Elastic and Plastic Strains Misfits During the Reverse Martensitic Transformation

Abstract

The properties of technological interest of ferromagnetic shape memory alloys (FSMAs) are linked to the occurrence of a thermoelastic martensitic transformation (MT). In the particular case of FePd alloys, a structure of twinned variants is formed by cooling as a result of the MT to minimize the internal stresses developed during the fcc–fct transformation. Different models have been proposed to analyze the internal strains and stresses but only describing the state after the complete MT. In this work, a mean-field model based on the Eshelby inclusion problem (EIP) is developed for the reverse MT characterization. This model allows determining the internal elastic and plastic strains during the evolution of this transition. The developed model is applied to a Fe66.8Pd30.7Mn2.5 alloy. For this study, the reorientation of martensite variants responsible for the plastic effect in Fe66.8Pd30.7Mn2.5 alloy was considered. It should be highlighted that the elastic and plastic response during the reverse MT could be monitored by the evolution of two elastic and elastoplastic misfit coefficients. The highest values of internal elastic and elastoplastic strains are obtained at the maximum of the internal stresses; this occurs when both transforming phases coexist around the midpoint of transformation. Moreover, the internal strain in martensite is higher than in austenite in the whole reverse MT temperature range in Fe66.8Pd30.7Mn2.5 alloy. According to the model, the maximum plastic strain is less than 10 pct of the total strain. In this context, the present study has been applied to analyze the effect of microstructure changes due to precipitation on the internal stresses linked to the reverse MT.