The effects of nonmetallic solute atoms including C, H, O and N on (0001)[1120] and (0001)[1010] basal slip ofα-Ti were investigated by first-principles method based on density func-tional theory. The calculation results showed that nonmetallic solutes C, H, O and N atoms de-creased the generalized stacking fault energy (GSFE) of the (0001)[1010] basal slip. The re-duced unstable stacking fault energy (γus ) resulted in the improvement in mobility of slip, while the reduction of intrinsic stacking fault energy could facilitate the formation of stacking faults I2 . Other than H atom, the C, N and O atoms increased GSFE of the ( 0001 ) [ 1120 ] basal slip, implying that the increased energy barriers of (0001)[1120] basal slip prompted the dislocation dissociation. Further more, Bader charge, differential charge density distribution and local density of states ( LDOS) were given to reveal the micro-mechanism for effects of nonmetallic solutes on basal slip. It was showed that stable Ti-X ( X = C、H、O and N) covalent bonds played an essential role in basal slip.

Employing first-principles calculations, the point defect in C15 Laves phase MgCu2 is studied. The formation enthalpies of point defects and defective C15 MgCu2 compound show that the dominating defect structure is MgCu and CuMg antisite defect on Mg-rich and Cu-rich side of off-stoichiometry, respectively. The concentration of various point defects as a function of composition of MgCu2 is analyzed from grand canonical statistics. The calculated local geometrical variation around defect demonstrates an apparent size effect, and the electronic densities of states and charge density difference distributions indicate the gradually weaker bonding from MgCu to VCu and from CuMg to VMg systems. Furthermore, doping of Zn atom in defective MgCu2 has been investigated. It is found that Zn has a stronger preference for occupying the Cu sublattice than for Mg sublattice because of the relative lower formation enthalpy. The electronic structure further reveals that the stability of Zn-doped system is associated with the stronger covalent bonds between Cu–Cu and Zn–Cu atoms.

The elastic properties of L12-type Al3Sc at high temperature have been studied using the first-principles calculations combined with the quasi-harmonic approximation. The obtained elastic constants and polycrystalline elastic moduli exhibit a very gentle descent trend with increasing temperature. Elastic anisotropy of Al3Sc is slightly enhanced with increasing temperature, while the brittle nature is insensitive to temperature. The relevant important thermodynamic properties including thermal expansion coefficient and heat capacity are further derived and discussed. The moderate variation of elastic properties and the thermal properties may be attributed to the slightly weakened Al–Sc covalent bonding with temperature increasing. Our results would be valuable for further experimental study of high temperature mechanical properties of the relevant Al-based alloys.

Abstract The elastic properties of L1 2 -type Al 3 Sc at high temperature have been studied using the first-principles calculations combined with the quasi-harmonic approximation. The obtained elastic constants and polycrystalline elastic moduli exhibit a very gentle descent trend with increasing temperature. Elastic anisotropy of Al 3 Sc is slightly enhanced with increasing temperature, while the brittle nature is insensitive to temperature. The relevant important thermodynamic properties including thermal expansion coefficient and heat capacity are further derived and discussed. The moderate variation of elastic properties and the thermal properties may be attributed to the slightly weakened Al–Sc covalent bonding with temperature increasing. Our results would be valuable for further experimental study of high temperature mechanical properties of the relevant Al-based alloys.

The density function theory calculations are performed to study the native point defect in MgZn2. The calculated defect formation energy depends largely on the atomic chemical potentials of Mg and Zn, and suggests that Mg anti-site on Zn2 site is dominant under strong Mg-rich condition, while Zn2 vacancy is favorable under moderate Mg-rich condition. For Zn-rich side Zn anti-site on Mg sublattice is energetic. Lattice vibration effect is further considered, it is found that incorporation vibration contribution into defect formation energy results in obvious temperature effect. The present results indicate that defect configuration is sensitive to the environment conditions, reasonably explain the different experimental observations. The derived point defect concentration undergoes a dramatic change from Zn- to Mg-rich near the stoichiometry. The locally atomic geometry around point defect and electronic structure are also studied, which further reveal the underlying mechanism for structure of point defect.

The native point defects in C14 Mg2Ca Laves phase are studied from the first-principles density functional theory calculations within GGA approximation. The defect formation energies indicate that anti-site defects are energetically favored over vacancies. Under Mg-rich and even general Ca-rich condition, defect MgCa of Mg anti-site on Ca sublattice is favorable owing to the lowest formation energy. The CaMg2 defect of Ca anti-site on Mg2 sublattice is also likely dominant only under extreme Ca-rich environment. The present results could explain reasonably the asymmetric off-stoichiometry of Mg2Ca. The effective point defect concentrations of Mg2Ca as a function of composition and temperature at experimental range are also calculated from a canonical statistical model, and the derived results show a linear relationship between the logarithm of defect concentration and T−1. Geometrical factor is further studied, and it is found that atomic size possesses an obvious influence on the structure of point defect in Mg2Ca. The electronic feature is further studied to reveal underlying mechanism for formation of point defects.

A comparative study of single- and co-substitution of Ti on dehydrogenation of Mg2NiH4 has been carried out from first-principles calculations based on density functional theory. In comparison with Ti single-substitution, the formation enthalpy in co-substituted Mg2NiH4 is higher, showing the lower thermodynamic stability. In Ti co-doped Mg2NiH4, the average NiH bond length of entire unit cell is larger, implying weaker NiH bonds and lower stability of complexes NiH44−. Ti co-substitution further shows a more favorable dehydrogenation effect on Mg2NiH4 due to lower hydrogen dehydrogenation energy, and significantly reduces the dehydrogenation reaction enthalpy of Mg2NiH4 by about 30% compared to the pure state. Further calculated electronic structure demonstrates that the co-substituted Mg2NiH4 displays a metallic behavior with the Fermi level locating at the doping band, and the underlying mechanism for improving dehydrogenation properties of co-substituted Mg2NiH4 can be attributed to the weakened NiH interactions together with the narrowed energy gap.

Eight possible native point defects in Mg24Y5 are studied from the density functional theory (DFT) calculations within GGA approximation. The energetic results show that anti-site defects with lower formation energies are energetically favored over vacancies. Under both Mg-rich and Y-rich conditions, MgY1 defect is dominant due to the lowest defect formation energy, followed by YMg2, MgY2 and YMg1, which reasonably explains the Mg-rich off-stoichiometry of Mg24Y5 alloys. For vacancy defects, the formation energies on the two Mg sub-lattices are smaller than ones on two Y sub-lattices, and VMg1 has the lowest formation energy, while VY2 is most unlikely due to the highest formation energy originating from the large mismatch in atomic size and the strong interaction of Y with surrounding atoms. The defect concentration distribution as a function of temperature and stoichiometry is further obtained from grand canonical statistics. The size factor and electronic structure are further discussed, demonstrating that Mg24Y5 alloy is typical system in which the point defects are affected by strong size effect due to the large atomic size radius and strong electronic factor caused by the intensive interaction of Y with the surrounding atoms.