Nanoindentation hardness of bulk metallic glass: Smaller is softer or harder?

Abstract

One of the issues often focused in many researches through the nanoindentation experiments with a geometrically self-similar pyramidal indenter is so-called indentation size effect (ISE), i.e., an increase in hardness with decreasing penetration depth. Based on Ashby`s suggestion that geometrically necessary dislocations (GNDs) would nucleate for accommodating plastic strain gradients in bending or flat-punch indentation, many works on the ISE proposed possible relationship between the GNDs and the ISE (e.g., a popular mechanism-based Nix-Gao model). Since these models are mainly based on Taylor`s dislocation strengthening, one might imagine that the models do not hold valid for bulk metallic glasses (BMGs) which do not contain crystalline defects such as dislocations and grain boundaries. Despite the expectation of the size-independent hardness or strength in BMGs due to the absence of the dislocation, some experiemtal indentation studies reported the possible ISE. Additional controversy over the size effect of BMGs arose in recent studies performing microcompression tests: While some suggested `smaller is softer,` `smaller is stronger` is reported by others. Also, very recent micro-pillar test showed that the strength of BMG is indeed size-independent. To shed light on the dispute on the size effect of BMG, we systematically analyzed the ISE of BMG through nanoindentations with a series of three-sided pyramidal indenters having different indenter angles. The purpose of this presentation is to report our recent experimental results, which led us to somewhat surprising observation on the issue. * This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant (No. R01-2008-000-20778-0) funded by the Korea government, MEST.