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CdSiP2 (CSP) with outstanding mid-infrared nonlinear optical properties in the 3–5 μm and 8–12 μm windows has always been expected for application requirement, whereas the inadequate of absorption losses cannot be ignored as practical usage. The defect formation energies, electronic structure characteristics, structure distortion and related optical properties of CSP with some traditional and basic dopants M= (Mg, Al, Cu, Fe, and Mn) substituting Cd or Si sites are investigated by employing density functional theory (DFT). The calculations show that the energetically and thermally stable states in CSP crystals are Mg0, Al1+, Cu4+, Fe4+, Mn4+ at Cd sites and FeSi1+ at SiP2 reference states. Fe and Mn dopants at Cd sites bring abundant valences, including 0, +1, +2 and +4 charged states, and result in a mean absorption spectrum increase. New peaks appear in the density of states as the CuCd4+, FeCd4+, and MnCd4+ take shape, and thus affect the absorption spectrum. According to the calculation results, highly localized d orbitals of dopants should be mainly responsible for the absorption aberrance, because they not only lead to a mean absorption spectrum increase, but also bring new absorption peaks affecting the conversion efficiency along X/Y or Z directions.

The volume modeling of the sigma phase is an indispensable complement to the integrated computational material design of technologically important materials, such as high-alloy steels and Ni-based superalloys. The molar volume of the sigma phase is influenced by both the atomic mixing (the volume variation affected by this factor is caused by composition alteration rather than site occupation change) and atomic order (i.e. atomic constituent distribution or site occupancy preference on inequivalent sites of a crystal). In the present work, we developed a new integrated thermodynamic and molar volume model to consider physically both mixing and order factors. The integrated model was built within the compound energy formalism (CEF), enabling the thermodynamic calculations to determine equilibrium site occupancies for the subsequent volume calculations. The model parameters of the CEF were assigned by using the first-principles calculated energies and molar volumes of the complete sets of ordered configurations of the sigma phase, as well as the extrapolated molar volumes of the pure elements in the hypothetic sigma phase structure. Such extrapolation for pure elements is based on the experimental data from the literature and the first-principles calculations. We applied the integrated model to study the binary compounds, e.g. Cr-Co, Cr-Fe, Cr-Mn, Mo-Fe, Mo-Mn, Mo-Re, Re-Cr, Re-Fe, Re-Mn, Nb-Al, Ta-Al, V-Fe, V-Ni, and ternary compounds Cr-Fe-X (X = Co and Ni). The integrated thermodynamic and molar volume databases can predict successfully the molar volume of the binary and ternary sigma compounds. As most experimental volume data were measured at room temperature and atmospheric pressure, and the first-principles calculations were performed at 0 K, the present model parameters are valid at about room temperature and atmospheric pressure.

Local chemical ordering influences significantly the mechanical properties of refractory high entropy alloys. In this work, the effects of local chemical ordering on the structural stability and mechanical properties of AlNbVTiZr are investigated by using first-principles simulations. It is shown that the Al Zr bond dominates the chemical ordering for ordered configurations. The local ordering enhances the structural stability, elastic property and ideal strength. By taking into account the effects of temperature and pressure, we found that chemical ordering mediates the thermodynamic properties of the AlNbVTiZr alloy, but it has little effect at high pressure.

Solute atoms such as Fe, Co, and Ni diffuse abnormally fast in α-Ti, which influences significantly the mechanical properties of the titanium alloys. Various mechanisms (e.g., the interstitial diffusion mechanism and solute-vacancy complex mechanism) have been proposed to account for the fast diffusion of these solutes in α-Ti. To elucidate such diffusion mechanism, a first-principles method is employed to calculate the formation energies, migration energy barriers, and solute-vacancy binding energies of the substitutional and interstitial solute atoms including Al, Si, Sn, V, Mn, Fe, Co, Ni, and Cu in α-Ti. Based on the calculated parameters, the diffusion mechanisms are discussed. Comparing the formation energies of the substitutional and interstitial solutes, we find that all the solute atoms prefer the substitutional configuration to the interstitial one. The interstitial migration energy barriers are quite low for all the solutes. Al and Sn diffuse mainly through normal vacancy mediated mechanism due to the high substitutional to interstitial preferential energy that leads to very low fraction of interstitial solutes (about 10 − 15 ∼ 10 − 16 at 1000 K) at thermal equilibrium state and high interstitial diffusion activation energy. The 3d metal solute atoms, especially Mn, Fe, and Co, are fast diffusers and the diffusion coefficients are dominated by the interstitial mechanism because of their sizable thermal equilibrium interstitial fractions (several percent at 1000 K). The solute-vacancy complex mechanism is not likely to account for the fast diffusions in α-Ti. We show that the direct chemical interaction between the solute and matrix atoms determines the site-occupancy of the solute atoms in α-Ti besides the atomic size effect that was commonly believed to be responsible for the fast diffusions.

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.

The critical resolved shear stress (CRSS) and its related plastic deformation of titanium with hexagonal close packed structure are highly anisotropic, leading to low ductility of the material. Understanding the alloying effect on the CRSS is crucial for the improvement of the mechanical properties through rational composition design. Accurate prediction of the CRSS is not straightforward due to the atomic randomness of the alloy. The generalized stacking fault energies (GSFEs) for the basal and prismatic plane 〈 a 〉 slips of random α-Ti1−xAlx alloys ( 0 ≤ x ≤ 0.1875 ) are calculated by using first-principles methods including exact muffin-tin orbitals (EMTO) and plane-wave psuedopotential (VASP) methods in this work. The random distribution of Al in the alloy is treated by using both coherent potential approximation (CPA) for EMTO and special quasirandom structure (SQS) techniques for VASP. The CRSSs are then evaluated within the frame work of semi-discrete variational Peierls-Nabarro model. The VASP-SQS calculations with atomic relaxation generate reasonably good GSFE and CRSS compared with the EMTO-CPA and VASP-SQS calculations without atomic relaxation. For pure Ti, the unstable stacking fault energy ( γ u s f ) and CRSS ( τ a ) for the basal 〈 a 〉 slip are higher than those for the prismatic 〈 a 〉 slip. With increasing Al concentration x, γ u s f b for the basal 〈 a 〉 slip decreases whereas γ u s f p for the prismatic 〈 a 〉 increases. The CRSSs for the basal 〈 a 〉 slip ( τ a b ) and the prismatic 〈 a 〉 slip ( τ a p ) both increase with x while τ a p increases faster than τ a b such that τ a b approaches to τ a p . The calculated CRSSs explain successfully our recently measured mechanical properties (yield strength, fracture toughness, ultimate strength, and elongation) of the α-Ti1−xAlx alloy against x.

The stacking fault energy (SFE) plays an important role in the deformation mode selection and consequently the mechanical properties of high entropy alloys. In the present work, the SFEs of face-centered cubic (fcc) CoCrFeNi and Al0.57CoCrFeNi HEAs are calculated by using first-principles calculations. The influence of the local lattice distortion (LLD) on the SFE is explored. We show that the addition of Al to CoCrFeNi alloy increases the intrinsic SFE (ISFE, γisf) but decreases the unstable SFE (USFE, γusf) such that the energy barrier (γusf - γisf) for the 〈11 2 ( _ ) 〉(111) slip decreases. The lattice distortion contributes significantly to the ISFE but has limited influence on the USFE, leading to decline of the slip energy barriers of both alloys. The ISFE and slip energy barrier differences between the two alloys are enhanced by the lattice distortion. Based on the calculated ISFE and slip energy barrier, the effects of Al alloying and lattice distortion on the mechanical properties of CoCrNiFe HEA are discussed.

Ductility enhancement up to about 28–39% in fracture elongation with simultaneous strengthening was achieved in fine-grained magnesium and magnesium alloy sheets by multi-pass rolling with lowered temperature suitable for industrial fabrication. Specifically, high strengths at yielding following slip-dominated Hall–Petch relations were achieved owning to the naturally twinning's replacement by pyramidal under intense basal texture. These Hall–Petch relations had a high slope of 190 MPa μm 1/2 and frictional stresses ranging from 40 MPa to 144 MPa. The variation in frictional stress matched well with solid solution hardening and/or precipitation hardening. Enhanced uniform elongation was strongly correlated with improved strain-hardening rate via introducing solid solution and/or fine precipitates in Mg alloys compared with pure Mg, and conversely post-necking elongation (local elongation) as well as cold-workability under compressive stress was strikingly weakened by the restriction of dynamic recovery.

Introducing high symmetric cubic disordered β or ordered β 0 phases by alloying β-stabilizers improves the strength and high-temperature deformability of intermetallic γ-TiAl. However, the β phase is prone to decomposition of lower symmetric hexagonal phases such as ω 0 and ω ’ ’ , which are extremely brittle and harmful to the ductility of the alloy. Therefore, understanding the effect of the β-stabilizers on the phase stability of TiAl is crucial for the rational design of TiAl based alloys. In the present work, the equilibrium lattice structures, bulk moduli, and heats of formation of β, β 0 , ω ’ ’ , and ω 0 of TiAl with some typical β stabilizers V, Nb, Ta, Cr, Mo, W, and Mn were calculated by using a first-principles method, with which the relative phase stability were discussed. We showed that the phase stability sequence is β β 0 ω 0 ω ’ ’ for Ti4Al3Nb and Ti4Al3Ta, β ω 0 β 0 ω ’ ’ for Ti4Al3V, Ti4Al3Mo and Ti4Al3W, and β ω 0 ω ’ ’ β 0 for Ti4Al3Cr and Ti4Al3Mn. The β-stabilizing effect of the alloying element X increases from Nb/Ta to V/Mo/W to Cr/Mn. The relative stability of the various phases is roughly in accordance with the trends of the unit volume and bulk modulus against the phases with a few exceptions. The phase diagrams of the alloys associated with these phases were estimated by comparing the free energies against the temperature of various phases, evaluated with the calculated unit volumes and bulk moduli.

A study on the kinetics of grain growth of a fine-grained, hot-rolled AZ31 magnesium alloy was carried out by isothermal and isochronal annealing treatments. The grain size of the as-rolled and subsequently annealed sheet samples was determined by metallographic observation using the linear intercept method. It was found that for annealing at temperature in the range of 523–723 K, the effect of annealing temperature, T, and time, t, on the grain growth could be well interpreted by the kinetic equation, D n − D 0 n = k t , where k = k 0 e − E g / R T . Based on the experimental data on grain growth obtained by annealing treatments, the grain growth exponent, n, and the activation energy, Eg, were figured out, and these grain growth kinetic parameters were compared with those for other magnesium alloys reported in recent literatures.

The hydrogen absorption kinetics of porous Ti6Al4V alloys in the temperature range of 623–1023 K have been investigated. The obtained hydrogen absorption kinetic curves were fitted using a series of mechanism functions to reveal the kinetic parameters and reaction mechanisms of the hydrogen absorption process. The results show that the starting temperature of hydrogen absorption reaction in porous Ti6Al4V alloys is around 623 K, which is lower than that in dense Ti6Al4V alloys but higher than that in Ti6Al4V powders. Additionally, both the initial hydrogen absorption rate and equilibrium hydrogen pressure increase with increasing temperature. The three-dimensional diffusion process is the intrinsic rate-limiting step of hydrogen absorption between 623 K and 1023 K. The apparent activation energy calculated according to Arrhenius equation for the absorption of 1 mol H2 into porous Ti6Al4V alloys is 14.545 kJ, which is smaller than that in dense Ti alloys.