Using in-situ nanoindentation experiments it is possible to study the dislocation mechanisms which unfold under an indenter.Large-scale atomistic simulations of the same are possible due to similarities in length scale, provided that defects can be included in the simulation. Yet, nanoindentation simulations have so far been mostly undertaken on defect free samples, while studies with pre-existing defects are few. The latter show that the average hardness is not affected by the presence of pre-existing defects, which justifies the use of ideal crystals in such simulations. However, this observation is counter-intuitive, as indenter-defect interactions should lead to work hardening and manifest themselves in hardness calculations. Our simulations along with a new look at the evolution of dislocations under the indenter, show for the first time, that hardness in atomistic simulations is influenced by pre-existing defects in the sample. Utilizing a face-centred tetragonal TiAl bicrystal with misfit dislocations at the interface, to populate the sample with defects, we correlate the contact-pressure variations to defect-indenter interactions. We show that the measured contact-pressure is affected by the presence and nature of defects under the indenter. Dislocation pile ups lead to intermittent rise in contact pressure, while seamless growth leads to steady convergence. The sensitivity to detect such defect interactions depends upon indenter size while convergence to average hardness is a result of curvature accommodation near the surface. Our findings prove that pre-existing defects have a profound influence on calculated hardness in indentation simulations which also corroborates with experimental observations in the literature.

使用原位纳米压痕实验可以研究在压头下展开的位错机制。由于长度尺度的相似性，如果可以在模拟中包含缺陷，则可以进行大规模原子模拟。然而，迄今为止，纳米压痕模拟主要是在无缺陷样品上进行的，而对预先存在的缺陷的研究很少。后者表明平均硬度不受预先存在缺陷的影响，这证明在此类模拟中使用理想晶体是合理的。然而，这种观察是违反直觉的，因为压头-缺陷相互作用会导致加工硬化并在硬度计算中表现出来。我们的模拟以及对压头下位错演变的新看法，首次显示，原子模拟中的硬度受样品中预先存在的缺陷的影响。利用在界面处具有错配位错的面心四方 TiAl 双晶，用缺陷填充样品，我们将接触压力变化与缺陷-压头相互作用相关联。我们表明，测量的接触压力受压头下缺陷的存在和性质的影响。位错堆积导致接触压力间歇性上升，而无缝增长导致稳定收敛。检测这种缺陷相互作用的灵敏度取决于压头尺寸，而平均硬度的收敛是表面附近曲率调节的结果。