Incubation times and entropy barriers in martensitic kinetics: Monte Carlo quench simulations of strain pseudospins

Abstract

Martensitic materials quenched from the austenite phase can show hugely different conversion kinetics: explosively rapid ("athermal"), or slowly incubated ("isothermal"). This traditional sharp distinction was queried by experiments finding conversion-incubation delay tails even in athermal martensites, at temperatures where only austenite should exist. To understand martensitic kinetics, we perform systematic Monte Carlo temperature-quench simulations of a protoypical martensitic model of S=0, ±1 strain pseudospins, with compatibility-induced, power law anisotropic interactions, and no extrinsic disorder. We find both athermal or isothermal behaviour in the same model, depending on parameters. In the athermal regime, the puzzling experimental temperature-time behaviour for conversions is reproduced: explosive conversions (below a spinodal), do indeed coexist with rising incubation-delay tails. A Vogel-Fulcher divergence at transition is predicted, in a region of tweed-like precursors. Incubations are explained as searches for rare, finite-scale transitional states, that are explicitly identified. Although complex textural changes occur during incubation, the energies are quite flat, in a signature of entropy barriers. The model suggests systematic quench experiments in martensites.