Investigation of Plastic Strain Recovery and Creep in Thin Film Nanocrystalline Metals

In this study an automatically controlled plane-strain bulge test system is used to characterized the mechanical behavior of nanocrystalline thin film samples. The free-standing thin film of copper with average grain size of 35nm is fabricated with thermal evaporation or sputtering. The tests are performed to measure Young's modulus, determine the strain rate for creep and monitor plastic strain recovery at room temperature and at 100°C Based on the experimental strain rate during the creep, The value for diffusion coefficient of copper is obtained. This value is in agreement with the diffusion coefficient resulted from numerical simulation for nanocrystalline copper film in another work and is about 4 orders of magnitude more than the value for conventional coarse grain one. By monitoring the plastic strain recovery, it is observed that it occurs in two rates, a fast temporary one follow by a slower rate. This phenomena can be explained due to grain boundary based deformation mechanisms for this grain size. We also develop a continuum model to prescribe grain boundary diffusion as the dominant deformation mechanism for nanocrystalline thin film with a preexisting void. The model is implemented using FEA software Abaqus. The numerical result indicates that plastic strain recovery occurs and it has two rates. A parametric study on different factors which can affect the recovery is performed and the strain recovery rates obtained from each parameter are compared with the experimental one.