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The structural, mechanical and electronic properties of (TaNbHfTiZr)C high entropy carbide are studied by using density functional theory in conjunction with special quasi-random structures. The proposed lattice constant difference as an empirical criterion for high entropy compounds and mixing enthalpy show the formation of (TaNbHfTiZr)C solid solution. The derived elastic stiffness constants also indicate the mechanical stability of (TaNbHfTiZr)C high entropy carbide. At zero pressure, the calculated elastic mechanics obeys the rule of mixture, whereas Vickers hardness is slightly larger than the average value of constituent binary carbides. The computed elastic parameters show that (TaNbHfTiZr)C is brittle, similar to constituent binary carbides. Under high pressure, the lattice constants decrease slightly, and mechanical properties are improved, even the brittleness-ductility transition takes place. The calculated electronic structures show that covalence in (TaNbHfTiZr)C is relatively weaker than ionic bonding. With increasing pressure, covalence in (TaNbHfTiZr)C decreases while ionicity increases. The present research will be valuable for understanding and designing of high-entropy carbides.

The crystal structure, elastic and magnetic properties of important ternary Mg-based alloys NdMgT4 (T = Co, Ni, Cu) have been studied using reliable ab initio calculations. Both cohesive energy and charge density difference suggest that three alloys have good structural stability with the order: NdMgCo4 > NdMgNi4 > NdMgCu4. It shows that NdMgCo4 alloy has magnetic moments with the Co atoms being the main contribution, which is also in agreement with the calculated electronic structures. We find that NdMgT4 (T = Co, Ni, Cu) alloys are all ductile materials with bulk-to-shear modulus (B/G) values higher than 1.75. The trends of calculated values for the shear moduli Cs and C44 are consistent with that of shear modulus G and young's modulus E, proving that NdMgT4 (T = Co, Ni, Cu) alloys exhibit good plasticity with the trend: NdMgNi4 > NdMgCu4 > NdMgCo4. These calculated results give the basis guidance for the design of rare earth-magnesium-transition metal (R-Mg-T) alloys with improved mechanical properties.

To predict and compare the main reinforcement effects of the key precipitation phases Mg2Cu3Si, Mg2Si and MgCu2 in Mg-Cu-Si alloy, the structural, mechanical and electronic properties of these phases have been studied by ab initio calculations. The lowest formation enthalpy and cohesive energy indicate that Mg2Cu3Si has the strongest alloying ability and structural stability. The mechanical modulus indicates that Mg2Cu3Si has the strongest resistance to reversible shear/volume distortion and has maximum hardness. The characterization of brittle (ductile) behavior manifests that MgCu2 has favorable ductility. Meanwhile the evaluation of elastic anisotropy indicates that Mg2Si possesses elastic isotropy. Debye temperature prediction shows that Mg2Si and Mg2Cu3Si have better thermal stability. To achieve an unbiased interpretation on the phase stability and mechanical behavior of these precipitation phases, the density of states and differential charge densities are also analyzed. The current study deepens the comprehensive understanding of main reinforcement effects of these precipitation phases on Mg-Cu-Si alloys, and also benefits to optimize the overall performances of Mg-Cu-Si alloy from the hardness, ductility and thermal stability by controlling these second precipitation phases during the heat treatment process.

Temperature dependence of mechanical and thermodynamic properties of refractory high entropy alloys Ti(25+x)Zr25Nb25Ta(25-x) (x ≤ 20) are studied from ab initio quasi-harmonic Debye-Gruneisen model, and the influences of substitution of Ti for Ta are emphasized. The substitution of Ti for Ta results in the significantly reduced densities and the apparently enhanced specific elastic properties of Ti(25+x)Zr25Nb25Ta(25-x) alloys. Moreover, the increases of Ti content obviously improves ductility. At high temperature, the alloys with higher Ti content demonstrate the stronger thermal expansion behavior and thermal conductivity. Importantly, the alloys with higher Ti content possess the greater specific elastic properties at high temperature. Especially, in the studied temperature range, the present alloys exhibit good anti-softening ability based on temperature-dependent mechanical properties. Therefore, the replacement of Ta by Ti in refractory Ti(25+x)Zr25Nb25Ta(25-x) alloys is beneficial for formation of novel high temperature light-weight structural materials with lower density, better ductility and higher specific mechanical properties.

HfTiZrSc is a novel hexagonal close-packed refractory high entropy alloys, addition of Al as typical additives significantly enhances the strength and ductility. To reveal the effect of Al addition on thermal properties, we combine the EMTO-CPA ab initio calculations with quasi-harmonic Debye-Gruneisen model to studied the thermodynamic properties of the Hf0.25Ti0.25Zr0.25Sc0.25-xAlx (x ≤ 15 at.%) HEAs. With Al substitution for Sc, the strength of Hf0.25Ti0.25Zr0.25Sc0.25-xAlx (x ≤ 15 at.%) is enhanced significantly, despite essentially similar thermal softening trend. Thermal expansion coefficient of Hf0.25Ti0.25Zr0.25Sc0.25-xAlx increase with increasing Al content. Thermodynamic entropy of Hf0.25Ti0.25Zr0.25Sc0.25-xAlx (x ≤ 15 at.%) increases dramatically with temperature, the trend is to some degree suppressed with increase of Al content, which is discussed from vibrational, electronic and configuration entropy contribution. The influences of Al content on Debye-temperature and Gruneisen parameter are also further studied. The present investigation is valuable for optimal designing of overall performance of Hf0.25Ti0.25Zr0.25Sc0.25-xAlx alloys.

The effects of solute atoms Zn and Y on I1 and I2 stacking faults in Mg–Y–Zn solid solution have been studied within framework of density functional theory using Vienna Ab initio Simulation Package (VASP). The calculated results show that the addition of Y or Zn exhibit the opposite effect on the stable and unstable stacking fault energies, and the simultaneous addition of Y and Zn decreases the stable and unstable stacking fault energies dramatically. Then it is further found that in Mg–Y–Zn solid solution the extended dislocation configuration tends to be stabilized and the dissociation of dislocation into partials would happen more easily, and the ductility could also be obviously improved. The electronic structures reveal that the significant reduction of SFEs originates from the stronger interaction between slip planes due to the formation of covalent bonds.

New ternary magnesium alloys AMgNi 4 (A=Y, La, Ce, Pr and Nd) have been studied by First-Principles calculations within the generalized gradient approximation. The optimized structural parameters were in good agreement with the available experimental data. The calculated cohesive energies and formation enthalpies showed that these alloys had strong structural stability. Then the elastic constants C ij of these AMgNi 4 alloys were calculated, and the bulk modulus B , shear modulus G , Young's modulus E , Poisson's ratio ν and anisotropy value A of polycrystalline materials were derived from the elastic constants, the related mechanical properties were further discussed. The electronic structures were also calculated to reveal the underlying mechanism for the structural stability and the elastic property.

First-principles calculations have been performed on the face-centered cubic (FCC) magnesium–transition metal (TM) hydrides Mg 7 TMH 16 (TM=Sc, Ti, V, Y, Zr, Nb). The cohesive energies are calculated to analyze the stability, and the obtained enthalpies of formation for hydrides Mg 7 TMH 16 have been used to investigate the possible pathways of formation reaction. The calculated enthalpy changes show that the decomposition temperatures of Mg 7 TMH 16 are lower than that of MgH 2 . The electronic densities of states reveal that all the hydrides studied here exhibit metallic characteristics. The bonding nature of Mg 7 TMH 16 is investigated, showing stronger covalent bonding between TM and H than between Mg and H.