Phase transformation-induced strength softening in Ti/Ta nanostructured multilayers: Coherent interface vs phase boundary

In this work, hcp-to-bcc Ti phase transformation was uncovered in Ti/Ta nanostructured multilayers. The Ti nanolayers were fully transformed to bcc phase when the layer thickness (h) smaller than ~7.5 nm, accompanied with the formation of fully coherent interfaces. While in the greater layer thickness, Ti layers were partially phase-transformed, resulting in the simultaneous existence of bcc/bcc Ti/Ta coherent interfaces and bcc/hcp Ti/Ti phase boundaries. A thermodynamic model was proposed to account for the layer thickness h-dependent Ti phase transformation. The phase transformation caused an unusual size-dependent hardness, i.e., the smaller the layer thickness and the lower the hardness, which was quantitatively addressed by an ad hoc strengthening model.