Cubic to hexagonal iron phase transition promoted by interstitial hydrogen

TL;DR

This study uses ab-initio density functional theory to show how interstitial hydrogen influences the phase transition of iron from bcc to hcp, involving internal stress, magnetic changes, and volume variations that may cause embrittlement.

Contribution

It reveals the mechanism by which interstitial hydrogen promotes the bcc to hcp phase transition in iron through internal stress and magnetic reorganization.

Findings

Hydrogen induces internal stress facilitating phase transition.

Magnetic reorganization accompanies the structural change.

Volume variations may lead to embrittlement.

Abstract

Using ab-initio density functional theory we study the role of interstitial hydrogen on the energetics of the phase transformation of iron from bcc to hcp along Bain's pathway. The impurity creates an internal stress field that can be released through a tetragonal distortion of the lattice, promoting the bcc (ferromagnetic) $\rightarrow$ fcc (frustrated antiferromagnetic) $\rightarrow$ hcp (ferromagnetic) transition. The transformation between crystal systems is accompanied by a drastic magnetic reorganization and sudden variations of the unit cell volume, that can be one of the reasons for embrittlement and mechanical failure of iron upon hydrogen adsorption.