Thermodynamics Of Mn-based Binary Antiferromagnetic Shape Memory Alloys

The shape memory effect and high damping property in Mn-X(X=Cu,Fe,Ni)antiferromagnetic shape memory alloys are highly related to the structural transformations happening in these systems as well as the interfaces between phases.It is of great importance to investigate martensitic transformations and the heterophase interfaces from a thermodynamic perspective,since it can help us design new Mn-based antiferromagnetic shape memory alloys.In the present work,base on the sub-regular solution model of alloys,a model was proposed to calculate the undetermined thermodynamic parameters of martensites by using both the known parameters of parent phase and the equilibrium temperature-alloy composition curves.On this basis,a chemical-structural model was proposed to calculate the heterophase coherent interfacial energy,which includes the chemical interfacial energy as well as the structural interfacial energy.In the aspect of phase transformation thermodynamics,with the calculated parameters,thermodynamic issues related to the fcc-fct phase transformation in Mn-Cu and Mn-Fe alloys and the fcc-fct-fco phase transformation in Mn-Ni were studied,such as the dependences of Gibbs free energies on the temperature,the influences of alloy composition on critical driving forces,entropies,enthalpies and heat capacities.Also,the transition orders of these transformations were discussed.In the aspect of interfacial thermodynamics,heterophase coherent interfacial energies in Mn-based binary alloys were calculated by using chemical-structural model.In this model,the coherent heterophase interfacial energy consisted of chemical and structural parts.Resulting from the formation process of the heterophase interface,the chemical interfacial energy was expressed as the incremental value of bond energy,while the structural part was obtained by calculating the interfacial strain energy.Based on the proposed model,interfacial energy,interfacial entropy,interfacial enthalpy and interfacial heat capacity were found to be correlated with temperature and interfacial preferring orientation.Furthermore,the influences of alloy composition,modulus softening,and the index of habit plane on the results were discussed.