Size dependent phase transformation and mechanical behaviors in nanocrystalline Ta thin films

Nanocrystalline (NC) Ta thin films with various thicknesses (t = 600, 1200, and 2200 nm) were grown on Si (100) substrate by using magnetron sputtering system. A phase transformation from β-Ta to α-Ta was observed when the thickness reduced to 600 nm, which is rationalized by employing a thermodynamic model. It was interesting to find that the α-Ta phase exhibited tetrahedral pyramidal grain morphology, while the β-Ta had equiaxial grain. Hardness and strain rate sensitivity (SRS, m) were measured as a function of film thickness t. Compared with the reduction in SRS as the grain size decreased in the submicron bcc metals, the NC Ta thin films with average grain size smaller than ~72 nm showed an opposite trend, both in the α-Ta and in the β-Ta phase. Improved m was observed as the grain size reduced but the increasing trend was not continuous between different phases. This unusual variation trend of m in the NC Ta was explained by a model based on the traditional double-kink mechanism coupled with a GB-mediated dislocation process, which agrees well with the experiment results.