Low-temperature criticality of martensitic transformations of Cu nanoprecipitates in \alpha-Fe

TL;DR

This study uses atomistic simulations to explore how Cu nanoprecipitates in e undergo size-dependent martensitic transformations at low temperatures, revealing a unique size-temperature phase diagram influenced by entropy effects.

Contribution

It provides the first detailed atomistic mapping of the size-temperature phase diagram for Cu nanoprecipitates in e, highlighting the role of entropy and soft modes in phase stability.

Findings

Martensitic transformations occur at decreasing sizes with cooling.

Hysteresis disappears below 300 K, but the transition remains discontinuous.

Entropy from soft modes stabilizes the BCC-Cu phase under confinement.

Abstract

Nanoprecipitates form during nucleation of multiphase equilibria in phase segregating multicomponent systems. In spite of their ubiquity, their size-dependent physical chemistry, in particular at the boundary between phases with incompatible topologies, is still rather arcane. Here we use extensive atomistic simulations to map out the size-temperature phase diagram of Cu nanoprecipitates in \alpha-Fe. The growing precipitates undergo martensitic transformations from the body-centered cubic (BCC) phase to multiply-twinned 9R structures. At high temperatures, the transitions exhibit strong first-order character and prominent hysteresis. Upon cooling the discontinuities become less pronounced and the transitions occur at ever smaller cluster sizes. Below 300 K the hysteresis vanishes while the transition remains discontinuous with a finite but diminishing latent heat. This unusual size-temperature phase diagram results from the entropy generated by the soft modes of the BCC-Cu phase, which are stabilized through confinement by the \alpha-Fe lattice.