Geometry of Butterfly Martensite in Fe-18Ni-0.7Cr-0.5C Alloy

To clarify the geometry of the junction of martensite which forming butterfly-type martensite, electron microscopy, theoretical analysis using the phenomenological theory of martensite crystallography and rank-1 connection were carried out on Fe-18Ni-0.7Cr-0.5C (mass%) alloy. Martensite plates in this alloy exhibit {252}γ habit plane and K-S OR and are in good agreement with the double shear PTMC by Ross and Crocker. The frequency of formation of V1/V2 and V1/V16 (butterflies) pairs was 20% each, with these two pairs alone accounting for 40% of the total. Theoretical analysis using rank-1 connection of variants revealed that butterfly-pairing is not a geometrically compatible morphology, even for alloys in which butterfly martensite forms frequently. We have also revealed theoretically that the junction plane of the butterfly-pairing keeps (100)γ independent of the mode of lattice invariant shear (i.e. morphology of martensite plate). Preferential formation mechanism for butterfly-pairing is discussed based on the common lattice invariant deformation between V1 and V16.