纳米单晶氩机械拉伸性质的分子动力学模拟

采用分子动力学(MD)方法对纳米单晶氩杆进行了机械拉伸变形和断裂的模拟研究。在拉伸过程中可以观察到原子位错,间隙的形成,裂纹的出现,以及后来的断裂分离等现象,与宏观的拉伸试样相似。MD模拟的拉伸试样的真实应力应变图显示,纳米单晶氩杆随着应变的增加应力略有增加,超过某一应变值后,应力急剧增加到最大值。随后,在加载速率较大时试样突然脆性断裂,在加载速率较小时应力却有一个突然的下降变负过程。这说明加载速率对材料的强度影响很大,当加载速度分别为2.16m/s和6.49m/s时,纳米单晶氩材料相应的断裂强度为2.6GPa和6.6GPa。纳米单晶氩材料在断裂前经历了一个较大的塑性变形过程,但其断裂方式却是完全脆性的。另外,纳米单晶氩材料在自由边界条件下充分弛豫后在垂直于拉伸的方向有轻微的收缩,不同于Al,Cu,Ni等具有FCC晶格结构的纳米单晶材料由于内力的作用引起轻微膨胀。

Molecular dynamics simulation of tensile mechanical properties of nano-single crystal argon

Single-crystal FCC cubic argon at nanolevel at two different constant rates of uniaxial tensile loading (2.16m/s, 6.49m/s) is studied by molecular dynamics (MD) using L-J(12-6) inter atomic potential. Deformation and fracture of the work materials duo to voids formation, their coalescence into nanocracks, and subsequent fracture or separation were observed similar to their behavior at macroscale. The true stress-train diagrams obtained by the MD simulation of the tensile specimens show a long slow increase in stress with a gradual increase in strain, and a rapid increase in stress up to a maximum when the value of strain reaches a certain value followed by a sudden drop to negative or a sudden disappear when the specimen fails by brittle fracture. The rate of loading plays an important role in the ultimate strength of the nanomaterials. The ultimate strengths are 2.6GPa and 6.6GPa separately corresponding to the rates of loading 2.16m/s and 6.49m/s. The nanomaterial underwent much more deformation before its sudden rupture than ordinary brittle materials. The brittle rupture form is absolutely different from the ductile rupture form reported previously about ordinary nanomaterials. In addition, the specimens after relaxation were found to be slightly tensile due to internal forces, which is different from the compressed form reported recently for the specimens of FCC materials,such as Al,Cu,Ni,et al, after relaxation.