Residual strain in metals was typically considered to be irreversible. However, residual strain in nanocrystalline materials could be recovered over a period of time via diffusive mechanisms. In this study, free-standing copper films of sub-micron thickness with an average grain size of about 40nm were mechanically loaded via a plane-strain bulge test, and residual strain recovery at room temperature was characterized after unloading. The specimens recover their residual strain in a period of time that could 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 25MPa was reached after each recovery finishes. Two characteristic strain rates occurred during residual strain recovery, a transient strain recovery rate of the order of 10−7/s and a steady-state strain recovery rate of the order of 10−9/s. A model of the plastic strain recovery was presented which demonstrated the plausibility that grain boundary diffusion driven by chemical potential gradients due to residual stresses and the presence of voids could rationalize the transient and steady-state plastic strain recovery rates, respectively.

Residual Plastic Strain Recovery Driven by Grain Boundary Diffusion in Nanocrystalline Thin Films. X.Wei, J.W.Kysar: Acta Materialia, 2011, 59[10], 3937-45