A NEW MODEL EXPLAINABLE FOR BOTH THE ATHERMAL AND ISOTHERMAL NATURES OF MARTENSITIC TRANSFORMATIONS IN FE-NI-MN ALLOYS

A new model, which is able to explain systematically for both the natu res of athermal and isothermal martensitic transformations, has been p roposed, referring to the experimental results obtained on the effect of magnetic fields on those martensitic transformations in Fe-Ni-Mn al loys. The model has been derived by making three assumptions, in which a new condition is introduced that martensitic transformation may sta rt when some numbers of particles make a cluster and are simultaneousl y excited in any place in the austenite. According to the model, the f ollowing characteristics were well explained: No C-curve exists for th e athermal transformation and a C-curve does for the isothermal one, d epending 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 tr ansformation start, which was involved in the model as a parameter, wa s obtained to be about 1.4 nm in cube at most by best fitting with the T T T 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 is othermal martensitic transformation in an Fe-Ni-Mn alloy is predicted.