between the intrinsic stacking faults. We find that the data derived from ILPs are consistent with those obtained in direct ab initio calculations. Along the fcc ⟨111⟩ direction, we study the surface energy and surface relaxation using the ILPs. The phonon dispersions are also described. Our conclusions are as follows the EMTO-CPAab initioalloy theory can be used to understand and predict the fundamental properties of multicomponent alloys. the interlayer potentials based on the Chen-Möbius inversion method may provide a new way to investigate the properties related to layers in layered materials, the EMTO-CPA alloy theory combined with the Chen-Möbius inversion method offers a powerful technique to study the properties of complex alloys.QC 20131108
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Wang, Anbo. "Life Extension of High Temperature Structural Alloys by Surface Engineering in Gas and Vacuum Carburizing Atmospheres." Digital WPI, 2018. https://digitalcommons.wpi.edu/etd-dissertations/24.
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The heat-treating industry is in need of heat- treatment furnace materials and fixtures that have a long service life and reduced heat capacity. Based on microstructural analysis of components that were used until failure in carburization furnace application, it was found that the primary reason for failure was the excessive carburization that leads to “metal dustingâ€� and subsequent cracking. Aluminizing is widely used to increase the high temperature oxidation and carburization resistance of nickel- based alloys. In this dissertation, RA330, RA602CA, 304L/316L, Inconel 625 alloys were selected to study their performance in an industrial carburization furnace for times up to two years. These alloys were exposed in both the as-fabricated and aluminized condition. The test samples were exposed to Cp=0.7-1.3% carburizing atmosphere at approximately 900℃ for 3 months, 6months, 12months, 18 months and 24months. The oxidation properties and oxide stability at high temperatures will be presented. In addition, the analysis of microstructural development during long term exposure experiments in an industrial carburizing furnace will be presented. These samples were characterized using optical and scanning electron microscope, EBSD, and x-ray diffraction. It was found that the aluminized alloys exhibited lower weight gain and carbon uptakes.
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Yoshida, Shuhei. "Microstructure and mechanical properties of face-centered cubic high/medium entropy alloys：From a viewpoint of heterogeneity on atomic-scale." Doctoral thesis, Kyoto University, 2021. http://hdl.handle.net/2433/263618.
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Sharpe, Heather Joan. "Effect of Microstructure on High-Temperature Mechanical Behavior of Nickel-Base Superalloys for Turbine Disc Applications." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/16255.
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Engineers constantly seek advancements in the performance of aircraft and power generation engines, including, lower costs and emissions, and improved fuel efficiency. Nickel-base superalloys are the material of choice for turbine discs, which experience some of the highest temperatures and stresses in the engine. Engine performance is proportional to operating temperatures. Consequently, the high-temperature capabilities of disc materials limit the performance of gas-turbine engines. Therefore, any improvements to engine performance necessitate improved alloy performance.
In order to take advantage of improvements in high-temperature capabilities through tailoring of alloy microstructure, the overall objectives of this work were to establish relationships between alloy processing and microstructure, and between microstructure and mechanical properties. In addition, the project aimed to demonstrate the applicability of neural network modeling to the field of Ni-base disc alloy development and behavior.
A full program of heat-treatment, microstructural quantification, mechanical testing, and neural network modeling was successfully applied to next generation Ni-base disc alloys. Mechanical testing included hot tensile, hot hardness, creep deformation, creep crack growth, and fatigue crack growth. From this work the mechanisms of processing-structure and structure-property relationships were studied. Further, testing results were used to demonstrate the applicability of machine-learning techniques to the development and optimization of this family of superalloys.
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Tsao, Te Kang, and 曹德綱. "Alloy Design Cast High Entropy Superalloys for High Temperature Applications." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/7277mt.
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博士國立清華大學
材料科學工程學系
105
In this study, high entropy alloys have been developed toward high temperature applications. According to the most widely used high temperature material superalloys, the microstructure of face-centered cubic (FCC) γ matrix with uniformly distributed L12 γ′ precipitates implies the more balanced high temperature strength and ductility. So, the thermal stability and strength of highly alloyed Ni3Al were studied initially. The strengthening effect on developing a γ′ composition toward higher entropy is significant, due to higher anti-phase boundary energy of the order phase. However, the order-disorder transition temperature would be decreased with the more random atomic distribution in γ′ lattice. The microstructure stability of the γ - γ′ alloys with medium to high mixing entropy were then studied. It was found that the high temperature alloys cannot be solely designed by entropy term, but should also enhance the ordering enthalpy of γ′ phase, to avoid lowering the thermal stability of γ′ phase. Through alloy designs, we have also found that present alloys are quite different from the conventional Ni-, Co- or Fe-based alloy design, but is within a range of stable (Ni, Co, Fe)-rich system. This composition space has rarely been studied through the development of superalloys. In addition, such highly-soluted (Ni-Co-Fe) matrix can exhibit an enlarged solubility of alloying contents, while remains good phase stability till high temperatures. Therefore, they have been named as high entropy superalloys (HESA). Since grain boundaries might be drawbacks to the thermal properties, HESAs have been successfully casted into the directionally-solidified (DS) structure by Bridgeman method. In terms of the high temperature mechanical properties, HESAs can exhibit comparable high temperature hardness, tensile strength and creep resistance to that of commercial superalloys due to the stable γ - γ′ microstructure, high volume fraction of γ′ precipitates, high anti-phase boundary energy for γ′ strengthening and low stacking fault energy to hinder dislocation climb. Good surface stability of HESA in high temperature oxidizing and corrosive environments were also demonstrated, which can be attributed to the rapid formation of continuous Chromia or Alumina for surface protection. Furthermore, with less alloying of refractory elements, HESAs exhibit the apparent advantages in lower density and cost of materials. Nevertheless, there are still concerns such as the directional coarsening of γ′ for HESAs cannot contribute to the creep resistance, and the strength of γ matrix is still lower than that of superalloys. As a result, further rooms for composition optimization of HESA exist. To summary, the novel high entropy superalloys are with unique composition, good thermal properties and improved cost-performance, thus can be promising as a new type of high temperature alloy.
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Chien, Feng-Chih, and 簡逢志. "High-Temperature Oxidation Properties of Ni2FeCoCrAlX High entropy Superalloys." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/e5d4gz.
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碩士國立臺灣海洋大學
材料工程研究所
106
The main goal of this thesis is to investigate the high-temperature oxidation of four Ni2FeCoCrAlX high entropy superalloys (HESAs) in various oxygen-containing conditions. In the first part of the study, the oxidation behavior of Ni2FeCoCrAlX high-entropy superalloys (where X = 0, 0.5, 1.0 and 1.5) was studied over the temperature range of 700 ~ 900 oC in dry air. The oxidation kinetics of all the alloys followed the parabolic rate low, indicating that solid-state diffusion is the main controlling step of oxidation. The oxidation rates of all the alloys increased with increasing temperature, but decreased with increasing aluminum content. Based on microstructure and XRD analyses, the scales formed on the Al-free alloy consisted mostly of FeCr2O4 and NiCo2O4, while those of Al-containing alloys were composed of Al2O3. It was found that θ-Al2O3 formed on Al-containing alloys at 700-800 oC, while α-Al2O3 dominated at 900 oC. The formation of Al2O3 is responsible for the lower oxidation rates of Al-containing alloys. Secondly, the oxidation behavior of Ni2FeCoCrAl0.5(N5-0.5Al) was studied at various oxygen pressures from 10-4 to 1 atm at 900 °C. The oxidation kinetics of N5-0.5Al obeys the parabolic law at Po2 ≤ 0.21atm, having its oxidation rate increased with oxygen partial pressure. On the contrast, a mass-loss kinetics was observed at a pure oxygen pressure at 900 °C. The scale formed at Po2 ≤ 0.21atm consisted of an exclusine α-Al2O3 layer, having its thickness increased with oxygen partial pressure, while the oxide layer peeled off at a pure oxygen pressure. The scales formed at Po2 = 1atm were heterophasic, consisted of α-Al2O3, Cr2O3, (Fe,Cr)3O4, and (Ni,Co)3O4. Finally, the oxidation behavior of N5-0.5Al was studied at various ratios of CO/CO2 mixed gases at 700 - 900 °C. In general, the oxidation kinetics of the Ni2FeCoCrAl0.5 superalloy obeyed the parabolic law in all the gas mixture with its oxidation rate increased with increasing temperature and CO2 concentration. The scales formed on N5-0.5Al were also heterophasic, which were composed of intermixed α-Al2O3, Cr2O3, (Fe,Co)3O4, and (Fe,Ni)3O4 on top of the superalloy surface, and no continnous scale-layer was formed in any temperature and CO/CO2 gas mixture.
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Cantor, Brian. "Multicomponent and High Entropy Alloys." 2014. http://hdl.handle.net/10454/9855.
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YesThis paper describes some underlying principles of multicomponent and high entropy alloys, and gives some examples of these materials. Different types of multicomponent alloy and different methods of accessing multicomponent phase space are discussed. The alloys were manufactured by conventional and high speed solidification techniques, and their macroscopic, microscopic and nanoscale structures were studied by optical, X-ray and electron microscope methods. They exhibit a variety of amorphous, quasicrystalline, dendritic and eutectic structures.
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曹德綱. "Nb-containing superconductive high-entropy alloys." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/54866083914106246337.
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碩士國立清華大學
材料科學工程學系
100
This study aims to explore if the high-entropy alloys (HEAs) also possess superconducting behaviors as conventional metals and alloys possess. By doing this, one may be able to understand further the properties of HEAs. Before systematic investigation, we first selected some known Nb-containing alloy and HEAs, such as MoNbTaW, MoNbTaVW, and NbSiTaTiZr, to check the R(T) behavior down to 5 K; and found the superconducting-like behavior of NbSiTaTiZr near that temperature. Therefore, we continued to design and to prepare NbTaTiZr, HfNbTaTiZr, NbGeTaTiZr, NbSiTaTiZrV, NbGeTatiZrV, NbSiTaTiZrGe, and NbSiTaTiZrGeV alloys; and performed the electrical and magnetic properties, and Hall measurements of these ten 4- to 7-component alloys. Alloy samples were vacuum-arc-remelted, cut, and ground, and then they were performed SEM, EDS, XRD, and 5-300 K 4-probe resistivity (T), 2-300 K magnetization M(T), and 5 K- and 300 K-hysteresis M(H) measurements, as well as varied-temperature and -magnetic field Hall measurement and analysis. Microstructure of as-cast MoNbTaW, NbTaTiZr, MoNbTaVW, and HfNbTaTiZr are of single, simple, solid-solution, pseudo-unitary BCC with dendrite-interdendrite substructure. Other 6 as-cast alloys contain a structure with multi-structure and multi-phase in them. Of the most important is that all structures that contain Nb-Ta-riched, pseudo-unitary BCC, solid-solution phase possess superconductivity. Alloys with zero electrical resistance and their corresponding critical temperatures are NbTaTiZr (8.98 K), GeNbTaTiZr (9.16 K), HfNbTaTiZr (7.93 K), NbSiTaTiVZr (4.99 K), GeNbSiTaTiZr (8.10 K), and GeNbTaTiVZr (9.10 K). As to NbSiTaTiZr, it has a drastic drop in resistance near 5 K. Owing to the limitation of measurement that experimental temperature cannot be lowered further; it therefore shows zero-like resistance in the electrical resistance measurements. Single BCC-structured MoNbTaW and MoNbTaVW alloys are composed of all BCC elements, their electrical resistivity ranges 22~40 μΩ-cm, which is lower than that (100~200 μΩ-cm) of other non-all-BCC multi-component alloys measured as before. The 7-component NbSiTaTiZrGeV alloy has the highest resistivity of ~200 μΩ-cm among the ten alloys in this study. The value of residual resistivity ratio, RRR 290K/10K, is in 1.05 ~ 1.36, manifesting that the non-thermal effect, i.e., lattice defect, is greater than the thermal factor. The latter conclusion is in consistent with the one ever made in the similar experiments. From M(T) curves at 1 kOe, alloys that have superconductivity or the like in resistivity measurements show definite diamagnetism. These alloys with their corresponding critical temperatures are NbTaTiZr (7.98 K), NbSiTaTiZr (4.92 K), NbSiTaTiVZr (4.73 K), GeNbTaTiZr (8.61 K), GeNbTaTiVZr (6.34 K), GeNbSiTaTiZr (5.94 K), and HfNbTaTiZr (6.30 K). These M(T) experiments demonstrate the existence of superconductivity in these alloys. On the other hand, the alloys without zero resistivity show no diamagnetic behavior in M(T). Measurements of M(H) hysteresis at 5 K with superconducting alloys show a loop extended in 4 quadrants, demonstrating that these alloys are of the type II superconductors. By M(H), one is able to determine the critical magnetic fields of superconducting alloys. The lower critical magnetic field, Hc1, in alloys and their corresponding values are NbTaTiZr (400 Oe), NbSiTaTiZr (400 Oe), GeNbTaTiZr (300 Oe), GeNbTaTiVZr (300 Oe), GeNbSiTaTiZr (100 Oe), and HfNbTaTiZr (< 100 Oe). The higher critical magnetic field, Hc2, in almost alloys is exceeding 1 T (104 G). Only GeNbSiTaTiZr show small Hc2 (6 kOe). M(H) measurements at 300 K for all ten alloys show superparamagnetism or soft ferromagnetism. Hall measurements at 5 and 300 K and at 1 T to 9 T for these multi-component alloys demonstrate that most of the carriers are of hole-like, with concentration of 1022 cm-3 that is the same value measured as before. The mobility of the alloys is one or two orders of magnitude less than that of the pure metals. As temperature rises, the Hall resistivity increases. The summary in Hall measurements elucidates that there is a large amount of point defects in the pseudo-unitary lattice of the multi-component alloys. Alloy that has the lowest hardness (322 Hv) is NbTaTiZr. The hardness value increases after the individual addition of Hf, Si, and Ge in NbTaTiZr, while the V addition in it decreases the hardness of the alloy. The NbSiTaTiZrGeV alloy has the largest hardness value (760 Hv) that is ascribed to its largest number of multi-phases and a significant effect of the solid solution strengthening.
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Tsai, Kun-Yo, and 蔡坤佑. "On High Entropy Effect and Sluggish Diffusion Effect of High-Entropy Alloys." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/50070343947779605748.
Full textAbstract:
博士國立清華大學
材料科學工程學系
102
High-entropy alloys have four core effects: high entropy, sluggish diffusion, severe lattice distortion, and cocktail. The aim of this study is to demonstrate high entropy and sluggish diffusion effects in a quantitative way. High entropy effect has been found to enhance the formation of solid solutions whereas large atomic size difference and large negative mixing enthalpy between unlike atom pairs have been found to enhance the formation of ordered phases. In order to gain more understanding of such an order-disorder competition, this study proposes two parameters to analyze the correlation between alloy compositions and phase types. One is the modified thermodynamic parameter ε which represents the competition between mixing entropy and mixing enthalpy, while the other is the topological parameter δ which represents the atomic size difference. From the analyses of these two parameters of published high-entropy alloys, the well-defined criteria for the formation of random solid solutions and ordered phases are obtained. When δ is small and ε is large (i.e. the effect of mixing entropy dominates over that of mixing enthalpy), alloys tend to form random solid solutions. On the other hands, when ε < 1.1 (i.e. the effect of mixing enthalpy dominates over that of mixing entropy), alloys tend to form ordered phases. Furthermore, when δ is too high to retain simple structures alloys will form intermetallic phases with more complex structures. These criteria could provide a useful guideline for alloy design of high-entropy alloys. Beside, this study directly confirms the sluggish diffusion phenomenon by the measurement of diffusion parameters for the Co-Cr-Fe-Mn-Ni alloys using a quasi-binary diffusion couple method. Comparing the diffusion parameters of the five component elements measured in the present HEAs with that in the reference FCC metals, it can be found that the diffusion coefficients decrease with the number of constituent elements in the matrix, whereas the normalized activation energies Q/Tm increase with the number of constituent elements. These tendencies are certainly the direct evidences of the sluggish diffusion effect in HEAs. The mechanism behind such effect has also been proposed. The fluctuation of lattice potential energy (LPE) was calculated using quasichemical model. The larger LPE fluctuation in the whole-solute matrix of HEAs provides abundant sites with lower potential energy, which become the traps of atoms and cause higher normalized activation energies and lower diffusion rate.
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Huang, Ming-He, and 黃銘鶴. "Wear Properties of Al0.2Co1.5CrFeNb0.1Ni1.5TiV0.1 High-Entropy Alloys." Thesis, 2009. http://ndltd.ncl.edu.tw/handle/90403162473835838310.
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饒軒安. "Wear Behaviour of High-Entropy Hardfacing Alloys." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/66697752981321659613.
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Wu, Ko-Yuan, and 吳克元. "Superconductivity in NbTaTiZr-based high entropy alloys." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/7p46gp.
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碩士國立清華大學
材料科學工程學系
101
This study aims to explore superconducting HEAs and to find their critical parameters, Tc, Jc, & Hc, and characteristic parameters, ξ, λL, & κ, by experimental analyses and accompanied theoretical calculations, respectively. The materials selected were alloys that were added by Hf, V, Mo, or Fe to pseudo-unitary BCC quaternary NbTaTiZr-based HEAs, and were arc-melted in as-cast equi-molar specimens containing numbers of alloying elements from four to seven. Samples were homogenized at 1100oC for 24 h in a quartz tube filled with argon gas and then quenched in water. Both as-cast and homogenized samples were characterized by XRD, SEM-BEI, EDS, four-point probe resistivity (4.2 K ~ 300 K), magnetization vs. temperature curve (2 K ~ 300 K), magnetic hysteresis loop (2 K, 5 K and 300 K) and Hall (300 K) measurements. NbTaTiZr, NbTaTiZrHf, & NbTaTiZrHfV are single-pseudo-unitary BCC solid solutions in both their as-cast and homogenized states. As-cast and homogenized NbTaTiZrMo alloys are composed of one BCC and one HCP. As-cast NbTaTiZrHfVMo has two BCCs, while the HCP phase will appear after homogenization. One Laves phase, which is composed of small Fe atoms and large Nb, Ta, Ti, and Zr atoms, as well as one BCC phase, occurs in NbTaTiZrFe, because the negative mixing enthalpy of Fe with Nb, Ta, Ti, and Zr are large. Most HEAs in this study show superconductivity in resistivity vs. temperature measurements. Tc falls as number of alloying elements in alloys increases. Superconducting transitions are in temperature intervals, ΔTc, and ΔTc < 1 K. Values of residual resistivity ratio (RRR) range from 1.06 to 1.15, that is to say, the effect of “lattice defects” to electrical conducting is larger than does that of “temperature” in HEAs. For as-cast HEAs, except NbTaTiZrMo, magnetization at 100 Oe vs. temperature curves show diamagnetism. Values of critical temperatures, Tc, range from 3.49 K to 7.97 K. Except that in NbTaTiZrMo and NbTaTiZrHfVMo, Tc of as-cast state in other alloys is higher than their corresponding Tc of homogenization. Some factors that affect the values of Tc in superconducting HEAs are demonstrated. According to their contribution order, they are, successively, (1) Nb + Ta contents in superconducting phase, (2) e/a ratio, (3) lattice distortion, (4) electron density of state, D(εF), and (5) Debye temperature, θD. Because susceptibility of the as-cast superconducting HEAs does not change with temperature, and their magnetic hysteresis loops at 300 K show paramagnetism, accordingly, these HEAs are Pauli paramagnetism at their normal state. From the results of magnetic hysteresis loops at 2 K and 5 K, both as-cast and homogenized alloys are type II superconductors. Values of upper critical fields, Hc2, range from 0.50 T to 2.01 T. For as-cast alloys, Hc2 increases as number of alloying elements increases. Hc2 decreases after homogenization. Values of critical current densities, Jc, range from 0.6 x 104 A/cm2 to 1.5 x 105 A/cm2. As-cast NbTaTiZrHfVMo has the largest Jc, and Jc for alloys decreases after homogenization. The results of Hall measurement at 300 K show carriers of as-cast alloys are “hole-like”. Carrier concentrations are in order of magnitude of 1022 cm-3, which are similar to that of non-superconductig HEAs ever investigated. In this study, characteristic parameters, ξ, λL, & κ, are calculated from the above-mentioned carrier concentrations by relevant theories. Values of κ range from 2.3 to 12.5, which also show that the superconducting HEAs are type II superconductors.
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Hsieh, Wen-Tai, and 謝文泰. "Study of High-Entropy Alloys on Hardfacing Weld." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/5sm8g2.
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碩士國立中山大學
材料科學研究所
95
In recent years, series of high-entropy alloy have been well developed with high hardness and high temperature stability. These properties could apply in hard surface welding technology. The previous research showed that Al0.5CoCrCuFeNi based alloy contained excellent abrasive and adhesive wear resistant properties. According to the results of first year project, the post heat treatment is required for Type A (Al0.3CrFe1.5MnNi0.5 ) and B (Al0.5CrFe1.5MnNi0.5) alloys. It is not suitable for the industrial field service in certain repairing application. This research project will modify the Al0.5CrFe1.5MnNi0.5 base high-entropy alloy in the alloy content of Cr and Ni. These new alloy called Type D high entropy alloys include BCC and FCC two phases. We expect BCC part will provide the wear hardness and FCC part could improve the ductility during the wearing stage. The FCC phase may improve the manufacture of welding rods, also. The research contents include, (1) Type D high entropy alloys weld rod evaluation, (2) wear test, (3) microstructure analysis using electron micro-probe
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HSIAO, JUNG-WEI, and 蕭榮緯. "The Characteristics of the CoCrFeNiTax High-Entropy Alloys." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/h275wy.
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碩士中國文化大學
化學工程與材料工程學系奈米材料碩士班
107
This study investigated the effects of Ta-content on the microstructures, hardness and corrosion properties of CrFeCoNiTax alloys. The results indicated that adding of Ta-content would change the microstructures from single-phase (FCC) of CrFeCoNi alloy to dual-phase (FCC+HCP) of CrFeCoNiTax alloys; also increased the hardness of these CrFeCoNiTax alloys. This study used constant galvanostatic/ potentiometric to measure the polarization curves of CoCrFeNiTa0.1、CoCrFeNiTa0.3、CoCrFeNiTa0.5 alloys in the deaeration 1M sulfuric acid, 1M hydrochloric acid, 1M nitric acid and 1M sodium chloride solutions; the tested temperatures were fixed at 30°C, 40°C, 50°C, 60°C. The results of the CrFeCoNiTax alloys were also compared with commercial 304 stainless steel. The results indicated that the corrosion properties of these CrFeCoNiTax alloys were all better than that of commercial 304 stainless steel.
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Gan, Chai Yan, and 顏在延. "Corrosion properties of low-entropy Fe-Cr-Ni versus high-entropy AlxCrFe1.5MnNi0.5 alloys." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/65373200570336406036.
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碩士國立清華大學
材料科學工程學系
99
This study employs linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and immersion test for weight loss to investigate the corrosion behavior of several anticorrosive alloys, and uses SEM, EDS, XRD, ESCA, and ICP-MS to observe and analyze their microstructure and corrosion morphology, composition of the inert film, and dissolving amount of elements in the immersion solution so as to compare corrosion mechanism of high-entropy AlxCrFe1.5MnNi0.5MoyNbz with that of conventional stainless alloys and to understand the influence of elemental individuality and whole integrity property of alloys on the corrosion behavior. It is seen that in the LSV study, the corrosion current density of both the conventional and the high-entropy in sulfuric acid is greater than in the sodium chloride by about 100 times, whereas the transpassive potential of the two alloys is nearly the same; indicates that for both alloys the corrosion in sulfuric acid is a homogeneous type, while that in sodium chloride is mainly a local pitting. The corrosion resistance of alloys in sulfuric acid is principally influenced by Cr and Ni; the more amount in the alloy, the more corrosion resistance for the alloy. The polarization curves of the alloys in sodium chloride is much unstable (fluctuates) as compared to that in sulfuric acid in that formation of metastable pits competes the recovery of the passive film. The transpassive potential in sodium chloride is much lower than in sulfuric acid; is because of the high pitting ability in the former solution. The more amount the Cr and Mo in the alloys, the higher the transpassive potential of the alloys. EIS analyses show that the growth of the passive film in the high-entropy is slower than in the conventional; is believed to be due to that formation of Al- and Mn-oxides on the surface of the former alloys impedes the growth of a densified passive film on the alloy surface. From the immersion test, it is seen that the corrosion resistance of alloys is obviously affected by Mo because of the rapid formation of passive film by the presence of Mo. It shows from ICP-MS analysis that total dissolution of elements for the Mo-containing alloys, whatever the high-entropy or the low-entropy (the conventional), is 100 times less than that for the Mo-free ones. The Cr dissolution for the Mo-containing alloys is also less than that for the Mo-free ones. Analyses from ESCA data suggest that formation of Cr2O3 and Cr(OH)3, of oxides or complexes of Mo4+ and Mo6+, and of NiO and Ni(OH)2 for Cr, Mo, and Ni, respectively, are principally responsible to anticorrosion for the alloys. The corrosion resistance of the high-entropy alloys is inferior to that of the conventional. Two factors are responsible. One is the property of the element itself in the alloys, i.e., element individuality; the other is the whole property of alloy, i.e., alloy integrationality or cocktail property. Since the high-entropy alloys contain chemically active Al and Mn, and the formed passive film is not dense, hence they are readily corrosive. On the other hand, the stainless steels compose only one or two solid solution(s), whereas the high-entropy alloys, in addition to solid solution(s), have a large amount of corrosive nano-scaled Ni- and Al-rich phase and ρ phase. Therefore, the high-entropy alloys are less corrosion resistant than the conventional. In summary, it is preliminarily concluded that corrosion behavior of materials, especially alloys, depends on the subjective or the intrinsic property of materials themselves and also on the objective or environment conditions such as temperature, pressure, and/or corrosive medium. The corrosion behavior of a material in a specific corrosion environment is a resultant result of the interaction of the material and the environment. The corrosion relation between a material and an environment is a part-and counterpart relationship or a spear-and-shield one; they are of one against the other. There is always no superior or inferior side for each of both sides. It is concluded that the intensity of corrosion resistance for a material or an alloy is always referred to a specific environment only. Based on the electrochemical theory, it is concluded that a single-phase material is more corrosion-resistant than a multi-phase one because of the existence of the local-cell effect for the latter material; the single-phase solid solution that contains specific composition would have superior corrosion property against some specific environment, as was shown in this study. One can thus conclude that the high-entropy effect enlarges the solubility of elements in alloys, enables to obtain a single homogeneous solid-solution phase, and therefore, favors the anticorrosion property. In addition, for good anticorrosion property against a specific environment, this single solid solution must contain suitable composition. The above requirements have to fulfill so that the alloys are really corrosion-resistant. Finally, as to the corrosion resistance issue for high-entropy alloys, one may conclude that the cocktail or collective property from the high-entropy effect is referred to the appearance of a single homogeneous phase, whereas the individuality of each element in the high-entropy alloys appears still in its own anticorrosion property.
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Lei, Yao-Jen, and 雷堯仁. "Study on the Weldability of New High Performance High-Entropy Alloys." Thesis, 2006. http://ndltd.ncl.edu.tw/handle/25170099747236396508.
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Chia-Chin, Li, and 李家瑨. "High Temperature Oxidation Behavior of Quinary FeCoNiCr-based High-entropy Alloys." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/20411929893766315067.
Full textAbstract:
碩士國立臺灣海洋大學
材料工程研究所
104
The main goal of this thesis is to investigate high temperature oxidation of three quinary FeCoNiCr-based high entropy alloys (HEAs), containing FeCoNiCrAl (H5A), FeCoNiCrSi (H5S), and FeCoNiCrMn (H5M) in various tested conditions. First of all, the oxidation behavior of all the HEAs was studied at 700~ 950oC in dry air. In general, the oxidation kinetics of all the alloys followed the parabolic rate law, and the oxidation rate constants (kp values) of the alloys were strongly dependent on alloy composition. It was found that the Mn-additional alloy revealed the fastest oxidation rates over the temperature range of interest, while the kp values of the Si- and Al-additional alloys were nearly identical and much lower with respect to those of H5M. The scales formed on the H5A alloy consisted of α-Al2O3and θ-Al2O3, while Cr2O3 and SiO2 were detected on the H5S alloy. In addition, triplex scales formed on the H5M alloy, consisted of an exclusive outer-layer of Mn2O3 and an intermediate- layer of (Mn,Cr)3O4 and Cr2O3, and an exclusive inner-layer of Cr2O3 at 700~ 800oC. Triplex scales also formed on the same alloy at 900~ 950oC consisted of an exclusive outer-layer of Mn3O4 and the intermediate and inner-layers are similar to those at lower temperatures. The formation of Al2O3 and SiO2 was responsible for the lower oxidation rates of Al- and Si-containing alloys, as compared to those of the H5M alloy. Secondly, the oxidation behavior of H5M was further studied in four oxygen-containing atmospheres over the oxygen partial pressure range from 10-4 to 1atm at 950 oC. The oxidation kinetics of the alloy followed the parabolic rate law, and the kp values increased with increasing oxygen partial pressure. Triplex scales also formed on the alloy regardless of oxygen pressure, having the same scale constitution and phases described in dry air. Finally, the cyclic oxidation behavior of all the HEAs was systematically studied at 700~ 900oC in dry air. The results showed that the cyclic-oxidation kinetics of the HEAs followed by the fast to slow rank of H5M > H5A > H5S. The scales formed on the H5A alloy consisted of α-Al2O3and θ-Al2O3, however a minor amount of FeAl2O4 was observed beneath the spalled alumina scales, while those formed on the H5S and H5M alloys remained unchanged as those in the isothermal oxidation.
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Jao, Yu-Lun, and 饒育綸. "High Temperature Behaviors of High-Entropy Alloys: Estimation of Vacancy Concentration." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/f6e7qn.
Full textAbstract:
碩士國立交通大學
材料科學與工程學系所
106
Equiatomic CoCrFeNi and CoCrFeMnNi high entropy alloy(HEA) is single phase FCC material under room temperature and pressure, also known as the material with highest toughness. This research aims to investigate four-element HEA CoCrFeNi and five-element HEA CoCrFeMnNi equilibrium vacancy consentration by measureing the thermal expansion under high temperature using DIL402. Then study the equilibrium vacancy consentration state between CoCrFeNi and CoCrFeMnNi in order to study the addition of component number to HEA. The result shows the more component number obviously increased the strain deviation between equilibrium and non-equilibrium state at high temperature, the difference between the two expansion could be inferred that the more component number will reduce the energy required for vacancy generation and indirectly affect the higher equilibrium vacancy concentration at high temperatures; at the same time, by comparing the vacancy formation energy and the equilibrium vacancy concentration near the melting point of CoCrFeNi and CoCrFeMnNi high-entropy alloys with pure metal, observe the effect of entropy on vacancy formation energy and equilibrium vacancy concentration.
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WU, CHENG-DE, and 吳正德. "The Calculation of Phase Diagrams in High Entropy Alloys." Thesis, 2005. http://ndltd.ncl.edu.tw/handle/17726727421083515282.
Full textAbstract:
碩士國立臺灣大學
材料科學與工程學研究所
93
High entropy alloy, with excellent properties such as high temperature thermal stability, corrosion resistance, high strength, hardness, high oxidation resistance, has improved material properties compared to traditional alloy. High entropy alloy creates a brand-new field of metal materials and becomes a new potential material for development. In this study, the main focus is based on Cu-Co-Ni-Cr-Al-Fe system alloy with multi-components. Commercial software Thermal Calc. is used to set up the database of this multi-components system alloys and to select an appropriate thermodynamic module for the simulation of phase diagrams. Then, by changing the contents of Al and Fe, phase diagrams of Cu-Co-Ni-Cr-Al-Fe multi-components system alloys are plotted. At the same time, the designed alloy is synthesized by an arc-melting and casting method. These samples were observed under optical microscope and then identified by electron microscope analysis, X-ray analysis, and thermal analysis (high-temperature DSC) to verify the result of the simulation. Dendrite and inter-dendrite structures were observed in these systems of alloys. By energy dispersive spectrometry analysis, Cu enriched inter-dendrite structure IV was obtained. With more addition of aluminum (x=1), the dendrite subsequently transformed into net-like structure due to the decomposition of spinodal. The specimens are heat treated and quenched under a specific temperature determined by thermal analysis measurement and analyzed in order to assess the accuracy of the simulated phase diagram. X-ray analysis results indicated that each element would solute reciprocally in a single BCC or FCC crystal structure.Therefore, these results indicated that the experimental phase diagrams reflected roughly with the calculated phase diagrams.
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Du, Yu-Chin, and 杜宇欽. "Microstructures and Mechanical Properties of Al0.3CoCrFeNiCx High-Entropy Alloys." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/22516645313454856591.
Full textAbstract:
碩士逢甲大學
材料科學所
95
In this study, microstructures and mechanical properties of FCC Al0.3CoCrFeNiCx (x= 0, 0.01, 0.03, 0.05, 0.07, 1) high-entropy alloys in the condition of both as-cast and 600-1300oC heat treatment were investigated. In as-cast Al0.3CoCrFeNi alloy, profuse of homogeneously coherent grains with L12 ordered structure in diameter of 20-30 nm are found in matrix. After 700oC-72hr heat treatment, platelet-like prectipitates with L12 structure form in matrix. After 800oC-72hr heat treatment, 0.5-1.0μm NiAl-rich precipitates with B2 structure form in matrix and they exhibit K-S orientation relationship with FCC matrix. Upper temperature limils of L12 phase and B2 phase formation are 700-800oC and 1000-1100oC, respectively. These two ordered phases will not disappear in alloy with carbon addition. Cr-rich M23C6 and M7C3 eutectic carbides are found in as-cast Al0.3CoCrFeNiC0.03 and Al0.3CoCrFeNiC0.05 alloys, respectively. These eutectic carbides appear under 1300oC and have M7C3→M23C6 phase transform under 1200oC. In addition, M23C6 carbides will precipitate at 700-1200oC in two alloys. Cr-rich M7C3 eutectic carbides are observed in both as-cast Al0.3CoCrFeNiC0.07, 0.1 alloys and formation temperatures are all above 1300oC. Both alloys have M7C3→M23C6 phase transform under 1100oC. In the meantime, 1-4 μm M7C3 carbides precipitate after 1200oC-24hr heat treatment. In present alloys containing carbon, 1-3 μm particle type Cr-rich M23C6 carbides precipitate at 1000-1100oC and sub-micro rod or needle-like Cr-rich M23C6 carbides precipitate at 700-900oC. The mechanism of M23C6 carbides precipitations during heat treatment in that they can form either from soluble carbon or from thw degeneration of M7C3 carbide. The hardness of Al0.3CoCrFeNiCx alloys increases from Hv106 to Hv188 with carbon content increasing. Obviously strengthening effect of alloys after 700-900oC heat treatment can mainly correlated to M23C6 carbides precipitate in matrix. The as-cast Al0.3CoCrFeNiC0.07 alloy after 700oC-72hr heat treatment it is found the decreasing percentage of hardness is lower than both Stellite6 cobalt-base superalloy and M2 high speed tool steel from room temperature to 800oC. Moreover, its hardness is closed to Stellite6 at 600-800oC and has the value of Hv220 at 800oC. After tensile tests in room temperature, elongations of alloys decrease with increasing carbon content that can be attributed to increase the amount of entectic carbides. Elongations of Al0.3CoCrFeNiCx (x= 0, 0.01, 0.03, 0.05, 0.07, 1) alloys are 60%, 50%, 45%, 16%, 14.8% and 10.4%, respectively.Tensile elongations of all alloys are superior to Stellite6 which is 1.3%. As-cast Al0.3CoCrFeNiC0.07, 0.1 alloys after both 700oC-72hr and 900oC-72hr heat treatments, values of hardness, UTS, and YS increase clearly but elongation is decrease. These phenomena come from precipitation of M23C6 carbides in the matrix after heat treatments. Wear resistances of as-cast of Al0.3CoCrFeNiC0.07, 0.1 alloys are 1.26m/mm3 and 1.28 m/mm3 respectively. They are higher than stellite6 which is 1.2 m/mm3. These two as-cast alloys show lower hardness but significantly higher wear resistance. In the study, for as-cast alloys, the strain-hardness exponent n increases form 0.128 to 0.237 with carbon content increasing.
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Tu, Shang-Yi, and 杜尚益. "Study on deformation behaviors of CoCrFeMnNi high entropy alloys." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/w7wq9x.
Full textAbstract:
碩士國立中央大學
化學工程與材料工程學系
103
Equimolar CoCrFeMnNi High Entropy Alloy is a single phase Face-Centered Cubic alloy. In FCC dendrites, Co, Cr, Fe, Mn and Ni these five atoms are randomly distributed. Since these five atoms have different electronegativities and atomic radii, lattice distortion takes place and hence affects its mechanical properties. This study aims at investigating the variation of lattice strain under different conditions by measuring its mechanical properties at different temperature levels and observing the changes of diffraction profiles under different temperatures and with in-situ Neutron Diffraction. Moreover, by observing the surface microstructure under different temperatures with Transmission Electron Microscope (TEM), the relationship between the microstructure and mechanical properties of the material is investigated. From the result of tensile testing, it is known that CoCrFeMnNi High Entropy Alloy has both good strength and ductility at low temperature, even at room temperature, which is suitable for engineering material application [1]. From the result of in-situ Neutron Diffraction, it shows that the behavior of CoCrFeMnNi High Entropy Alloy is different from that of conventional alloys. The elasticity of High Entropy Alloys is usually smaller and the alloy is insensitive to temperature. Due to lattice distortion, the atoms are not at their lattice positions and hence the system experiences a distinct reaction from ordinary metals when stress is applied. Since high temperature thermal vibration cancels off the lattice strain, leading to the low sensation of the elastic modulus towards temperature. From the microscopic result of in-situ Neutron Diffraction, we can see that the elastic modulus of each crystal plane increases with the temperature and has two particular types of trends. Plane (111), (200), (311), (400), etc. are in one group as their elastic modulus decreases linearly when the temperature arises while plane (220), (331), (420) and (222) belong to another group as their elastic modulus first increases when the temperature reaches 200oC and 400oC and then decreases after 600OC. This is due to the lattice distortion of High Entropy Alloys: the higher the temperature elevates, the further the distance between the neutrons will be. As a result, the reduction in lattice distortion causes the elastic modulus to go back to its original position. Last but not least, this study collaborates with two fitting software applications, which are Elastic Visco-Plastic Self-Consistent model fitting (EVPSC) and Convolutional Multiple Whole Profile fitting (CMWP) respectively. EVPSC is used to identify other factors causing the slip mechanism of inner part in CoCrFeMnNi High Entropy Alloy besides the contribution from conventional alloys. The grain size and dislocation density of CoCrFeMnNi High Entropy Alloy can be determined with Convolutional Multiple Whole Profile fitting (CMWP).
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張競謙. "Corrosion and Electrochemical Properties of AlxCrMnFe1.5Niy High Entropy Alloys." Thesis, 2006. http://ndltd.ncl.edu.tw/handle/13832942702295866994.
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Lai, Hsu-Fan, and 賴絮凡. "The High-Entropy Alloys Phase Diagram Simulation of AlxCoCrCuFeNi." Thesis, 2008. http://ndltd.ncl.edu.tw/handle/35738529194762388025.
Full textAbstract:
碩士國立臺灣大學
材料科學與工程學研究所
96
In tradition, the development of an alloy system is almost based on a principal element as the matrix, such as iron-, copper-, and aluminum based alloys, limiting the number of applicable alloy systems. Traditional metallurgical theory suggests that multiple elements in an alloy system may lead to the formation of many compounds with complex microstructure and poor mechanical properties. Recently this paradigm has been broken by high entropy alloy, which has excellent material properties such as high temperature thermal stability, corrosion resistance, high strength, hardness, high oxidation resistance, will be a new potential material for development. In particular, not only were all XRD peak intensities of alloys with multiprincipal elements markedly lower than the corresponding ones of conventional alloys under the same XRD measurement conditions, and these XRD peaks were slightly broadened and superposed on a broad peak. In order to make more extensive applications of high entropy alloy, it is necessary to establish the phase diagrams of these alloys. We use commercial software, Thermal-Calc., and set up database for this system, select an appropriate thermodynamic module for the calculation of phase diagrams. By changing the contents of Al, the phase diagrams of AlxCoCrCuFeNi will be plotted. Experimentally, the designed alloy is synthesized by an arc-melting. These samples were examined with optical microscope, scanning electron microscope analysis, X-ray analysis, and thermal analysis to verify the result of the phase calculation. Dendrite and inter-dendrite structures were observed in these systems of alloys. By energy dispersive spectrometry analysis, Cu enriched inter-dendrite structure was obtained. With more addition of aluminum (x=1), the dendrite subsequently transformed into net-like structure due to the spinodal decomposition. With little aluminum addition, the alloys were composed of a simple fcc solid-solution structure. As the aluminum content reached x= 0.8, a bcc structure appeared and constructed with mixed fcc and bcc eutectic phases. Spinodal decomposition occurred further on when the aluminum contents were higher than x= 1.0. A single ordered bcc structure was obtained for aluminum contents larger than x= 2.8. Utilizing these invariant points as the object, and making further thermal analysis to confirm the phase transition temperature agreed with the phase diagram we simulated.
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Fan, An-Chen, and 范恩誠. "On the phase formation of CoCrFeNiX high entropy alloys." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/99273497616861057019.
Full textAbstract:
碩士國立中興大學
材料科學與工程學系所
103
Despite the vast numbers of reported high entropy alloys (HEAs), our understanding of the composition-phase relationship of HEAs is far from satisfactory. One of the main problems that hinder our progress is that most works study the effect of alloying by adding different elements into different base alloys. Because there can be substantial difference between base alloys, it becomes difficult to understand the effect of the alloying elements themselves. Here, we add ten different alloying elements to the same base alloy CoCrFeNi. The alloying elements are varied systematically from IIIB to VIB elements (X = Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W), leading to ten CoCrFeNiX alloys. Except for the cast ingots, the ten alloys have been processed by four heat treatments conditions. We try to understand that the results in the alloys with different adding elements by analysing the structures of these alloys with different conditions. We realize that existing phase formation theories often fail to explain the behavior of the CoCrFeNiX alloys. Besides, we find that atomic size difference is the key factor to the selection of intermetallic phase type. In this research, we create an idea to predict the possible main intermettallic phase in the CoCrFeNiX cast alloys by checking the binary phase diagrams. In other conditions, we find that most of the ten alloys have no phase transformations during 1100°C Homogenizing and 500°C annealing. However, most of the ten alloys have phase transformations during 900°C and 700°C annealing, and the common phase transformation in thes ten alloys during annealing is the formation of the σ phase.
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Tsai, Pei-Hua, and 蔡霈樺. "On the phases in CoCrFeNiA0.5B0.5 senary high-entropy alloys." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/39405822837612770848.
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Chen, Pin-Chih, and 陳品志. "Study on the Sintering of High Entropy Alloys Powders." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/bhtgt3.
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Chang, Yao-Jen, and 張耀仁. "Development of Wrought Precipitate-strengthened High Entropy Alloys for High Temperature Application." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/p2a78h.
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陳家裕. "Development of multicomponent high-entropy alloys for thermal spray coating." Thesis, 2002. http://ndltd.ncl.edu.tw/handle/47099338451611909570.
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Kao, Yih-Farn, and 高逸帆. "Corrosion of AlxCrFe1.5MnNi0.5Mo0.1 (x = 0.15, 0.3, 0.5) High-Entropy Alloys." Thesis, 2008. http://ndltd.ncl.edu.tw/handle/16542197147717714781.
Full textAbstract:
碩士國立清華大學
材料科學工程學系
96
In this study we use linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV),immersion test, a combined dynamic LSV(t)-OCP(t), OM, SEM, and glancing-angle XRD (GXRD) to investigate the corrosion properties of AlxCrFe1.5MnNi0.5Mo0.1 (x = 0.15, 0.3, 0.5) high-entropy alloys that is similar to 316 type stainless steel. LSV results show that (1) corrosion resistance of AlxCrFe1.5MnNi0.5Mo0.1 in 0.5 M sulfuric acid solution increases with x, which is contrary to that of AlxCrFe1.5MnNi0.5 (x = 0, 0.3,0.5) of Chang, and that of AlxFeCoNiCrMo0.5 (x = 0.5, 1, 1.5, 2) of Wu in 2006, suggesting that the corrosion properties of alloys is influenced by the environmental alloying composition. (2) The corrosion resistance of alloys in 0.5 M sulfuric acid and 0.25 NaCl mixing solution decreases with x, showing that the intrinsic alloy determines the corrosion property in this case of defective passive film. (3) There is irregular influence effect of x on corrosion property for alloys in 1 M NaCl solution. EIS results in 0.5 M sulfuric acid show the double-layer passive film is likely to form and the total electrical resistance increases as time increases, conforming results from SLV. CV tests in 0.5 M sulfuric acid +0.25 M NaCl solution show that positive hysteresis loop enlarges as Al content, manifesting the severe pitting corrosion, and conforming results from OM. There is an obvious deviation of about 100 times for results from conventional immersion test and from LSV, which has long been noticed but not explained in references in the past. By a combined dynamic LSV(t)-OCP(t) proposed by this study, we observe that an increase in corrosion rate due to the dissolution of the passive film after long-term immersion in corrosive solution, and the deviation between LSV and VII immersion results comes from this increase in corrosion rate. In this study we successfully reduce this deviation to a reasonable degree. Compared to the as-cast, each of all the other states, such as the as-solutionized & quenched, the as-furnace-cooled, the 500℃ x 2 h-aged,the 500℃ x 48 h-aged, the 900℃ x 2 h-aged, and the 900℃ x 48 h-aged,has no obvious beneficial or detrimental effects on the corrosion properties of alloys in this study. From G-XRD data we observe the composition of the corrosive film on the surface of the sample is related to the Al content in the alloy. Similar results are obtained for both x = 0.30 and 0.50, but different for 0.15.
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林家旭. "Electrochemical Properties of Fe2AlCoCrNiMo0.5Bx High Entropy Alloys under room Temperature." Thesis, 2005. http://ndltd.ncl.edu.tw/handle/73411412930016943253.
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