Composition-mediated abnormal phase evolution in Ta-W films with Cr buffer layers

With the monotonically increasing of W from 0 to 20 at. % in Ta-W films with Cr buffers, an abnormal phase evolution of alpha-Ta ->beta-Ta ->alpha-Ta was found, which is due to the breaking and rebuilding of alpha-Ta nucleation. A small amount of W addition could disturb the epitaxial growth of alpha-Ta on Cr layers by refining the nucleus size. Nevertheless, as a completely miscible system, Ta-W solid solutions are preferred to possess the alpha-Ta structure. With the increasing W content, the nucleation barrier of alpha-Ta is correspondingly lower, facilitating the rebuilding of alpha-Ta growth. The effect of phase evolution competes with that of solid solution strengthening in the controlling mechanism of Ta-W hardness. The hardness of the sample with low W contents is largely influenced by the proportion of beta-Ta, while the solid solution strengthening effect is more pronounced in high W.