Effect of magnetic state on the $\gamma -\alpha$ transition in iron: First-principle calculations of the Bain transformation path

TL;DR

This study uses first-principle calculations to explore how magnetic states influence the energetics and mechanism of the gamma-alpha phase transition in iron, highlighting the role of the Curie temperature.

Contribution

It provides a detailed computational analysis of the magnetic influence on the Bain transformation path in iron, emphasizing the significance of the Curie temperature in the transition mechanism.

Findings

Magnetic order affects the energetics of the gamma-alpha transition.

The transformation mechanism changes at the Curie temperature.

The results explain the proximity of the Curie temperature to the martensitic start temperature.

Abstract

Energetics of the fcc ($\gamma$) - bcc ($\alpha$) lattice transformation by the Bain tetragonal deformation is calculated for both magnetically ordered and paramagnetic (disordered local moment) states of iron. The first-principle computational results manifest a relevance of the magnetic order in a scenario of the $\gamma$ - $\alpha$ transition and reveal a special role of the Curie temperature of $\alpha$-Fe, $T_C$, where a character of the transformation is changed. At a cooling down to the temperatures $T < T_C$ one can expect that the transformation is developed as a lattice instability whereas for $T > T_C$ it follows a standard mechanism of creation and growth of an embryo of the new phase. It explains a closeness of $T_C$ to the temperature of start of the martensitic transformation, $M_s$.