Observation of {011} twins in Fe−Ni−C martensite using neutron powder diffraction

High resolution neutron powder diffraction measurements were performed on freshly quenched specimens of Fe-13 wt pct Ni-1.0 wt pct C martensite formed at subambient temperature and after aging for 1 hour to 293, 313, 333 and 353 K. The widths of the powder reflections in the unaged martensite indicate the presence of a large number of {011} twins in this tetragonal structure. The {011} twins were modeled as single layer randomly distributed twin faults. The model predicts powder peak breadths with the unmistakable reflection index dependence that is observed in the measured powder spectra. The magnitude of the observed peak broadening is consistent with a twin volume of 17(±4) pct. Upon aging, there is no detectable change in the number of {011} twins despite an observed decrease in the tetragonality. Previous transmission electron microscope (TEM) measurements have demonstrated the presence of these twins, but it was not clear if they formed during the martensitic transformation or upon warming from subambient temperature. This study confirms their presence immediately after transformation, indicating that they may play an important role in the transformation. B.D. B-uputler, formerly Research Assistant Materials Science Department, Northwestern University, is Postdoctoral Associate, Research School of Chemistry, Australian National University, Canberra, Australia.