材料期刊网

通过分子动力学方法, 研究了3种含相同半径、不同深度空洞的镍基单晶合金模型与理想模型纳米压痕过程的区别. 采用中心对称参数分析4种模型在不同压入深度时基体内部位错形核、长大的过程以及空洞和错配位错对纳米压痕过程的影响. 材料的压入荷载-压入深度曲线显示, 空洞最浅的模型与理想模型相差最大. 空洞对材料纳米压痕过程有2种作用, 当压入深度较浅时(h<0.375 nm), 空洞的存在会弱化材料, 而当压入深度处于0.375~0.567 nm之间时, 空洞表面的原子对位错的长大起到阻碍作用, 使得压入荷载增加; 空洞的坍塌会吸收一部分应变能, 减少γ相中层错的形成; 当空洞完全坍塌后, 位错会在空洞原始位置纠缠, 并产生大量层错, 使得压入荷载减小. γ/γ'相相界面存在空洞时, 当达到最大压入深度, 部分错配位错分解, 且被γ相表面吸收, 形成表面台阶. 处在最深位置的空洞并未对材料纳米压痕过程产生影响.

Nanoindentation of Ni-based single crystal alloy which has a void defect is simulated by the molecular dynamics method. Three models with different voids which have a same radius but different depth (H=1.5 nm, 3.0 nm, 4.5 nm) are contrasted to the perfect model respectively. The influence of a void and misfit dislocation on nanoindentation process are analyzed using center symmetry parameter. Nucleation and growth of dislocation on various indentation depth are researched simultaneously. After relaxation, misfit dislocations occur in all models, which indicates that the void does not affect the generation of misfit dislocation in γ/γ' phase. The indentation load-depth curves show the shallow void (H=1.5 nm) has the greatest influence on nanoindentation. The results demonstrate that the void has two different ways to affect the nanoindentation process. Initially, the void softens the materials when the indentation depth is less than 0.375 nm. However, it will hinder the growth of dislocations because of a kind of surface force, which causes the increase of indentation load while the indentation depth is between 0.375 nm and 0.567 nm. The collapse of a void absorbs the strain energy, so the amount of stacking faults nucleation in γ phase in model with the shallow void is less than which in the perfect model. The indentation load-depth curves show that the indentation load in the H=1.5 nm model is larger than load in the perfect model at 1.263 nm indentation depth. But when the void collapses completely, dislocations tangle around the original location of the void and more stacking faults generate comparing to the perfect model at the same indentation depth h=1.743 nm. So the indentation load declines and becomes smaller than load in perfect model. If the void locates at the interface of γ/γ' phase (H=3.0 nm), it influence the nanoindentation process later than H=1.5 nm model. Dissociation of misfit dislocations is observed when the indentation depth arrives the maximum value 1.748 nm in H=3.0 nm model. Stairs form on the surface of γ phase because of the dissociation of misfit dislocations. There is almost no influence on the nanoindentation of Ni-based single crystal alloy when the void locates in the γ' phase (H=4.5 nm).