Martensite habit planes in Fe-Mn-C alloys

Abstract

Habit planes have been determined for martensite plates in two high-carbon iron alloys, containing 11.0 and 1.9% manganese, respectively, and in an iron-nickel-carbon alloy. The high-carbon alloys have been shown to form plates with habits which range widely in their orientations; when all habits are converted to the same variant the mean habit occurs approximately mid-way between (225)_F and (259)_F for that variant. Consistent with observations for similar alloys, the Fe-Ni-C alloy was found to be a {3 10 15}_F type. Markings observed after metallographic preparation within plates of all the alloys have been identified as traces of {110}_F in the austenite which become {112}_M twinning planes in the martensite. In the high-carbon alloys, two sets of such markings from different {110}_F in the same plate were observed frequently. It is suggested that the anomalously wide ranges of habit-plane orientations may be consequences of martensitic transformations in which coupled, lattice-invariant shears occur on pairs of systems, either twinning in the martensite or their equivalent in the austenite.