Thermal activation in Au-based bulk metallic glass characterized by high-temperature nanoindentation

High-temperature nanoindentation experiments have been conducted on a Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 bulk metallic glass from 30 to 140 °C, utilizing loading rates ranging from 0.1 to 100 mN/s. Generally, the hardness decreased with increasing temperature. An inhomogeneous-to-homogeneous flow transition was clearly observed when the test temperature approached the glass transition temperature. Analyses of the pop-in pattern and hardness variation showed that the inhomogeneous-to-homogeneous transition temperature was loading-rate dependent. Using a free-volume model, the authors deduced the size of the basic flow units and the activation energy for the homogeneous flow. In addition, the strain rate dependency of the transition temperature was predicted.