Effect of spherical indenter radius and loading rate on the kinetic nanoindentation creep behavior of La-based metallic glasses

Abstract

Although the effect of the deformation field size on the mechanical strength of metallic glasses (MGs) has been investigated thoroughly, a few studies have investigated the effect of the deformation field size on the kinetic creep deformation behavior for these materials. This issue is addressed in the current work by investigating the nanoindentation creep kinetics of La-based MGs under the application of spherical indenters with two different tip radii and three different loading rates. The experimental data are fit to a viscoelastic model to extract the activation volume and relaxation time characteristics of the kinetic nanoindentation creep deformation of the samples. The results demonstrate that the nanoindentation creep kinetics are not only strongly dependent on the loading rate, but also vary significantly with the indenter tip radius. The deformation dynamics is interpreted by the stretched exponent β, indicating its relatively small value at a large loading rate along with a large indenter tip radius. Furthermore, the observed effect of the deformation field size can be attributed to an interplay between the deformation field under the spherical indenter and flow defects associated with the excess free volume, shear transition zone, or shear band embryo with an evolution that is essentially dependent on the loading rate. The current findings contribute to a fundamental understanding of the mechanism of shear softening during creep deformation in MGs.