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The incorporation of cubic β/β0 phase improves significantly the ductility and workability of TiAl-based alloys. However, the ductility deteriorates with the precipitation of the ordered hexagonal ω0 phase in the β/β0 phase. To avoid the formation of the ω0 phase in alloy design, fundamentals about the β/β0 to ω0 phase transition and its alloying element dependence have to be known. In the present work, we investigated the phase transition and phase stability in Ti4Al3Nb and Ti4Al3Mo alloys using a first-principles method based on density functional theory. Our calculations indicate that for the β0 to ω0 transition, the "collapse-diffusion" pathway is easier than the "diffusion-collapse" one; however, both pathways are quite difficult due to the high-energy barriers. The pa to co" transition occurs through the "diffusion-collapse" pathway without an energy barrier (except for the activation energy for the short-range diffusion needed for atomic rearrangement). Comparing the total energies of different phases, we find that the ω0 phase of Ti4Al3Nb is more stable than β0 and vice versa for Ti4Al3Mo whereas the relative stability of ω" to β0 for Ti4Al3Nb is stronger than that for Ti4Al3Mo, indicating that the addition of Mo inhibits the transition from the β0- to the ω-related phases in Ti4Al3Nb. Our calculations demonstrate that ω" is more stable than ω0 for both Ti4Al3Nb and Ti4Al3Mo, in contrast to the previously proposed picture based on some experimental observations, the possible origins of which are discussed in detail. [ABSTRACT FROM AUTHOR]

Structure searches for new high-pressure phases of Y metal have been performed by using evolutionary algorithms in conjunction with a first-principles, pseudopotential plane-wave method based on density functional theory. The oFl6-Fddd and hP3-P3\2\ phases are predicted to be energetically favorable at pressures over 97 GPa. These two phases are shown to be dynamically stable by computing their phonon dispersions. We thus propose that oF\6-Fddd and hP3-P3]2l are the most probable crystal structures Y may take in the 97-206 GPa range. The superconducting critical temperatures (Tc) of the new phases are estimated using the Allen-Dynes formula. The Tc is predicted to decrease with increasing pressure over about 100 GPa, in sharp contrast to its observed monotonie increase under lower pressure. The electronic origins of the stabilities of the proposed high-pressure phases have also been investigated. [ABSTRACT FROM AUTHOR]

The structural phase transitions of yttrium under high pressure up to 300 GPa have been studied by using the first-principles, pseudopotential plane-wave method based on density functional theory. The experimental rare-earth structure sequence with increasing pressure is successfully reproduced. The yet unknown distorted lace-centered cubic (dfcc) phase is determined to be of the R3̄m space group. In addition, the P6222 phase is predicted to be energetically more stable beyond the pressure range for dfcc. Accurate phonon-dispersion calculations demonstrate that the P6222 phase is dynamically stable beyond pressure of about 206 GPa. The corresponding projected band structures, density of states, and charge density differences are given in order to clarify the electronic origins of the stability of the P6222 phase under pressure. [ABSTRACT FROM AUTHOR]

Abstract: The Ising-like phase transition of two-dimensional fully frustrated XY (FFXY) model with time dependent Ginzburg–Landau (TDGL) dynamics is studied by using the short-time dynamics scaling analysis. The transition temperature and the dynamic and static critical exponents z, , and ν are estimated and are compared with the results of FFXY model under resistively-shunted junction dynamics (RSJ) and Monte Carlo (MC) dynamics. We find , z, and ν are roughly independent of dynamic mode but is dependent on the dynamic mode. [Copyright &y& Elsevier]

Abstract: The depinning phase transition of the Mullins–Herring equation with an external driving force and quenched random noise is studied in a short-time dynamic scaling scheme. Besides the critical driving force, all the critical exponents can be accessed, agreeing well with those in long-time steady-state simulations. The finite size effects on the critical exponents are also discussed. It is found that reasonable results can be achieved with a relatively small system, which highlights the advantage of the present approach. [Copyright &y& Elsevier]

The curve of the hardness of ternary nitride Ti 1- x Al x N against the Al content x is convex shaped with a maximum at x ∼0.6, of which the mechanism remains to be further explored. The present work scrutinizes the phase stability and elastic modulus of Ti 1- x Al x N against the Al content x by using a first principles method as they are highly pertinent to the hardness of the system. It is shown that the cubic B1 structure is more stable than the hexagonal structure (Bk) for x < ∼0.75 whereas the hexagonal structure (B4) is more stable than the cubic B1 structure for x > ∼0.75. The hardness of both B1- and B4–Ti 1- x Al x N evaluated with the calculated elastic modulus become higher with increasing x. B1–Ti 1- x Al x N is harder than B4–Ti 1- x Al x N. Besides the composition dependent hardness of the individual B1- and B4–Ti 1- x Al x N, the phase decomposition contributes to the convex shaped hardness of Ti 1- x Al x N against x. The work presents a clear picture on how the chemical composition and phase transition influence jointly on the hardness of Ti 1- x Al x N. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The interface energies and GSFE of β / ω interface in Ti alloy are calculated. • The strong anisotropy of the interface energy contributes to the ellipsoid-like morphology of β particles, with its major axis parallel to 〈1 1 1〉 β direction. • The β / ω interface is prone to crack initiation and propagation as compared to the bulk and contributes to the ω -embrittlement of β -Ti alloys. Abstract ω phase precipitating in a β -Ti alloy is generally detrimental to the ductility and plasticity of the alloy.To understand the influence of ω phase on the mechanical properties of β -Ti alloys, the property of the β / ω interface is demanded.In this regard, we calculate the interface energies ( γ int ), cleavage energies ( γ cl ), and generalized stacking fault energies (γ) of the β / ω interfaces with different orientations using a first-principles method.Our calculations predict interface energies of 0.32 J m−2 and 0.03 J m−2 for the (1 1 1 ¯) β / (0 0 0 1) ω and (1 1 2) β / (1 0 1 ¯ 0) ω interfaces, respectively.The strong anisotropy of the interface energy contributes to the ellipsoid-like morphology of ω phase with its major axis parallel to the 〈 1 1 1 〉 β direction.The influence of the interface on the ductility of the alloy is discussed with the calculated cleavage energies and the unstable stacking fault energies (γ us) extracted from the generalized stacking fault energy (GSFE). The γ cl / γ us ratio of the (1 1 2) β / (1 0 1 ¯ 0) ω interface is lower than those of the main slip planes of the bulk phases, indicating that this interface is prone to crack initiation and propagation as compared to the bulk and contributes to the ω -embrittlement of β -Ti alloys. [ABSTRACT FROM AUTHOR]

Graphical abstract Effects of the shear stress and composition on the ω -variant selection during the β → ω phase transformation of Ti- x Mo alloys from first-principles. Highlights • A model is developed to investigate the variant selection from first-principles. • Both bulk and interface contributions to the variant selection are considered. • The variant selection becomes stronger with increasing applied shear stress. • There may exists a critical concentration where there is no variant selection. Abstract Variant selection during solid–solid phase transformation in titanium alloys affects greatly the microstructure and mechanical properties of the alloys. Theoretical investigations of the variant selection were generally performed by using phase field simulation that considers solely the elastic coherency strain energy. In the present work, we develop a model to determine the variant selection directly from first-principles calculations. The source of the variant selection is considered to be the varying free energy gains (including both bulk and interface contributions) induced by the transformation from the parent phase to different variants of the product phase. This model is applied to investigate the effects of shear stress and alloy composition on ω variant selection during athermal β → ω transformation in binary Ti- x Mo (x ≤ 25 at.%) alloys. The random distribution of the atoms in the alloy is described by using virtual crystal approximation (VCA). We show that the tendency of variant selection becomes stronger with increasing shear stress. With increasing Mo concentration, the favorable ω variant transfers from one to another, except for the ω phase with very small particle size (e.g., radius R ≤ 1 nm) where one variant is always selected. The critical Mo concentration for the transfer of the favorable ω variant approaches to 5 at.% with increasing size of the ω phase particles and remains almost unchanged against the shear stress. At the critical Mo concentration, the considered variants have equal free energy gains and there is no variant selection. This finding opens the possibility of controlling the variant selection and the associated microstructure by changing the composition of the alloys. [ABSTRACT FROM AUTHOR]

Water ice is a molecular solid whose behavior under compression reveals the interplay of covalent bonding in molecules and forces acting between them. This interplay determines high-pressure phase transitions, the elastic and plastic behavior of H2O ice, which are the properties needed for modeling the convection and internal structure of the giant planets and moons of the solar system as well as H2O-rich exoplanets. We investigated experimentally and theoretically elastic properties and phase transitions of cubic H2O ice at room temperature and high pressures between 10 and 82 GPa. The time-domain Brillouin scattering (TDBS) technique was used to measure longitudinal sound velocities (VL) in polycrystalline ice samples compressed in a diamond anvil cell. The high spatial resolution of the TDBS technique revealed variations of VL caused by elastic anisotropy, allowing us to reliably determine the fastest and the slowest sound velocity in a single crystal of cubic H2O ice and thus to evaluate existing equations of state. Pressure dependencies of the single-crystal elastic moduli Cij(P) of cubic H2O ice to 82 GPa have been obtained which indicate its hardness and brittleness. These results were compared with ab initio calculations. It is suggested that the transition from molecular ice VII to ionic ice X occurs at much higher pressures than proposed earlier, probably above 80 GPa. [ABSTRACT FROM AUTHOR]

In this work, site-preference and elastic properties of TiNi-Nb austenite were investigated for various compositions, by using Exact Muffin-Tin Orbitals method in conjunction with Coherent Potential Approximation (EMTO-CPA). Moreover, composition-dependent elastic constants were calculated and used to explain the relation between composition and martensitic transition temperature. Our calculations suggested that Nb atoms prefer to occupy either Ni sites or Ti sites, depending strongly on composition. Variation of TiNi-Nb compositions is achieved through the formation of four sorts of defects: substituting Ti by Nb, substituting Ni by Nb, Ti anti-sites and Ni anti-sites. Our results suggested that c ′increases when Ni is substituted by Nb, while c 44 increases when Ni is substituted by Nb or Ni anti-sites form. Moreover, the maps of composition-dependent elastic constants were presented. TiNi-Nb with high c ′and c 44 could be obtained in Ni-deficient area. However, in Ni-excess area, only high c 44 could be obtained. Changing of martensitic transition temperature is sensitive to the variation of c 44 , rather than c ′. With the variation of c 44 , we explained the composition-dependent martensitic transition temperature for TiNi-Nb. [ABSTRACT FROM AUTHOR]

The phase stability and elastic property of binary Ti- x TM (TM = V, Cr, Nb, Mo) and ternary Ti-15TM- y Al alloys are investigated systematically by using a first-principles method. The coherent potential approximation is employed to describe the random distribution of the alloying atoms in the alloys. We show that the transition metal (TM) elements V, Cr, Nb, Mo increase the elastic stability of the β phase but decreases that of the α phase. The polycrystalline bulk modulus of the α phase increases with the concentration of the TM alloying elements whereas the Young's modulus and shear modulus are weakened. For the β phase, all the polycrystalline elastic moduli hardens with the addition of the TM alloying elements. The influence of the simple metal (SM) element Al on the elastic properties is much weaker than the TM ones. The TM alloying elements increase the stability of the β relative to the α and ω phases. Al stabilizes significantly the β phase over the ω phase. Our calculations suggest that the d electron density dominates the elastic modulus but not the relative phase stability. The interaction between the TM and SM alloying elements in titanium alloys matters to the relative stability. [ABSTRACT FROM AUTHOR]