Unified thermodynamic analysis of the stress-assisted γ→ε martensitic transformation in Fe-Mn-Si alloys

A unified thermodynamic analysis has been developed which allows the combined complex effects of applied stresses and alloy composition on the martensite start temperature, M-s(sigma), to be explained and predicted. In this unified analysis. a thermodynamic analysis of the effect of applied stress on the relative stability of the high and low temperature phase is combined with existing thermodynamic descriptions of the alloy system. The calculated results are compared with experimental results for the gamma --> epsilon martensitic transformation in Fe-Mn-Si shape memory alloys. For single crystals, the comparison shows that the influence of applied stresses can be directly predicted with a reasonable accuracy for a wide composition range. By integrating a polycrystallinity parameter in the analysis it is shown that the calculations are also valid for polycrystalline alloys. Application of this analysis to experimental values of M-s(sigma) can also generate useful data for future optimizations of the thermodynamic description of several alloy systems. (C) 1998 Acta Metallurgica Inc.