Transient strain rate sensitivity exponent from nanoindentation creep at constant depth

Abstract

Activation parameters derived from nanoindentation creep tests, such as the transient creep exponent, strain rate sensitivity (SRS) exponent and activation volume, typically show indentation depth or load dependence. This has led to differing interpretations of the underlying deformation mechanisms in nanoindentation creep literature. In this study, we investigate the determination of the SRS exponents from constant load hold (CLH) nanoindentation creep experiments using two distinct approaches: (i) the widely used single creep segment method, in which the plastic zone volume continuously expands during the hold period, and (ii) a newly proposed constant depth method that utilizes multiple creep curves, thereby maintaining approximately constant plastic zone volume. The SRS exponents obtained from both approaches are compared with those derived from the more established nanoindentation strain-rate jump (SRJ) tests. All experiments were performed on nanocrystalline nickel (nc Ni) with a spherical tip. While both creep-based approaches show an increase in SRS with indentation depth, the constant-depth method exhibits a substantially weaker dependence. The observed trends suggest that the increase in SRS during nanoindentation creep may be associated with the evolution of primary creep exponent itself.