The effects of elastic cycling in nanoindentation of a metallic glass

Abstract

We use nanoindentation to perform elastic cycling on, and to characterise, the bulk metallic glass Cu46Zr46Al7Gd1. From multiple loading curves, cumulative distributions are determined for several properties, including the initial yield load, the hardness and the indentation modulus. The distributions are characterised by the median property value and by their width. The effects of elastic cycling under the indenter tip are compared with those of annealing and of cryogenic thermal cycling applied to the bulk glass. We confirm that elastic cycling significantly increases the initial yield load, and find evidence for localised strengthening. In the first reports of this apparent strengthening, it was attributed to stimulation of relaxation of the glass to a more ordered structure, an effect that would be similar to that of annealing. In contrast, we conclude that the increased yield load is not due to structural ordering in the glass, but to compressive residual stress resulting from anelastic strains. While there may be effects of elastic cycling on the structure of metallic glasses, these are not revealed by loading in nanoindentation.