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

Abstract 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.