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In ferrous 0 martensite (bcc or bct structure), there are several different morphologies, ie, lath, butterfly, lenticular and thin plate, each of which is unique in crystallographic features as well as in substructure. 1) Among these ferrous 0 martensite, the substructure of lenticular martensite is much more complicated than that of other morphologies and it consists of the following three distinctive regions: midrib, twinned region and untwinned region. 2, 3) The midrib is contained in the middle of lenticular martensite plate. Although it is well known that midrib is the region in which high density of transformation twins exist, 2–5) the origin and nature of midrib has not been clear yet. However, it is considered that the formation of lenticular martensite is triggered by the formation of midrib. There are some experimental evidences supporting this view. Neuhäuser and Pitch6) studied the effect of incoherent precipitate particle (Fe3P or Ni3P) on the martensitic transformation in an Fe-31Ni-1P alloy and showed that small austenite is retained around the incoherent precipitate particle in martensite. Since the shapes of these retained austenite are opposite on each side of midrib, they proposed that midrib is the plane from which martensitic transformation starts. Kakeshita et al. 7) studied the growth behavior of lenticular martensite in an Fe-30Ni-0.4 C alloy by repeating subzero-cooling and microstructure observation. They reported that midrib, which is thin plate shape, appears in the earliest stage of martensitic transformation and martensite morphology becomes lenticular by further growth. Once lenticular martensite plate reaches the final size, the martensite …