Computational study of nanoscale mechanical properties of Fe–Cr–Ni alloy

Mechanical properties of Fe–Cr–Ni alloy nanowires have been investigated using molecular dynamics simulation with embedded atom method and first principles approach. Various cases of uniaxial tension, compression and shear deformations have been performed and studied in this work. From the first principles calculations, the higher magnitudes of uniaxial and shear deformations resulted in higher probability of martensitic transformations. Before the first yielding, nanowires preserved the elastic stage and then the mechanical deformation proceeded in alternating quasi-elastic and yielding stages. The plastic behaviour was not observed in compression while both tensile and shear deformations showed apparent plastic behaviour. In shear deformation, due to the martensitic phase transformation, the plastic behaviour persisted for total strain of 0.6 which was much larger than that during tensile and compression. This validated the previous experimental observations. In the studied Fe–Cr–Ni nanowires, deformations were controlled by dislocations. Dislocation-mediated twinnings were captured by common neighbour analysis. Twin quantification showed that the twin activity increased with increasing strain rate. Twinnings originated from stacking faults led by 1/6 <112> Shockley partial dislocations. At elevated temperature (beyond 500 K), the materials softening happened, and 316L nanowire became more plastic under a lower stress.

本研究采用嵌入原子法（Embedded Atom Method）与第一性原理（First Principles）方法，结合分子动力学（Molecular Dynamics）模拟，对Fe-Cr-Ni合金纳米线的力学性能展开了系统性探究，本工作针对单轴拉伸、压缩与剪切形变的多种工况开展了研究与分析。通过第一性原理计算可知，单轴与剪切形变的幅度越高，马氏体相变（Martensitic Transformation）的发生概率便越高；在首次屈服前，纳米线始终处于弹性阶段，随后机械形变以准弹性阶段与屈服阶段交替的方式进行；压缩形变过程中未观测到塑性行为，而拉伸与剪切形变均表现出显著的塑性特征，在剪切形变场景下，得益于马氏体相变，塑性行为可维持至总应变为0.6的水平，远高于拉伸与压缩形变对应的总应变，该结果验证了此前的实验观测结论；在所研究的Fe-Cr-Ni合金纳米线中，形变过程由位错主导，通过共近邻分析（Common Neighbour Analysis）可捕捉到位错介导的孪晶现象，孪晶定量分析结果显示孪晶活性随应变速率的提升而增强，孪晶起源于由1/6 <112>型肖克莱不全位错（Shockley Partial Dislocations）引发的堆垛层错；当温度升高至500K以上时，材料发生软化现象，316L纳米线在更低的应力下便展现出更强的塑性。