Role of Mn and Cr on structural parameters and strain energy during FCC-HCP martensitic transformation in Fe-Mn-Cr shape memory alloys

Fe-Mn-based alloys show the shape memory effect which is mainly related to the FCC-HCP martensitic transformation. Cr is one of the additional elements which improve the properties of these alloys. In the present work structural data are obtained for the FCC austenite, and both martensitic structures, HCP and BCC, for an extended composition range where the FCC-HCP transition takes place. Lattice parameters are determined by X-Ray diffraction measurements performed at room temperature. The volume change between the austenite and each martensitic structure plays a significant role on relevant properties for martensitic transformations, like the strain energy associated to the transition. The effect of Mn and Cr on lattice parameters and volume change between FCC and HCP is determined and modeling of the data is presented. This result allows estimating the strain energy associated to the phase change. By using this information, the strain energy contribution to the balance of energy for the HCP nucleation is discussed. The addition of Cr decreases the volume change between FCC and HCP for contents larger than 12wt% Cr which leads to a decrease of the strain energy. Both effects favor an increased shape memory effect associated to the FCC-HCP martensitic transition.