High-density and nanosized deformation twins in face-centered cubic (fcc) materials can effectively improve the combination of strength and ductility. However, the microscopic dislocation mechanisms enabling a high twinnability remain elusive. Twinning usually occurs via continuous nucleation and gliding of twinning partial dislocations on consecutive close-packed atomic planes. Here we unveil a completely different twinning mechanism being active in metastable fcc materials. The transformation-mediated twinning (TMT) is featured by a preceding displacive transformation from the fcc phase to the hexagonal close-packed (hcp) one, followed by a second-step transformation from the hcp phase to the fcc twin. The nucleation of the intermediate hcp phase is driven by the thermodynamic instability and the negative stacking fault energy of the metastable fcc phase. The intermediate hcp structure is characterized by the easy slips of Shockley partial dislocations on the basal planes, which leads to both fcc and fcc twin platelets during deformation, creating more twin boundaries and further enhancing the prosperity of twins. The disclosed fundamental understanding of the complex dislocation mechanism of deformation twinning in metastable alloys paves the road to design novel materials with outstanding mechanical properties.

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. Herein this case, I investigate the basic properties of paramagnetic (PM) HEAs, including the magnetic properties, Curie temperatures, electronic structures, phase stabilities, and elastic properties 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. To understand and model the mechanical properties of face-centered cubic (fcc) HEAs, I also study the generalized stacking fault energy (GSFE), negative stacking fault energy (SFE) and twinning mechanism of various HEAs. Thesis focuses mainly on AlxCrMnFeCoNi (0 ≤ x ≤ 5, in molar fraction) and related HEAs.Whenever possible, I compare the theoretical predictions to the available experimental data in order to verify the employed ab initio methodology. I make use of the previous theoretical investigations carried out on AlxCrFeCoNi HEAs to reveal and understand the role of Mn in AlxCrMnFeCoNi 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 body-centered cubic (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 and the elastic anisotropy is unusually high for both cubic 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 negative SFE of fcc medium-entropy alloys (MEAs) and HEAs originate from the metastable character of the fcc phase. I argue that the common models underlying the experimental measurements of SFE fail in metastable alloys. Considering various metals including concentrated solid solutions, I demonstrate that in contrast to the experimentally measured SFEs, the SFEs obtained by DFT calculations correlate well with the primary deformation mechanisms observed experimentally in these alloys. In the case of negative SFE (or in metastable fcc alloys), the transformation-mediated twinning (TMT) is the predominant mechanism instead of the layer-by-layer twinning mechanism. It provides a continuous avenue for strain accommodation and strain hardening, realizing the joint transformation-induced plasticity and twinning-induced plasticity in the same system, and thus enabling the simultaneous improvement of strength and ductility. For the fcc CrMnFeCoNi HEA, upon Al addition or temperature increase, the intrinsic and extrinsic stacking fault energies increase, whereas the hexagonal close packed (hcp)/fcc interfacial energy stays almost constant.The work and results presented in this thesis give a good background to go further and study the plasticity of fcc 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, elasticity and GSFE can provide enough information to turn my plan regarding ab initio description of the thermo-plastic deformation mechanisms in fcc HEAs into a successful research. The novel TMT mechanism disclosed for the first time by myself and my colleagues advances our knowledge in plasticity and paves the road to design novel alloys with outstanding mechanical properties using quantum metallurgy.

Högentropi-legeringar (HEA) representerar en speciell grupp av fasta lösningar som innehåller fem eller flera huvudelement. Den helt nya designstrategin och de utmärkta egenskaperna hos HEA har lockat stor uppmärksamhet från materialvetenskapssamhället. Formgivning och utveckling av HEA med önskade egenskaper har blivit ett viktigt ämne inom materialvetenskap och teknik. För att förstå de grundläggande egenskaperna hos HEA, undersöker vi de magnetiska egenskaperna, Curietemperaturen, den elektronstrukturen, fasstabiliteten och elastiska egenskaper hos paramagnetiska rymdcentrerat kubisk (bcc) och ytcentrerat kubisk (fcc) AlxCrMnFeCoNi (0 ≤ x ≤ 5) högentropi-legeringar baserat på den första princips EMTO-metoden.De teoretiska gitterkonstanterna ökar med ökande x, vilket är i god överensstämmelse med experimentella data. Den magnetiska övergångstemperaturen för bcc-strukturen minskar kraftigt med x, medan för fcc-strukturen uppvisar den övergångstemperaturen svag kompositionberoende. Inom sina egna stabilitetsfält förutsägas båda strukturerna vara paramagnetiska under normala omgivningsförhållanden. Vid tillägg av Al ändras kristallstrukturen från fcc till bcc med en bred tvåfasfältregion, vilket är i linje med observationer. Bain-väg beräkningar stöder att båda faserna är dynamiskt stabila inom den duplexregionen.Jämförelse med tillgängliga experimentella data visar att den här använda metoden beskriver den elastiska modulen med god noggrannhet. De elastiska parametrarna uppvisar komplexa kompositionberoende, även om de förutsagda gitterkonstanterna ökar monotont med Al tillsats. Den elastiska anisotropin är ovanligt hög för båda faser. De spröda / duktila övergångarna formulerade i form av Cauchy tryck och Pugh förhållandet blir konsekventa endast när man tar den starka elastiska anisotropin med i beräkningen. Det negativa Cauchy trycket hos CrMnFeCoNi beror på de relativt låga bulkmodulen och C12 elastiska konstanten, som i sin tur är konsekventa med den relativt låga kohesionenergin. Denna resultaten i kombination med experimentella data antyder anomalös metallisk karaktär för HEA system.För att förstå och modellera de mekaniska egenskaperna hos fcc HEA studerar jag också generaliserad staplingsfelenergi (GSFE), negativ staplingsfelenergi (SFE) och tvillingmekanism för olika HEA. Denna avhandling fokuserar huvudsakligen på AlxCrMnFeCoNi och relaterade HEA.

Using first-principles methods, we investigate the effect of Al on the generalized stacking fault energy of face-centered cubic (fcc) CrMnFeCoNi high-entropy alloy as a function of temperature. Upon Al addition or temperature increase, the intrinsic and extrinsic stacking fault energies increase, whereas the unstable stacking fault and unstable twinning fault energies decrease monotonously. The thermodynamic expression for the intrinsic stacking fault energy in combination with the theoretical Gibbs energy difference between the hexagonal close packed (hcp) and fcc lattices allows one to determine the so-called hcp-fcc interfacial energy. The results show that the interfacial energy is small and only weakly dependent on temperature and Al content. Two parameters are adopted to measure the nano-twinning ability of the present high-entropy alloys (HEAs). Both measures indicate that the twinability decreases with increasing temperature or Al content. The present study provides systematic theoretical plasticity parameters for modeling and designing high entropy alloys with specific mechanical properties.

With the increase of time, the shrinkage of materials at fixed temperature could enhance the failure of fasteners. We report a potential way to alter the volume/length of alloy automatically through isothermal aging due to pseudospinodal decomposition mechanism. The volume of Ti-10V-2Fe-3Al alloy first shrunk and then expanded during isothermal aging at 550 degrees C. It can fit tightly and make up for volume loss. Transmission electron microscopy observation exhibits no obvious coarsening of intragranular alpha phase with the increasing time. However, composition evolution with time shows a gradual change through energy dispersive spectrometer analysis. The result shows that beta stabilizers, V and Fe, are prone to diffuse to the beta matrix, while alpha stabilizers, Al, prefer to segregate to the alpha phase. First principle calculations suggest that the structure transition for beta to alpha cause the first decrease of volume, and the diffusion of V, Fe and Al is the origin of the later abnormal increase of volume.

The stacking fault energy (SFE) is often used as a key parameter to predict and describe the mechanical behaviour of face centered cubic material. The SFE determines the width of the partial dislocation ribbon, and shows strong correlation with the leading plastic deformation modes. Based on the SFE, one can estimate the critical twinning stress of the system as well. The SFE mainly depends on the composition of the system, but temperature can also play an important role. In this work, using first principles calculations, electron backscatter diffraction and tensile tests, we show a correlation between the temperature dependent critical twinning stress and the developing microstructure in a typical austenitic stainless steel (316L) during plastic deformation. We also show that the deformation twins contribute to the strain hardening rate and gradually disappear with increasing temperature. We conclude that, for a given grain size there is a critical temperature above which the critical twinning stress cannot be reached by normal tensile deformation, and the disappearance of the deformation twinning leads to lower strain hardening rate and decreased ductility.

We report the existence of secondary hardening behavior in Ti-10V-2Fe-3Al (wt.%) (Ti-1023) for the first time. Through controlling the ageing temperature window between 550 degrees C and 575 degrees C, alloys are found to show the existence of two hardness peaks with aging time. This heat treatment with secondary hardening phenomenon exhibits unusual increase of hardness and strength. Further experimental observations show that the first hardness peak corresponds with the well precipitated alpha phase at very short time. Further increase of aging time does not influence the morphology of these intragranular alpha precipitates, but accompanying with a continuous composition change, which could be the origin of the secondary hardness peak. This structure change followed by composition diffusion is the typical characteristic of pseudospinodal mechanism. Phase field and ab-initio calculations based on pseudospinodal mechanism analyze the detailed microstructure/composition evolution and related solid solution strength, which confirm the role of composition on the appearance of secondary hardening behavior.

Using ab initio calculations, we investigate the elastic properties of paramagnetic AlxCrMnFeCoNi (0 <= x <= 5) high -entropy alloys (HEAs) in both body-centered cubic (bcc) and face-centered cubic (fcc) structures. Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli. The predicted lattice constants increase monotonously with Al addition, whereas the elastic parameters exhibit complex composition dependences. 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 due to the relatively low bulk modulus and C-12 elastic constant, which in turn are consistent with the relatively low cohesive energy. The present findings in combination with the experimental data suggest anomalous metallic character for the HEAs system.