Phase Transition in Iron Thin Films Containing Coherent Twin Boundaries: A Molecular Dynamics Approach.

Binjun Wang,Chun Xu,Yunqiang Jiang

doi:10.3390/ma13163631

Abstract

Using molecular dynamics (MD) simulation, the austenitic and martensitic phase transitions in pure iron (Fe) thin films containing coherent twin boundaries (TBs) have been studied. Twelve thin films with various crystalline structures, thicknesses and TB fractions were investigated to study the roles of the free surface and TB in the phase transition. In the austenitic phase transition, the new phase nucleates mainly at the (112)bcc TB in the thicker films. The ()bcc free surface only attends to the nucleation, when the film is extremely thin. The austenitic transition temperature shows weak dependence on the film thickness in thicker films, while an obvious transition temperature decrease is found in a thinner film. TB fraction has only slight influence on the austenitic temperature. In the martensitic phase transition, both the (fcc free surface and (111)fcc TB attribute to the new body-center-cubic (bcc) phase nucleation. The martensitic transition temperature increases with decreased film thickness and TB fraction does not influent the transition temperature. In addition, the transition pathways were analyzed. The austenitic transition obeys the Burgers pathway while both the Kurdjumov–Sachs (K–S) and Nishiyama–Wassermann (N–W) relationship are observed in the martensitic phase transition. This work may help to understand the mechanism of phase transition in the Fe nanoscaled system containing a pre-existing defect.

Highlights

Due to the industrial and scientific importance of the solid–solid phase transition, in particular the α/γ phase transition in pure iron (Fe) or in steel, significant efforts have been made to reveal the transition mechanisms [1,2,3,4]
Meiser and Urbassek [12] studied systematically the influence of different orientated face-center-cubic and bcc surfaces on the austenitic and martensitic phase transitions using molecular dynamics (MD) simulation. They found that the some surfaces, for instance, the (111)bcc surface, do not assist the phase transition as nucleation sites, while most of the free surfaces serve as the sites of the new phase nucleation
We aim to reveal the roles of free surface and twin boundaries (TBs)

Summary

Introduction

Due to the industrial and scientific importance of the solid–solid phase transition, in particular the α/γ phase transition in pure iron (Fe) or in steel, significant efforts have been made to reveal the transition mechanisms [1,2,3,4]. Compared to the bulk materials, such systems exhibit different features in phase transition due to their high surface–volume ration, for instance, surface nucleation could be often observed and some interesting phenomena such as structural reorientation or back transition to the original phase at high tensile strains in strain induced phase transition have been reported [7,8,9,10,11]. Meiser and Urbassek [12] studied systematically the influence of different orientated face-center-cubic (fcc) and bcc surfaces on the austenitic and martensitic phase transitions using molecular dynamics (MD) simulation They found that the some surfaces, for instance, the (111)bcc surface, do not assist the phase transition as nucleation sites, while most of the free surfaces serve as the sites of the new phase nucleation

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