The Influence of Aging on the Mechanical Behavior of Sputter Deposited Ni-Mo-W Thin Films with Mo Content Above the Solubility Limit

Alloying elements that i) segregate toward grain boundary or ii) facilitate the generation of low-energy boundaries (e.g. low-angle grain boundary, twin boundary) have shown to be effective approaches for stabilizing nanocrystalline materials. However, in practice, many of these successes were limited by the formation of intermetallic compound or elemental precipitates at elevated temperatures. Therefore, the temperature at which secondary phases starts to form and the attendant changes on its mechanical behavior are crucial information in designing stable nanostructured metals with exceptional strength.<br/>Here we report the microstructural and mechanical property of sputter deposited, freestanding Ni-Mo-W alloy thin films with Mo concentration above the solubility limit (20 at.%). The as-deposited films exhibit single-phase solid solution with grains filled with nanoscale twin boundaries and possess high tensile strength of approximately 3 GPa. Upon post-annealing, grain growth and nucleation of MoNi phase was observed. Interestingly, films retained its tensile strength of 2.5-3.0 GPa even after heat treatment up to 700°C, while the elastic modulus started to increase at temperature above 500°C. We discuss the influence that precipitate can impart, including the changes in tensile and fracture properties.