Droplet Fluctuations in the Morphology and Kinetics of Martensites

TL;DR

This paper develops a theoretical framework to understand droplet configurations, kinetics, and morphology of martensites, highlighting the role of a slow vacancy mode and quench rate effects on phase stability.

Contribution

It introduces a new free-energy functional including a slow vacancy mode and explains how quench rates influence martensite formation and morphology.

Findings

Slow quenches lead to equilibrium product formation.

Fast quenches produce metastable martensite with lens-shaped nuclei.

Structural and kinetic relations are naturally explained by the theory.

Abstract

We derive a coarse grained, free-energy functional which describes droplet configurations arising on nucleation of a product crystal within a parent. This involves a new `slow' vacancy mode that lives at the parent-product interface. A mode-coupling theory suggests that a {\it slow} quench from the parent phase produces an equilibrium product, while a {\it fast} quench produces a metastable martensite. In two dimensions, the martensite nuclei grow as `lens-shaped' strips having alternating twin domains, with well-defined front velocities. Several empirically known structural and kinetic relations drop out naturally from our theory.