Residual plastic strain recovery driven by grain boundary diffusion in nanocrystalline thin films

Residual strain in metals is typically considered to be irreversible. However, residual strain in nanocrystalline materials can be recovered over a period of time via diffusive mechanisms. In this study, free-standing copper films of submicron thickness with an average grain size of about 40 nm are mechanically loaded via a plane-strain bulge test, and residual strain recovery at room temperature is characterized after unloading. The specimens recover their residual strain in a period of time that can range from a few days to more than 1 month depending upon the surface conditions and heterogeneous residual strain distributions in multiple cycles of recoveries. A constant tensile stress of about 25 MPa is reached after each recovery finishes. Two characteristic strain rates occur during residual strain recovery, a transient strain recovery rate of the order of 10−7 s−1 and a steady-state strain recovery rate of the order of 10−9 s−1. A model of the plastic strain recovery is presented which demonstrates the plausibility that grain boundary diffusion driven by chemical potential gradients due to residual stresses and the presence of voids can rationalize the transient and steady-state plastic strain recovery rates, respectively.