zA62镁合金中AB_2型金属间化合物的结构稳定性与弹性性能的第一原理计算

采用基于密度泛函理论Castep和Dmol程序软件包, 计算了ZA62镁合金中AB₂型金属间化合物 MgZn₂, Mg₂Sn和MgCu₂的结构稳定性、弹性性能与电子结构. 合金形成热和结合能的计算结果显示:  Mg₂Sn具有最强的合金化形成能力, 而MgCu₂结构最稳定; 体模量(B)、弹性各向异性系数(A)、Young's模量(E)、剪切模量(G)和Poisson比(ν)的计算结果表明: MgZn₂和MgCu₂为延性相, 而Mg₂Sn为脆性相, MgZn₂的塑性最好; 采用弹性常数、体模量和结合能的经验公式计算金属间化合物的熔点, 实验值均在采用弹性常数(±300 K)和体模量(±500 K)计算熔点预测的范围内, 采用弹性常数比采用体模量和结合能预测熔点的平均相对误差小, 其中采用弹性常数计算Mg₂Sn的熔点与对应的实验值十分接近, 相对误差仅为0.31%. 不同温度下热力学性质的计算结果表明, 在298-573 K温度范围内, MgCu₂的Gibbs自由能始终最小, 其结构热稳定性最好, 结构稳定性的强弱顺序并不随温度的升高而消失; 而对MgZn₂和Mg₂Sn, 以475 K为临界, 结构稳定性的强弱顺序随温度的升高发生了变化; 态密度和Mulliken电子占据数的计算结果表明: MgCu₂ 结构最稳定的原因主要在于体系在Fermi能级以下区域成键电子存在强烈的离子键作用.

FIRST-PRINCIPLES CALCULATIONS OF STRUCTURAL STABILITIES AND ELASTIC PROPERTIES OF AB_2 TYPE INTERMETALLICS IN ZA62 MAGNESIUM ALLOY

Structural stabilities, elastic properties and electronic structures of Mg₂Sn, MgZn₂ and MgCu₂ in ZA62 magnesium alloy have been determined from first-principles calculations by using Castep and Dmol program based on the density functional theory. The calculated heats of formation and cohesive energies showed that Mg₂ Sn has the strongest alloying ability and MgCu₂ the highest structural stability. The calculated bulk moduli (B), anisotropy values (A), Young's moduli (E), shear moduli (G) and Poisson ratio (ν) showed that MgZn₂ and MgCu₂ both are ductile, on the contrary, Mg₂ Sn is brittle, and among the three phases MgZn₂ is a phase with the best plasticity. Their tested melting temperatures are within the ranges calculated from elastic constants (±300 K) and bulk moduli (±500 K), the estimated values from elastic constant have the smallest average relative error, the calculated melting temperature of Mg₂Sn phase is in well agreement with the experimental one and the error relative to the experiment result is about 0.31%. MgCu₂ has higher melting temperature, i.e. better structural stability among the three compounds. The calculations of thermodynamic properties show that the Gibbs free energy of MgCu₂ is also the smallest within 298-573 K range, indicating the structural stability of MgCu₂  does not change with the elevated temperature. The calculations of the density of states (DOS) and Mulliken electronic populations showed that the reason of MgCu₂ having highest structural stability in ZA62 magnesium alloy attributes to MgCu₂ phase having more ionic bonds below Fermi level compared with those of Mg₂ Sn and MgZn₂ phases.