Time-dependent nature of displacive transformations in Fe–Ni and Fe–Ni–Mn alloys under magnetic field and hydrostatic pressure

Abstract

We have investigated athermal and isothermal martensitic transformations (typical displacive transformations) in Fe–Ni and Fe–Ni–Mn alloys under pulsed and static magnetic fields and hydrostatic pressures in order to understand the time-dependent nature of martensitic transformation, i.e. the kinetics of martensitic transformation. Following results were obtained, (i) in Fe–Ni alloys, which exhibit athermal martensitic transformations, isothermal holding at a temperature above respective M s temperature induces a martensitic transformation isothermally after some finite incubation time. (ii) In an Fe–32.6at.%Ni alloy, which originally does not show any athermal martensitic transformation, a martensitic transformation occurs isothermally after some finite incubation time. Moreover, in the alloy, a pulsed magnetic field induces an athermal martensitic transformation. (iii) A hydrostatic pressure changes the transformation process from athermal to isothermal one in Fe–31.7 and –32.3at.%Ni alloys. (iv) In an Fe–24.0Ni–4.0Mn (at.%) alloy, which exhibits an isothermal martensitic transformation, a static magnetic field lowers the nose temperature and reduces the incubation time and a hydrostatic pressure raises the nose temperature and increases the incubation time. These findings can be explained by the phenomenological theory, which gives a unified explanation for the two transformation processes previously proposed by our group.