Thermodynamic study of fcc-fct-fco multi-step structural transformation in Mn-Ni antiferromagnetic shape memory alloys

Abstract Shape memory effect in Mn-Ni antiferromagnetic alloys is closely related to the structural transformations that happen in this system, especially the fcc→fct→fco multi-step structural transformation. In order to discuss the change of Gibbs free energy of this alloy system during the multi-step transformation process, thus study the mechanism of shape memory effect from the standpoint of thermodynamics of fcc→fct→fco transformation, the unknown thermodynamic parameters of fct and fco phases in Mn-Ni alloys must be determined. In this work, based on the application of subregular solution model of the binary system and equilibrium relationships between different phases given in the Mn-Ni phase diagram, these unknown thermodynamic parameters were determined by solving overdetermined equation systems, and dependencies of Gibbs free energies of every phase in Mn-Ni antiferromagnetic alloys on temperature were obtained. The results of calculations show that the critical driving force of fcc→fct martensitic transformation is about 10 J/mol and higher than that of fct→fco transformation (about 1 J/mol), while the enthalpy, entropy and heat capacity of fcc→fct transformation are several times higher than the equivalent parameters for the fct→fco transformation. The dependences of critical chemical driving force, enthalpy, entropy and heat capacity on the alloy composition and temperature were calculated and discussed for the fcc→fct→fco multistep structural transformation. Based on these results, the orders of the two transformations in the fcc→fct→fco multi-step transformation happening in this system were compared with that of the single-step phase transformation, and were found to be all of first-order and to be weakened during the multi-step transformations.