Solid-solid transformations via nanoscale intermediate interfacial phase: Multiple structures, scale and mechanics effects

Solid-solid (SS) phase transformations via nanometer-size intermediate melts (IMs) within the SS interface, hundreds of degrees below melting temperature, were predicted thermodynamically and are consistent with experiments for various materials. A necessary condition for the appearance of IMs, using a sharp interface approach, was that the ratio of the energies of SS and solid-melt (SM) interfaces, k(E), were >2. Here, an advanced phase-field approach coupled with mechanics is developed that reveals various new scale and interaction effects and phenomena. Various types of IM are found: (i) continuous and reversible premelting and melting; (ii) jump-like barrierless transformation to IMs, which can be kept at much lower temperature even for k(E) <2; (iii) unstable IMs, i.e. a critical nucleus between the SS interface and the IM. A surprising scale effect related to the ratio of widths of SS and SM interfaces is found: it suppresses barrierless IMs but allows IMs to be kept at much lower temperatures even for k(E) <2. Relaxation of elastic stresses strongly promotes IMs, which can appear even at k(E) <2 and be retained at k(E) = 1. The theory developed here can be tailored for diffusive phase transformations, formation of intergranular and interfacial phases, and surface-induced phase transformations. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.