Coexistence pressure for a martensitic transformation from theory and experiment: revisiting the bcc-hcp transition of iron under pressure

TL;DR

This paper investigates the coexistence pressure of the bcc-hcp transition in iron, combining theoretical calculations and experimental data to account for non-hydrostatic effects and hysteresis in phase transformations.

Contribution

It introduces an inequality for comparing theoretical and experimental coexistence pressures, revisits decades of experimental data, and provides a refined electronic-structure calculation of the transition pressure.

Findings

Theoretical coexistence pressure of 8.4 GPa for bcc-hcp transition in iron.

Accounted for non-hydrostatic stresses and hysteresis effects in experimental data.

Constructed an equation of state for phases under pressure.

Abstract

The coexistence pressure of two phases is a well-defined point at fixed temperature. In experiment, however, due to non-hydrostatic stresses and a stress-dependent potential energy barrier, different measurements yield different ranges of pressure with a hysteresis. Accounting for these effects, we propose an inequality for comparison of the theoretical value to a plurality of measured intervals. We revisit decades of pressure experiments on the bcc - hcp transformations in iron, which are sensitive to non-hydrostatic conditions and sample size. From electronic-structure calculations, we find a bcc - hcp coexistence pressure of 8.4 GPa. We construct the equation of state for competing phases under hydrostatic pressure, compare to experiments and other calculations, and address the observed pressure hysteresis and range of onset pressures of the nucleating phase.