Phase stability and fcc/hcp martensitic transformation in Fe–Mn–Si alloys: Part II. Thermodynamic modelling of the driving forces and the MS and AS temperatures

Abstract

This paper presents the second part of a study of the fcc/hcp relative phase stability and the fcc↔hcp martensitic transformation (MT) in the Fe–Mn–Si system. In part I, an experimental database was built up using dilatometric measurements, which covers the composition range in which the fcc↔hcp MT is detected. This new information is analysed in the present work using models for the molar Gibbs energy (Gm) of the various phases. Hcp is a metastable phase in the system, but we show that its properties can be inferred from selected pieces of experimental data, using the concept of T0 temperature. The assessed Gm functions of the ternary fcc and hcp phases are used to evaluate the so-called resistance-to-start-the-transformation energy (RSTE). According to our results the RSTE in this system vary smoothly with composition, and we account phenomenologically for that using low-order polynomials in the atomic fractions. Finally, the optimum Gm and RSTE functions are also used to calculate the MS and AS temperatures for arbitrary compositions in the ternary system. The extensive comparisons between calculations and measurements presented in this work show a very good agreement, which adds to the credibility of the present approach.