激光冲击下CoCrFeMnNi高熵合金微观塑性变形的分子动力学模拟

激光冲击强化技术可以有效地提高材料的疲劳寿命, 被广泛应用于航空航天领域. CoCrFeMnNi高熵合金作为一种经典的高熵合金体系, 研究其激光冲击强化后的微观组织变化以及冲击动态响应对该材料未来在航空航天领域中的应用具有重要意义. 采用分子动力学方法, 对CoCrFeMnNi高熵合金进行了冲击模拟, 发现冲击时弹、塑性双波分离现象以及微结构演化具有明显的取向相关性. 沿[100]方向进行冲击时未出现双波分离结构, 并且塑性变形过程中会产生中间相; 而沿[110]与[111]方向冲击时产生了双波分离结构, 并且受冲击区域存在大量的层错以及无序结构, 高位错密度是产生无序结构的重要原因. 双波分离现象与可开动滑移系个数有关, 而沿不同取向冲击时的Hugoniot弹性极限和发生塑性变形的临界冲击速度与其可开动滑移系的Schmid因子大小有关. 此外, 冲击诱导了梯度位错结构的产生, 位错密度沿冲击深度先增加后减小, 在沿原子密排方向冲击时产生了更高的位错密度. 冲击之后在模型两侧存在残余压应力, 芯部为残余拉应力, 残余应力的大小具有明显的取向相关性. 最后, 与具有相同尺寸及取向的纯Ni进行对比, 发现CoCrFeMnNi高熵合金在冲击过程中由于晶格畸变效应产生了较纯Ni更多的无序结构. 

MICROPLASTIC DEFORMATION OF CoCrFeMnNi HIGH-ENTROPY ALLOY UNDER LASER SHOCK: A MOLECULAR DYNAMICS SIMULATION

Laser shock processing (LSP) can effectively improve the fatigue life of materials, which is widely used in the aerospace field. CoCrFeMnNi high-entropy alloy is a classic high-entropy alloy system, so the studies on microstructure evolutions and shock wave responses after LSP play an important role in the application of this material in the aerospace field. The molecular dynamics method is used to simulate the shock of CoCrFeMnNi high-entropy alloy, and it is obtained that the elastoplastic two-wave separation phenomenon is related to the shock direction, showing obvious orientation-dependence. It is found that there is no two-wave separation structure when shocking along the [100] direction, and an intermediate phase will be produced in the process of plastic deformation. But, when shocking along the [110] and [111] directions, a two-wave separation structure is produced, and there are a large number of stacking faults and disordered structures in the impacted area, the high dislocation density is an important reason for the disordered structure. The phenomenon of two-wave separation is related to the number of active slip systems, the Hugoniot elastic limit and the critical impact velocity for plastic deformation when impacted along different orientations are related to the Schmid factor of the active slip systems. In addition, a gradient dislocation density structure is induced due to the shocking loading, the dislocation density first increases and then decreases along with the shock depth, and a greater dislocation density is produced when shocked in the close-packed direction. After the shock, there is residual compressive stress at the both ends of the model, the residual tensile stress is at the core of the model, and the magnitude of residual stress has obvious orientation dependence. Finally, compared with pure Ni with the same size and orientation, it is found that there are more disordered structures in CoCrFeMnNi high-entropy alloy than pure Ni during the impact process due to the lattice distortion effect.