Evaluation of Grain Boundary Strength through Nanoindentation Technique

Abstract

A plasticity resistance by a grain boundary was evaluated by nanoindentation measurement in the vicinity of the grain boundary for austenitic stainless steels. Converting a load (P) - displacement (h) relation to a P/h - h curve, the plasticity resistance can be evaluated in the parameter α with hardness dimension obtained as the slope on the loading curve. The slope α is constant until the plastic zone underneath the indenter reaches to the grain boundary at a certain penetration depth (stage I), and then turns into a higher value in the deeper penetration depth range (stage II). The lower α in the stage I (α_I) means the deformation resistance in the grain interior, and that in the stage II (α_II) includes the deformation resistances of both the grain interior and the grain boundary. Therefore, Δα = α_II − α_I could correspond to the plasticity resistance by the grain boundary. It has been found that the average of Δα is higher for the nitrogen-added sample than that for the nitrogen-free one, suggesting a nitrogen effect including segregation on the plasticity resistance by a grain boundary.

Fig. 4　(a) Typical load (P)-displacement (h) curves for grain interior and near grain boundary and (b) P/h-h curves for grain interior and near grain boundary.