An interpretation of the kinetics of martensitic transformation in a Ni45Co5Mn36.5In13.5 alloy

We investigated the kinetics of martensitic transformation (MT) using a Ni45Co5Mn36.5In13.5 alloy. The MT start temperature Ms of this alloy decreases gradually with increasing applied magnetic field up to 1.5 T, and Ms vanishes under 2 T. Under this magnetic field, the MT shows a clear C-curve in the time–temperature transformation (TTT) diagram. To understand this behavior, we evaluated the difference in the free energy Δg of the parent phase and the martensite phase by measuring the specific heat of the alloy. We found that Ms largely deviates from the equilibrium temperature T0 as the magnetic field increases. We also found that Δg at Ms increases as Ms decreases. Using Δg thus evaluated and a thermal activation model for the nucleation of martensite, we calculated the TTT diagram for various magnetic field strengths so as to reproduce the experimentally obtained one. We estimated the size of the thermally activated cluster, which may be regarded as the nuclei of martensite, to be (5 nm)3.