The kinetics of martensite formation in small particles

Abstract

The kinetics of martensite nucleation in “atomized” particles of Fe-24.2 Ni-3.6 Mn and Fe-22 Ni-0.49 C have been investigated as a function of particle size (10 to 140 μ) and reaction temperature. The dependence of particle fraction transformed on particle size indicates that martensite nucleates at surface or near-surface sites for the Fe−Ni−C powder and throughout the bulk for the Fe−Ni−Mn powder. It is shown that, in contrast to kinetic measurements on bulk samples where autocatalysis predominates, the present technique measures reaction rate due solely to the sites present initially. The measurements show that there is no detectable incubation time for nucleation and that the nucleation sites have a specific distribution of activation energies. A method for extracting the distribution from experimental measurements is given and the result is used to develop a revised equation for describing isothermal martensite formation. The isothermal kinetics of martensite formation in Fe-22 Ni-0.49 C are investigated despite the fact that such measurements are not possible in bulk samples because the alloy transforms by “bursting”. It is found that the apparent activation entropy for martensite nucleation is significantly higher for this alloy than for Fe-24.2 Ni-3.6 Mn. This suggests that the dislocation-dislocation interactions at the critical nucleation step are longer-range in the Fe−Ni−C alloy than in the Fe−Ni−Mn alloy.