高压下BCC金属钨和钼力学性质的第一性原理研究

First-principles study of mechanical properties of BCC metals tungsten and molybdenum under high pressure

摘要:

本文采用基于密度泛函理论的第一性原理方法研究了体心立方金属钨和钼的体积、弹性常数、弹性模量、声子色散曲线以及广义层错能在0-100GPa压强下随压强的变化关系，并讨论了高压下两种材料的力学结构稳定性以及高压对材料韧脆性以及剪切形变难易程度的影响.首先，通过0-100GPa压强下的弹性常数发现，两种材料在不同压强下的弹性常数皆满足材料力学稳定性的判定条件，而且两种材料在100GPa下的声子色散曲线中并没有出现虚频，因此两种材料的结构在0-100GPa压强下都是力学稳定的.此外，通过研究不同压强下体模量与剪切模量的比值B/G发现，两种材料的韧性和随压强的增加而增强，并且Mo的韧性强于W.最后，通过研究两种材料的广义层错能、沿<111>密排方向的剪切模量G111以及材料的各向异性比A发现，随着压强增加，广义层错能和G111逐渐增大，A整体趋于1，这说明高压会使得<111>密排方向的剪切形变变得困难，而且同时也削弱了材料的各向异性.

Abstract:

By using the first principles method based on the density functional theory, the volume, elastic constants, elastic modulus and the phonon dispersion curve as well as the generalized stacking fault energies of bcc metals W and Mo have been investigated at the pressure from 0GPa to 100GPa, and the mechanical stability, the brittle-ductile properties and the shear deformation of the two materials have also been studied at the same pressure. Firstly, by calculating the elastic constants of the two materials at the pressure from 0-100GPa, it is found the elastic constants of each material satisfy the conditions of mechanical stability, moreover, the frequency of the phonon dispersion curves at 100GPa is positive and there is no imaginary frequency, therefore, both the structures of the W and Mo are mechanical stable at the pressure from 0-100GPa. Besides, through analyzing the ratio of the bulk modulus and the shear modulus, it is found that the high pressure can make the ductility of the W and Mo stronger, and the ductility of Mo is better than that of W. Finally, the generalized stacking fault energies, shear modulus G111 along the <111> direction and the anisotropy ratio A of the two materials are all investigated and it is found that, both the generalized stacking fault energies and G111 as well as A all increase when the pressure becomes higher, and the value of A is very close to 1 at 100GPa, all these indicate that the high pressure makes the shear deformation become more difficult and the weakens the anisotropy of W and Mo.