Abstract
A new model, which is able to explain systematically for both the natures of athermal and isothermal martensitic transformations, has been proposed, referring to the experimental results obtained on the effect of magnetic fields on those martensitic transformations in Fe–Ni–Mn alloys. The model has been derived by making three assumptions, in which a new condition is introduced that martensitic transformation may start when some numbers of particles make a cluster and are simultaneously excited in any place in the austenite. According to the model, the following characteristics were well explained: No C-curve exists for the athermal transformation and a C-curve does for the isothermal one, depending on a difference in their temperature dependence of the Gibbs chemical free energy difference between the austenitic and martensitic states. The minimum size of the cluster for making the martensitic transformation start, which was involved in the model as a parameter, was obtained to be about 1.4 nm in cube at most by best fitting with the TTT diagram of the isothermal martensitic transformation previously examined. Based on the model, the influence of external forces, such as uniaxial stress, magnetic field and hydrostatic pressure, on the isothermal martensitic transformation in an Fe–Ni–Mn alloy is predicted.
