纳米单晶γ-TiAl合金应变速率效应分子动力学模拟

Molecular Dynamics Simulation of Nano Single Crystal Gamma-TiAl Alloy Strain Rate Effect

采用分子动力学方法模拟不同应变速率下纳米单晶γ-TiAl合金中裂纹的扩展,利用速度加载方式对预置裂纹的单晶γ-TiAl合金进行动态单向拉伸,模拟过程中施加应变速率为5.0×10~7～7.5×10~9 s^(-1)。结果表明:不同的应变速率范围下裂纹的扩展形式差异很大。在不敏感区(ε≤4×10~8s^(-1)),裂纹呈解理扩展;在敏感区(4.0×10~8s^(-1)<ε≤1.0×10~9s^(-1)),前期呈现解理扩展特征,后期裂纹扩展通过裂尖发射滑移位错,位错塞积萌生空洞,空洞形核长大形成子裂纹,ε≤5.0×10~8s^(-1)时,子裂纹发生偏向,与主裂纹呈45°方向串接,5.0×10~8s^(-1)<ε≤1.0×10~9s^(-1)时,子裂纹与主裂纹同向串接,最终导致裂纹扩展直至断裂;在突变区(ε≥1.0×10~9s^(-1)),因应变强化作用使裂纹不在应力最大时刻启裂,出现裂纹扩展后应力持续增加一段时间后减小的现象,高应变速率导致裂尖前端多处区域的原子结构局部非晶化,最终在原子结构混乱处萌生微裂纹,微裂纹扩展导致"试件"多处开裂。

The propagation form of the preset edge crack in the nano single crystal gamma-Ti Al at different strain rates were simulated by molecular dynamics. Dynamic uniaxial tension for preset edge cracks was carried out by velocity loading in the single crystal gamma-Ti Al within 5.0×10^7～7.5×10^9 s-1. The results show that the crack propagation form varies with the range of strain rate. In the non-sensitive region （ ε ≤4×10~8 s-1）, the crack propagates in a brittle cleavage manner. In the sensitive region （4.0×10^8 s-1 ε ≤1.0×10^9 s-1）, it exhibits cleavage extension feature early; in the latter stage, the slip dislocation is emitted from the crack tip, and the void defect initiates the place where dislocation pile up and then grow to the child crack, the child crack links the main crack when it bias 45o when ε ≤5.0×108 s-1, but it connects the main crack at the same direction under 5.0×10^8 s-1 ε ≤1.0×10^9 s-1, thus leading to the fracture finally. In the sharp-change region （ ε ≥1.0×10^9 s-1）, the crack does not craze at the maximum stress for the reinforcement of strain, and the stress decreases after it continues to increase for some time. The non-crystallizing of the atom structure occurs near the crack tip because of high strain rate, and then the micro crack initiates in the area of structure disordered, the micro crack growth causes fracture of the ＂test specimen＂ eventually.