Alloying aluminum offers the possibility of creating low-density high-entropy alloys (HEAs). Several studies that focus on the system AlCoCrFeNiTi differ in their phase determination. The effect of aluminum on the phase composition and microstructure of the compositionally complex alloy (CCA) system AlxCoCrFeNiTi was studied with variation in aluminum content (molar ratios x = 0.2, 0.8, and 1.5). The chemical composition and elemental segregation was measured for the different domains in the microstructure. The crystal structure was determined using X-ray diffraction (XRD) analysis. To identify the spatial distribution of the phases found with XRD, phase mapping with associated orientation distribution was performed using electron backscatter diffraction. This made it possible to correlate the chemical and structural conditions of the phases. The phase formation strongly depends on the aluminum content. Two different body-centered cubic (bcc) phases were found. Texture analysis proved the presence of a face-centered cubic (fcc) phase for all aluminum amounts. The hard η-(Ni, Co)3Ti phase in the x = 0.2 alloy was detected via metallographic investigation and confirmed via electron backscatter diffraction. Additionally, a centered cluster (cc) with the A12 structure type was detected in the x = 0.2 and 0.8 alloys. The correlation of structural and chemical properties as well as microstructure formation contribute to a better understanding of the alloying effects concerning the aluminum content in CCAs. Especially in the context of current developments in lightweight high-entropy alloys (HEAs), the presented results provide an approach to the development of new alloy systems.

The investigation of high-entropy alloys (HEAs) has revealed many promising properties. HEAs with a high share of Al and Ti are suitable for the formation of lightweight materials. Investigations of the alloy system AlCoCrFeNiTi showed high strength, hardness, ductility, and wear resistance, which makes this special alloy interesting for surface engineering and particularly for thermal spray technology. In this study, the suitability of inert gas-atomised HEA powder for high-velocity-oxygen-fuel (HVOF) thermal spray is investigated. This process allows for high particle velocities and comparatively low process temperatures, resulting in dense coatings with a low oxidation. The microstructure and phase composition of the atomised powder and the HVOF coating were investigated, as well as the wear behaviour under various conditions. A multiphase microstructure was revealed for the powder and coating, whereas a chemically ordered bcc phase occurred as the main phase. The thermal spray process resulted in a slightly changed lattice parameter of the main phase and an additional phase. In comparison with a hard chrome-plated sample, an increase in wear resistance was achieved. Furthermore, no brittle behaviour occurred under abrasive load in the scratch test. The investigation of wear tracks showed only minor cracking and spallation under maximum load.

In this study, the wear behaviour of a powder metallurgically produced AlCoCrFeNiTi0.5 high-entropy alloy (HEAs) is investigated at elevated temperatures. Spark plasma sintering (SPS) of inert gas atomised feedstock enables the production of dense bulk material. The microstructure evolution and phase formation are analysed. The high cooling rate in the atomisation process results in spherical powder with a microstructure comprising two finely distributed body-centred cubic phases. An additional phase with a complex crystal structure precipitates during SPS processing, while no coarsening of microstructural features occurs. The wear resistance under reciprocating wear conditions increases at elevated temperatures due to the formation of a protective oxide layer under atmospherical conditions. Additionally, the coefficient of friction (COF) slightly decreases with increasing temperature. SPS processing is suitable for the production of HEA bulk material. An increase in the wear resistance at elevated temperature enables high temperature applications of the HEA system AlCoCrFeNiTi0.5.

Hochentropielegierungen stellen einen neuen Entwicklungsansatz metallischer Werkstoffe ohne ein eigenschaftsbestimmendes Hauptelement dar. Die zielgerichtete Übertragung der bisher überwiegend an Massivwerkstoffen ermittelten Eigenschaften in die Beschichtungstechnik erfordert die Kenntnis der bestimmenden Einflussfaktoren. Für die Schichtherstellung werden die Verfahren des thermischen Spritzens betrachtet. Hierfür wird eine geeignete Prozessroute ermittelt. Die detaillierten Untersuchungen zu den Prozess-Struktur-Eigenschaftsbeziehungen erfolgen an Legierungen mit variabler Struktur. Diese werden anhand von thermodynamischen Parametern sowie Untersuchungen an schmelzmetallurgisch hergestellten Massivwerkstoffen ausgewählt. Zur Bewertung des Einflusses der Größe der Strukturmerkmale, der Heterogenität und möglicher Ungleichgewichtszustände werden schmelz- und pulvermetallurgisch hergestellte Massivwerkstoffe als Referenz betrachtet. Die geplanten Forschungsarbeiten tragen zu einem Verständnis der Prozess-Struktur-Eigenschaftsbeziehung von Hochentropielegierungen bei. Weiterhin wird eine geeignete Prozessroute für die pulvermetallurgische Verarbeitung sowie für Anwendungen in der Oberflächentechnik ermittelt.

Les alliages à haute entropie (HEAs) sont des alliages métalliques composés de 5 éléments ou plus, présents en proportions equimolaire ou presque. Depuis leur apparition dans le domaine de la métallurgie, leurs propriétés intéressantes ont causé un intérêt croissant de la part de la communauté scientifique pour essayer de les comprendre et les prédire. Plusieurs de ces propriétés peuvent aussi être observées dans d'autres systèmes cristallins ayant moins d'éléments, comme les solutions solides concentrées (CSAs) composées de FeNiCr. Ce mémoire présente les effets du calcul des préfacteurs par l'approximation harmonique de la théorie de l'état de transition (hTST) sur la diffusion d'une lacune dans une CSA en FeNiCr modélisée par un algorithme Monte-Carlo cinétique (KMC) hors réseau. Ce travail est motivé par les nombreux débats qui entourent la diffusion lente dans les HEAs et autres CSAs hautement désordonnés. Bien que cette caractéristique ait été proposée et utilisée pour expliquer certaines des propriétés les plus intéressantes des HEAs, les mécanismes de diffusion dans ceux-ci sont encore mal compris. Dans des travaux précédents, il a été démontré que les préfacteurs hTST dans une CSA FeNiCr peuvent avoir des valeurs qui s'étalent sur plusieurs ordres de grandeur. En partant de ces résultats, l'influence de cette variation de préfacteurs sur la diffusion d'une lacune est étudiée. Grâce à une analyse comparative entre des simulations utilisant des préfacteurs hTST et constants, le rôle de l'entropie dans la diffusion est étudié. Plus de un millions d'évènements au total sont trouvés dans les 96 simulations effectuées dans chaque type de simulation, fournissant une base statistique solide. Ces simulations KMC ont été performées par l'algorithme d'activation-relaxation cinétique (kART) couplé au potentiel Bonny-2013 pour les calculs de surface d'énergie potentielle (PEL). Nous démontrons que, en plus de l'entropie configurationnelle, le désordre affecte aussi l'entropie vibrationnelle, et que ce phénomène peut être à la base de diverses propriétés de ces systèmes, y compris leur diffusion lente. Les résultats présentés ne peuvent être obtenus que grâce à une analyse cinétique du système. En effet, la dynamique obtenue ne peut pas être extraite directement de l'évaluation statique du PEL, car la corrélation de sélection des événements est construite à partir des contributions combinées du préfacteur et des barrières énergétiques. Cette combinaison affecte la loi de compensation qui est mesurée, selon si le calcul de cette loi est effectué avec les évènements qui sont disponibles ou avec les évènements sélectionnés. Une introduction, ainsi que deux chapitres sur les HEAs et sur la théorie de l'état de transition débutent ce travail, suivis de la méthodologie, présentée au chapitre 4, et de l'article au chapitre 5. / High entropy alloys (HEAs) are metallic alloys composed of 5 or more elements, present in equimolar or near equimolar proportions. Since their appearance in the field of metallurgy at the beginning of the XXIst century, their properties have caused a growing interest from the scientific community in order to understand and predict certain of these properties. Many of them can also be observed in other crystalline systems with fewer elements, such as concentrated solid solution (CSAs) composed of FeNiCr. This masters' thesis presents the effect that the computation of prefactors by the harmonic approximation of the transition state theory (hTST) has on the diffusion of a single vacancy in a FeNiCr CSA, simulated by a kinetic Monte Carlo algorithm (KMC). The debate around a sluggish defect diffusion, proposed as a core effect of HEAs and CSAs with high amounts of disorder motivates this research. Indeed, even though this characteristic is often used to explain some of the most interesting properties of HEAs, the diffusion mechanisms are still not fully comprehended. In a previous study, it was demonstrated that hTST prefactors span over several orders of magnitude. Based on these results, we study the impact of hTST on diffusion. Through a comparative analysis between simulations using hTST and constant prefactors, the role of entropy on diffusion is studied. More than one million events in total are found in the 96 simulations performed for each type of simulation, providing a solid statistical basis for this analysis. These KMC simulations were performed by the kinetic activation-relaxation technique (kART) coupled with the Bonny-2013 potential for potential energy landscape (PEL) calculations. We demonstrate that both disorder and configurational entropy strongly affect the vibrational entropy, and that this can be responsible for various properties of these systems, including their sluggish diffusion. Presented results can only be obtained by a kinetic study of the system. The kinetic patterns that are observed can not be obtained by only the static analysis of the PEL for the combination of both prefactors and energy barriers affects event selection. This selection of events can change the compensation law that is measured whether it is computed using available events or selected events. Two chapters on HEAs and transition state theory, as well as a chapter on the methodology are presented before these results.

<p dir="ltr">Advancements in material science are accelerating technological evolution, driven by initiatives like the Materials Genome Project, which integrates computational and experi- mental strategies to expedite material discovery. In this work, we focus on the reliability of advanced materials under extreme conditions, a critical area for enhancing their technological applications.</p><p dir="ltr">Multi-principal component alloys (MPEAs) exhibit remarkable properties under extreme conditions. However, their vast compositional space makes a brute-force exploration of potential alloys prohibitive. We address this challenge by employing a Bayesian approach to explore the oxidation resistance of hundreds of alloys, applying computational techniques to accurately calculate and quantify errors in the melting temperatures of MPEAs, and investigating the compositional biases and short-range order in their nucleation behaviors.</p><p dir="ltr">Furthermore, we scrutinize the role of wide bandgap semiconductors, which are essential in high-power applications due to their superior breakdown voltage, drift velocity, and sheet charge density. The lack of lattice-matched substrates often results in strained films, which enhances piezoelectric effects crucial for device reliability. Our research advances the pre- diction of piezoelectric and dielectric responses as influenced by biaxial strain and doping in gallium nitride (GaN). Additionally, we delve into how various common defects affect the formation of trap states, significantly impacting the electronic properties of these materials. These studies offer significant advancements in understanding MPEAs and wide bandgap semiconductors under extreme conditions. We also provide foundational insights for developing robust and efficient materials essential for next-generation applications.</p>

High hardness and good wear resistance have been revealed for the high-entropy alloy (HEA) system AlCoCrFeNiTi, confirming the potential for surface protection applications. Detailed studies to investigate the microstructure and phase formation have been carried out using different production routes. Powder metallurgical technologies allow for much higher flexibility in the customisation of materials compared to casting processes. Particularly, spark plasma sintering (SPS) enables the fast processing of the feedstock, the suppression of grain coarsening and the production of samples with a low porosity. Furthermore, solid lubricants can be incorporated for the improvement of wear resistance and the reduction of the coefficient of friction (COF). This study focuses on the production of AlCoCrFeNiTi composites comprising solid lubricants. Bulk materials with a MoS2 content of up to 15 wt % were produced. The wear resistance and COF were investigated in detail under sliding wear conditions in ball-on-disk tests at room temperature and elevated temperature. At least 10 wt % of MoS2 was required to improve the wear behaviour in both test conditions. Furthermore, the effects of the production route and the content of solid lubricant on microstructure formation and phase composition were investigated. Two major body-centred cubic (bcc) phases were detected in accordance with the feedstock. The formation of additional phases indicated the decomposition of MoS2.

Yes / This 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.

<p dir="ltr">Solid-polymer electrolytes constitute an attractive alternative to flammable liquid electrolytes, but their low ionic conductivity (σ) and transference number (<i>t</i>₊) are not sufficient to replace current liquid electrolytes. In turn, the rational design of new materials is of ultimate importance to overcome the main limitation to high ionic conductivity: the close relationship between ion transport and polymer segmental relaxation. On one hand, the strategy to overcome such issue is designing new composite polymer electrolytes (CPEs) where ceramic particles can modify the properties of the polymer host by increasing the amorphous fraction, enhance the dissociation of salts, hinder the diffusion of anions, and/or create new Li⁺ conduction pathways at the interface ceramic/polymer. One of the main obstacles to achieving higher performance is the limited understanding of transport mechanism and the effect of ceramic filler on the physical properties, ion transport, and interactions with the CPE constituent materials. The dielectric properties of polymers play a critical role in the ability of the polymer to dissolve salts and mediate the electrostatic interactions between the cations and the polymer chain. To further study the effect of the CPE dielectric constant and its impact on ionic conductivity, in this thesis the effect on incorporating High Entropy Oxides (HEO) that possess colossal permittivity into PEO/LiTFSI matrixes is reported. The results show that particles of 700 nm average diameter yield ionic conductivities > 10^‑4 S cm^‑1. Measurements of the complex dielectric function reveal an increase in the rate of relaxation of the ion-coupled chain dynamics. This is in line with the reduced Tg observed in DSC analysis. DSC also reveals no significant change in the degree of crystallinity and results based on FTIR do not indicate a significant dissociation of Li-salt compared to the PEO-based SPE. Finally, the addition of these high dielectric constant fillers of smaller size produces a radical change in the polymer microstructure because of their integration with the polymer matrix. In summary, these results suggest that the improvement in IC is likely due to the formation of efficient Li-pathways involving fast-moving amorphous polymer. Further studies are needed to determine the effect of the HEO fillers on the bonding interactions between the Li cations and the oxygen groups of the polymer. An additional strategy to overcome limitations in ion-transport of polymer electrolytes was pursued in this work through the design of new polymer structures. Single ion-conducting polymer electrolytes (SICPEs) can restrict the diffusion of anions which is responsible for the development of polarization gradients in rechargeable batteries, under high charge/discharge conditions. The design of poly (Li-FAST-<i>alt</i>-DEG) is intended to regulate other aspects such as Li⁺ concentration and free volume of the polymer. Whereas the synthesis of oligomers was successfully accomplished, challenges in the synthetic process hindered the fabrication of the polymer.</p>

<p>Co and Ni are two major elements in high temperature structural alloys that include superalloys for turbine engines and hard metals for cutting tools. The recent development of complex concentrated alloys (CCAs), loosely defined as alloys without a single principal element (e.g. CoNiFeMn), offers additional opportunities in designing new alloys through extensive composition and structure modifications. Within CCAs and Co- and Ni-based superalloys, solid-solution strengthening and stacking fault energy engineering are two of the most important strengthening mechanisms. While studied for decades, the potency and quantitative materials properties of these mechanisms remain elusive. </p> <p><br></p> <p>Solid-solution strengthening originates from stress field interactions between dislocations and solute of various species in the alloy. These stress fields can be engineered by composition modification in CCAs, and therefore a wide range of alloys with promising mechanical strength may be designed. This thesis initially reports on experimental and computational validation of newly developed theories for solid-solution strengthening in 3d transition metal (MnFeCoNi) alloys. The strengthening effects of Al, Ti, V, Cr, Cu and Mo as alloying elements are quantified by coupling the Labusch-type strengthening model and experimental measurements. With large atomic misfits with the base alloy, Al, Ti, Mo, and Cr present strong strengthening effects comparable to other Cantor alloys. </p> <p> </p> <p>Stacking fault energy engineering can enable novel deformation mechanisms and exceptional strength in face-centered cubic (FCC) materials such as austenitic TRIP/TWIP steels and CoNi-based superalloys exhibiting local phase transformation strengthening via Suzuki segregation. We employed first-principles calculations to investigate the Suzuki segregation and stacking fault energy of the FCC Co-Ni binary alloys at finite temperatures and concentrations. We quantitatively predicted the Co segregation in the innermost plane of the intrinsic stacking fault (ISF). We further quantified the decrease of stacking fault energy due to segregation.  </p> <p><br></p> <p>We further investigated the driving force of segregation and the origin of the segregation behaviors of 3d, 4d and 5d elements in the Co- and Ni-alloys. Using first-principles calculations, we calculated the ground-state solute-ISF interaction energies and revealed the trends across the periodic table. We discussed the relationships between the interaction energies and the local lattice distortions, charge density redistribution, density of states and local magnetization of the solutes. </p> <p><br></p> <p>Finally, this thesis reports on new methodologies to accelerate first-principles calculations utilizing active learning techniques, such as Bayesian optimization, to efficiently search for the ground-state energy line of the system with limited computational resources. Based on the expected improvement method, new acquisition strategies were developed and will be compared and presented. </p>