Morphological and mechanical stability of HCP-based multilayer nanofilms at elevated temperatures

The thermal stability of Mg/Ti multilayer nanofilms was investigated by examining their microstructure and nanoindentation hardness after annealing at various temperatures and time periods. The multilayers with individual layer thickness h ≥ 5 nm exhibit excellent capability of maintaining the lamellar microstructure and high strength up to 200 °C for annealing time up to 2.0 h. The annealed multilayer films with h = 2.5 nm are still highly textured but characterized with discontinuous layer interfaces, in which the transition of atomic arrangement from hexagonal close-packed (HCP) to body-centered cubic (BCC) structure was observed at columnar boundaries. The degradation of uniform lamellar microstructure is related to the decrease of hardness with annealing temperature at this size scale. A diffusion based instability mechanism was proposed for this typical HCP-based nanoscale multilayer system.