Depth dependency of indentation hardness during solid-state phase transition of shape memory alloys

We conducted the measurement of the hardness-depth relationship of NiTi shape memoryalloy with a sharp Berkovich indenter. Different from most ductile metals, NiTi reacts to the mechanical load of indentation through phase transition underneath the indentation tip. We found that the hardness decreases rapidly with the increase of the indentation depth and eventually approaches a constant. To understand the depth dependency, we performed energy analysis involving the bulk and the interface energies of the transformation zone. We derived the hardness-depth relationship which well explains the experimental results. The finding is useful in hardness measurement of materials involving solid-state phase transitions.