Nanoindentation creep behavior of nanocrystalline Ni and Ni-20 wt% Fe alloy and underlying mechanisms revealed by apparent activation volumes

Nanoindentation creep behavior and underlying mechanisms of nanocrystalline (NC) Ni and NC Ni-20 wt% Fe (Ni-Fe) alloy were investigated at room temperature by a new testing method. The continuous creep strain rate versus creep stress data and hence the continuous apparent activation volume versus creep stress data in the entire holding stage on each single sample were achieved under a loading rate-controlled mode by a cooperative use of the continuous stiffness measurement (CSM) technique. Furthermore, the dependence of the apparent activation volume on the creep stress was interpreted by performing a theoretical analysis. Based on the above experimental and theoretical results and the high-resolution transmission electron microscope observation of the microstructures, the effects of the loading rate and stacking fault energy on the nanoindentation creep behavior of NC Ni and NC Ni-Fe alloy and the underlying mechanisms were analyzed. It was demonstrated that NC Ni-Fe alloy shows higher creep resistance in the transient regime, but lower creep resistance in the steady-state regime compared to NC Ni. The contact stiffness data obtained by the CSM technique in a strain rate-controlled mode can be used to determine the creep stress data from the creep displacement data obtained in a loading rate-controlled mode