Role of magnetic degrees of freedom in a scenario of phase transformations in steel

TL;DR

This paper investigates how magnetic degrees of freedom influence phase transformations in steel, demonstrating that magnetic states of iron critically control the transformation pathways and resulting microstructures.

Contribution

It introduces a computational model combining first-principles calculations and phase-field simulations to elucidate the magnetic influence on steel phase transformations.

Findings

Magnetic state of iron governs transformation character

Proposed model aligns well with experimental transformation curves

Simulations reproduce typical microstructure patterns

Abstract

The diversity of mesostructures formed in steel at cooling from a high-temperature austenite ("gamma") phase is determined by the interplay of shear reconstructions of crystal lattice and diffusion of carbon. Combining first-principles calculations with large-scale phase-field simulations we demonstrate a decisive role of magnetic degrees of freedom in the formation of energy relief along the Bain path of "gamma"-"alpha" transformation and, thus, in this interplay. We show that there is the main factor, namely, the magnetic state of iron and its evolution with temperature which controls the change in character of the transformation. Based on the computational results we propose a simple model which reproduces, in good agreement with experiment, the most important curves of the phase transformation in Fe-C, namely, the lines relevant to a start of ferrite, bainite, and martensite transformations. Phase-field simulations within the model describe qualitatively typical patterns at these transformations.