Characterisation of microstructure of isothermal martensite formed at low temperature in powder of Fe-23Ni-3.3Mn alloy by Rietveld method

The present study attempts to characterise and compare the microstructures of martensite in powders prepared by hand-filing Fe-23Ni-3.3Mn alloys at room temperature with those of the martensites formed at low temperature in the austenite powder and bulk forms of the alloy. The X-ray diffraction line profiles obtained from the powder and the bulk specimens of the material have been analysed by Rietveld's whole profile fitting method incorporating preferred orientations of the crystallites. The results reveal that the martensite formed in cold-worked powder has higher value of the stacking fault density compared to that of the martensite formed at 160 K in annealed powder of Fe-23Ni-3.3Mn. The martensites have higher value of dislocation density (similar to 10(11) m(-2)) with smaller crystallite sizes and larger r.m.s, strain values. The bulk martensite has higher hardness value. The volume percentage of martensites formed are 24, 79, 84% for the austenite powder, cold worked powder and bulk sample, respectively. (C) 1999 Elsevier Science S.A. All rights reserved.