, W Gerberich, K A. Yoder
Nanoindentation of single crystals has been a topic of recent investigations. This is a result of their ability to withstand near theoretical stresses without showing signs of plastic deformation. When plasticity occurs, it produces a yield point, a sudden discontinuous increase in indenter displacement and decrease in contact pressure. It has been suggested that dislocation nucleation is the controlling mechanism for the initiation of plasticity under these conditions, while the importance of an oxide or contaimination layer has largely been discounted. This study combines atomic force microscopy (AFM) with nanoindentation to focus on the roles that oxide and asperities play in the yield point process. Time dependent and instantaneous yield point properties were investigated for single crystals of tungsten and Fe 3%-Si in variable temperature and variable humidity environments. AFM observations indicate that the presence of aperities has a dramatic effect on the time dependent yield point properties. AFM measurements also provide evidence for plasticity in the absence of a yield point, suggesting that dislocation nucleation can occur well before a yield point is observed. Measurements on the dependence of yield point load on oxide film thickness are used to develop a fracture mechanics based model in which oxide fracture controls the yield point process. The results suggest that dislocation egress occurs upon oxide fracture, resulting in a yield point.
Surface Constrained Plasticity: Oxide Rupture and the Yield Point Process
nanoindentation, oxide films, plasticity, single crystal