Dislocation nucleation in the initial stage during nanoindentation.

The microstructure origin of the elastic-plastic response of a Cu substrate during nanoindentation is studied using molecular dynamics simulation. The elastic response is found to deviate from the Hertzian solution observed experimentally. The departure can be traced to the small tip radius used in the simulation. Further penetration sees the development of an inhomogeneous microstructure. Even at the same strain rate, different parts of the contact surface deform via different mechanisms: some elastically, some via the dislocation bow-out and some via the nucleation and growth of Shockley partials that sometimes interact to form stair-rod locks. The resultant effect produces the observed quasi-elastic behaviour on the load-displacement curve, characterized by interspersed minor yields. The present computer simulation shows in some detail the corresponding dislocation structure development. The stair-rod lock formation is found to provide a more satisfactory explanation to the experimentally observed time-delayed occurrence of pop-in below the spontaneous pop-in load. [ABSTRACT FROM AUTHOR]