Interactions between Coherent Twin Boundaries and Phase Transition of Iron under Dynamic Loading and Unloading

TL;DR

This study explores how coherent twin boundaries influence phase transitions in iron under dynamic conditions, revealing mechanisms of twin-assisted transitions, memory effects, and implications for material strengthening.

Contribution

It provides the first detailed investigation of twin boundary interactions with phase transitions in BCC iron using molecular dynamics and nudged elastic band methods.

Findings

Identified orientation relationships between BCC and HCP phases.

Discovered the 'memory' effect in reverse phase transition.

Analyzed twin spacing and energy barriers in phase transformation.

Abstract

Under high pressures, phase transition, as well as deformation twins, are constantly reported in many BCC metals, whose interactions are of fundamental importance to understand strengthen mechanism of these metals under extreme conditions. However, the interactions between twins and phase transition in BCC metals are remain largely unexplored. In this work, interactions between coherent twin boundaries and $\alpha \to \epsilon$ phase transition of iron are investigated using both non- equilibrium molecular dynamics simulations and nudge elastic band method. Mechanisms of both twin-assisted phase transition and reverse phase transition are studied and orientation relationships between BCC and HCP phase are found to be <11-1>BCC || <-12-10>HCP and <1-10>BCC || <0001>HCP for both cases. The twin boundary corresponds to after the phase transition. It is amazing that the reverse transition seems to be able to "memory" and recover the initial BCC twins. The memory would partly loss when plastic slips take place in the HCP phase before the reverse transition. In the recovered initial BCC twins, three major twin spacing are observed, which are well explained in terms of energy barriers of the transition from HCP phase to BCC twin. Besides, variant selection rule of the twin assisted phase transition is also discussed. The results of present work could be expected to give some clues for producing ultra-fine grain structure in materials exhibiting martensitic phase transition.