Effects of particle and grain sizes on martensitic transformation in an Fe–30.5 at.%Ni alloy

Abstract

Abstract We have investigated martensitic transformation in micrometer-sized single and poly crystalline particles of an Fe–30.5 at.%Ni alloy by their temperature dependences of magnetic susceptibility measurement and X-ray diffraction. Following results are obtained: (1) poly crystalline particles show a “so-called” athermal martensitic transformation and M s temperature decreases from 215 K to 151 K with decreasing particle size from 1000 μm (average grain size is 250 μm) to 18 μm (average grain size is 3.7 μm). The single crystalline particle with its size of 7.6 μm also shows a “so-called” athermal martensitic transformation as in the poly crystalline particles, but its M s temperature decreases drastically ( M s = 77 K). (2) The single crystalline particle with its size of 4.6 μm has no M s temperature and shows a “so-called” isothermal martensitic transformation. However, M s temperature appears again in this particle when we introduce grain boundaries and dislocation in the particle by sintering and/or deforming. Considering these results, we conclude that the suppression of martensitic transformation in the micrometer-sized particles of an Fe–Ni alloy is due to the lack of lattice defects for the nucleation, such as grain boundary and dislocation. Furthermore, the single crystalline particle with its size of 4.6 μm shows the athermal martensitic transformation again under the magnetic field and its chemical driving force of the martensitic transformation is estimated to be about 2.0 kJ/mol.