The influence of time, temperature, and grain size on indentation creep in high-purity nanocrystalline and ultrafine grain copper

Microhardness measurements have been carried out on high purity Cu samples with average grain sizes ranging from $\sim 10$ to $\sim 200\mathrm{nm}$ , over temperatures from liquid nitrogen to ambient, and dwell-times of the indenter in the sample from 5 s to 39 h. The Vickers hardness diminishes approximately linearly with the logarithm of the dwell-time. At short dwell-times the hardness increases significantly with decreasing grain size and with decreasing temperature, but the influence of these variables substantially diminishes at longer times. Investigation by transmission electron microscopy shows that rapid grain growth under the indenter most likely is responsible for the strong and long-lasting indentation creep.