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On the role of pre-existing defects in influencing hardness in nanoscale indentations — insights from atomistic simulations
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.