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. / <p>QC 20131108</p>

Structure-tribological property relations have been studied for five high entropy alloys (HEAs). Microhardness, room and elevated (100°C and 300°C) temperature sliding friction coefficients and wear rates were determined for five HEAs: Co0.5 Cr Cu0.5 Fe Ni1.5 Al Ti0.4; Co Cr Fe Ni Al0.25 Ti0.75; Ti V Nb Cr Al; Al0.3CoCrFeNi; and Al0.3CuCrFeNi2. Wear surfaces were characterized with scanning electron microscopy and micro-Raman spectroscopy to determine the wear mechanisms and tribochemical phases, respectively. It was determined that the two HEAs Co0.5 Cr Cu0.5 Fe Ni1.5 Al Ti0.4 and Ti V Nb Cr Al exhibit an excellent balance of high hardness, low friction coefficients and wear rates compared to 440C stainless steel, a currently used bearing steel. This was attributed to their more ductile body centered cubic (BCC) solid solution phase along with the formation of tribochemical Cr oxide and Nb oxide phases, respectively, in the wear surfaces. This study provides guidelines for fabricating novel, low-friction, and wear-resistant HEAs for potential use at room and elevated temperatures, which will help reduce energy and material losses in friction and wear applications.

No description available.

High entropy alloys (HEAs) are a relatively new class of solid solution alloys that contain multiple principal elements to take advantage of their high configurational entropy, sluggish diffusion, lattice distortion, and the cocktail effect. In recent development, work hardening mechanisms known as twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) have been found active in Al0.3CoCrFeNi (molar fraction) and Fe50Mn30Co10Cr10 (at %) HEA compositions. Friction-stir processing was done to increase the mechanical properties and improve the microstructure of the alloys for the purpose of high strain rate performance. Quasi-static tensile tests as well as top-hat geometry Split-Hopkinson pressure bar tests were conducted to view the mechanical properties as well as view the microstructural evolution at dynamic strain rates. Overall, the Al0.3CoCrFeNi condition after friction-stir processing and heat treatment has proved to have the best mechanical properties, and selecting from the conditions in this study, Al0.3CoCrFeNi has better shear localization resistance.

In the current study, we have investigated the cyclic loading behavior of conventional as well as novel alloy system exhibiting fine and ultrafine-grained structure. While in case of conventional alloy systems (here aluminum alloy AA5024), the effect of three different grain sizes was investigated. Improvement in fatigue properties was observed with decreasing grain size. The unique microstructure produced via Friction stir processing was responsible for the improved fatigue response. Additionally, microstructures consisting of a high fraction of special boundaries within the fine and ultrafine-grained regime were also subjected to cyclic loading. The hierarchical features introduced in the eutectic high entropy alloy deflected the persistent slip bands, responsible for fatigue cracking, thus resulted in delayed crack initiation and improved fatigue life. The selective nature of fatigue was learnt in the fine grain Al0.5CoCrFeNi, where the introduction of hierarchical features did not result in improved fatigue properties. The weak links in the microstructure, while not affecting the tensile properties, got exposed during cyclic loading. Further study on the medium entropy alloy revealed the inherent reason for the improved fatigue properties. The medium entropy alloys utilized the benefit of UFG single-phase FCC matrix. The UFG matrix showed signs of transformation of FCC phase into the HCP phase during fatigue deformation and hence exhibited improved work-hardening. Alongside atomic scale transformation, stacking faults and nano-twins can also be attributed for obtained cyclic properties.

No description available.

This project presents an attempt to construct a setup and implement a reliable technique for measuring the magnetocaloric effect (MCE) on various materials via a direct method for the acquisition of the data. The main objective of the latter is to produce a ∆Tadiabatic vs T graph over a reasonable temperature span (-100◦C up to 220◦C) by thermal monitoring of a magnetic material exposed to an oscillating magnetic field with a maximum strength of 1.2T. The setup consists of a vacuum-insulated glass tube containing the sample placed between two electromagnets of a vibrating-sample magnetometer (VSM) and increasingly heated by a resistance wire, while the temperature is recorded directly by a thermocouple. The first experiments are performed on Gadolinium (Gd) samples as a reference material in order to verify the overall reliability of the system. The obtained results on Gadolinium show that meaningful data can be acquired with this direct method, although the initially-extracted ∆Tadiabatic near room temperature stands at the accuracy limit (25%) generally accepted with this method. Unexpected interference signals from the thermocouple are encountered for high temperatures and are shown to be due to magnetic dependence from one of its constituents. Data from high temperatures can however be reliably corrected with respect to a baseline signal from a neutral nonmagnetic material. As such magnetocaloric properties of two Manganese-rich high entropy alloys are investigated with one exhibiting at most ∆Tadiabatic = 0.2◦C at its Curie temperature TC = 60◦C. Suggestions regarding the possibility of operating the setup at sub-zero temperatures are put forward and promising results from a new spot- welded thermocouple show a significantenhancement of the initial setup accuracy. / Detta projekt presenterar ett försök att konstruera en installation och implementera en pålitlig teknik för att mäta den magnetokolorisk effekten (MCE) på olika material via en direkt metod för insamling av data. Det sistnämnda syftet är att producera en ∆Tadiabatisk vs T över ett rimligt temperaturintervall (-100◦C up to 220◦C). Detta genom en termisk övervakning av ett magnetiskt material utsatt för ett oscillerande magnetfält med en maximal magnitud på 1.2 T. Utrustningen utgörs av ett vakuumisolerade glasrör som innehåller provet, vilket är placerat mellan två elektromagneter från en vibrating-sample magnetometer (VSM) och som stegvis värms upp av en resistanstråd, medan temperaturen registreras direkt av ett termoelement. De inledande experimenten utförs på prover av Gadolinium (Gd) som referensmaterial för att verifiera systemets totala tillförlitlighet. De erhållna resultaten från Gadolinium proverna visar att meningsfulla data kan produceras med denna direkta metod. Även om de extraherade ∆Tadiabatisk vid rumstemperatur befinner sig inom precisions gränsen (25%), vilken är allmänt accepterad med avseende på den direkta metoden. Oväntade missvisande signaler från termoelementet uppträder vid höga temperaturer och visar sig bero på magnetiskt beroende från instumentet. Data från höga temperaturer kan emellertid pålitligt korrigeras med en baslinjesignal från ett neutralt icke-magnetiskt material. Därmed undersöks de magnetokoloriska egenskaper hos två Mangan-rika hög entropi legeringar, där en uppvisar som högst ∆Tadiabatisk = 0.2◦C vid dess Curie-temperatur TC = 60◦C. Förslag beträffande möjligheten att använda installationen vid temperaturer under noll läggs fram. Lovande resultat från ett nytt punktsvetsat termoelement visar en betydande förbättringav den inledande installationens noggrannhet.

Ziel der Arbeit ist es, die Eignung pulvermetallurgischer Verfahren für die spätere industrielle Nutzung, die Werkstoffeigenschaften sowie das Sinterverhalten für High-Entropy Alloys (HEAs) zu evaluieren. Aufgrund des einphasigen Gefüges ist CoCrFeMnNi eine geeignete Modelllegierung, um die grundlegenden Eigenschaften von HEAs zu analysieren und wird im Rahmen dieser Dissertation exemplarisch für die High-Entropy Alloys verwendet. Verdüstes CoCrFeMnNi-Pulver wird mit drucklosem Sintern und Spark Plasma Sintern unter Verwendung von geeigneten Prozessparametern kompaktiert. Ausgewählte mechanische und physikalische Eigenschaften von CoCrFeMnNi werden bestimmt und im Vergleich mit konventionellen Legierungen bewertet. Die festigkeitssteigernden Mechanismen Kornfeinung und Ausscheidungshärtung werden diskutiert. Durch die Zugabe von Titan zu dem inertgasverdüsten HEA-Pulver wird dafür eine aushärtbare Legierung hergestellt. Zum besseren Verständnis der beim Sintern in HEAs ablaufenden Diffusionsvorgänge wird ein Modell zum Ausheilen isolierter Poren anhand eines vereinfachten Systems für einen binären Mischkristall entwickelt und diskutiert. Des Weiteren wird die Aktivierungsenergie beim Sintern anhand von Schwindungsversuchen berechnet und anschließend mit konventionellen Legierungen verglichen.

No description available.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23157号 / 工博第4801号 / 新制||工||1751(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 乾 晴行, 教授 安田 秀幸 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM