Characterisation of microstructure of isothermal martensite in Fe-23Ni-3.8Mn by Rietveld method

The lattice defect-related features of the microstructure evoluted due to lattice invariant shear of martensitic transformations at low temperature (77 K and 143 K) in Fe-23Ni-3.8Mn alloy have been characterised by analysing the X-ray profiles originating from the parent and isothermally transformed martensite phases. The X-ray profiles have been analyzed employing Rietveld's whole powder pattern fitting technique, incorporating preferred orientations of the crystallites. The martensite phase has lower values of the crystallite sizes and higher value r.m.s. strain and higher dislocation densities compared to those of the austenite phase, both in the bulk and deformation induced transformed powder samples. The annealed powders, however, could not be transformed isothermally at low temperatures (77 K and 143 K), unlike the bulk specimens. The bulk martensite has an increased value of hardness compared to the austenite. (C) 1998 Elsevier Science S.A. All rights reserved.