Influence of Interfaces on the Phase Stability in Nanostructured Thin Film Multilayers

摘要

Nanostructured thin film multilayers, comprising of alternating A/B layers, can exhibit metastable structures in one or both layers. From a classical thermodynamic viewpoint, the reduction interfacial energy is primarily responsible for this stabilizing effect. Based on this idea, a model has been constructed in which phase stability regions are represented as functions of both the bilayer thickness and volume fraction of the one the layers. Applying this classical thermodynamic model to a single, previously reported hcp to bcc transformation in Zr for Zr/Nb multilayers, a phase stability diagram was proposed. Various Zr/Nb multilayers with different bilayer thicknesses and volume fractions have been sputtered deposited. hcp to bcc transformations in the Zr layer were confirmed by x-ray and electron diffraction. Furthermore the Zr/Nb stability diagram predicted a novel hcp Nb phase which was subsequently verified experimentally. Using Zr/Nb as a guide, a similar phase stability diagram was constructed and experimentally determined for Ti/Nb multilayers. For each multilayer system, the reduction in interfacial energy was calculated from the experimentally determined diagram. These values were then compared to estimations of the structural component of the interfacial energy. The structural component was based on the energy per unit area of a misfit dislocation network constructed by an o-lattice. This simple assesment suggests that the reduction of the structural component of the interfacial energy is sufficient to drive the transformation.