The effect of crystal orientation on the stochastic behavior of dislocation nucleation and multiplication during nanoindentation

Current methods to measure the theoretical shear strength of metals using nanoindentation often present a stochastic view of the applied stresses needed to nucleate dislocations. In this study a combination of molecular dynamics simulations and experimental nanoindentation tests were used to explore the coupled effects of indenter size, crystallographic orientation, and the presence of internal structural defects on the resulting distribution of loads at the onset of plastic deformation in face-centered cubic metals. In this case stacking fault tetrahedra have been selected as a representative structural, rather than chemically distinct, defect. The sensitivity of the crystal to the presence of internal structural defects depends strongly on its crystallographic orientation. Simulations of indentations in the presence of a stacking fault tetrahedron show the highest reduction in the pop-in load for the (1 1 1) orientation, while experimentally the effect of orientation is dependent on the size of the indenter used, and hence the volume of material under stress. The simulations suggest that indenting near a defect can cause small, sub-critical events to occur which then lead to a large “pop-in” at higher loads, and thus the first event observed experimentally may not correspond to the first plastic deformation event. As internal defects are almost inevitable in materials, a defect-based model can be used to explain the stochastic pop-in loads in nanoindentation tests.