Atomistic prediction of the temperature- and loading-rate-dependent first pop-in load in nanoindentation

Abstract

Nanoindentation has been used for a long time to investigate the mechanical properties of materials. A well-known displacement burst, the so-called “pop-in,” is usually observed during nanoindentation tests. In particular for the first pop-in has been well studied because it should be directly related to the fundamental mechanical strength of the local volume beneath the indenter. The first pop-in load measured for a defect-free local volume is corresponding to the onset load of homogeneous dislocation nucleation, while measured for a local volume with immobile defects is that of heterogeneous dislocation nucleation. The first pop-in is a thermally activated event under loading; thus, the pop-in load exhibits both temperature and loading-rate dependencies. Although theoretical studies have successfully explained the temperature and loading-rate dependencies, to the best of our knowledge, an atomistic prediction for specific materials which takes into account the stress state beneath the indenter has not yet been reported. In this study, we propose an atomistically informed multiscale (two-scale) modeling to predict the temperature and loading-rate dependencies of the first pop-in load for the homogeneous dislocation nucleation considering an atomistically estimated stress state beneath the indenter and stress-dependent activation energies of dislocation nucleation. Body-centered-cubic Fe and Ta are chosen as target metals, although the method is generally applicable to any ductile material. In addition to the homogeneous dislocation nucleation, we also developed a heterogeneous nucleation theory to explain the difference between the experimentally and atomistically obtained homogeneous nucleation pop-in loads.