Inhomogeneity and size of Microstructure - A new paradigm in understanding deformation mechanism of nanocrystalline metals

We present experimental evidence showing that plastically deformed polycrystalline free standing thin metal films with average grain size of 50-65 nm recover a large fraction of plastic strain under macroscopically stress-free condition. We hypothesize, inhomogeneities in the microstructure (grain size and orientation variations) create non uniform local stress fields during loading as relatively larger or favorably oriented grain deform plastically while smaller or unfavorably oriented grains accommodate the strain elastically. Upon unloading, the elastically deformed grains, in order to reduce their strain energy, induce reverse plasticity in the larger grains leading to time dependent strain recovery in these films.