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Classification of Twin Arrangements in Butterfly Martensite Grains and Analysis of Relationship between Twin Arrangement and Butterfly Wing Angle in Medium-Carbon Steel

Butterfly martensite grains formed in medium-carbon steel consists of two coarse grains colliding with each other. The present study investigates the relationship between the wing angle of the butterfly and its substructures. The substructures (especially the arrangement of twins in butterfly wings) were observed using transmission electron microscopy (TEM). The observation results show that twins pile up along the longitudinal or width direction in a butterfly wing. This result indicates that butterfly martensite grains can be classified into three patterns by the twin piling direction and that the wing angle of the butterfly is determined depending on the substructure patterns. We also proposed a method to estimate the substructure patterns using data from the electron backscatter diffraction measurement without TEM observation. In the most common butterfly grains, twins pile up in the longitudinal direction in one wing and in the width direction in the other wing, and the wing angle is obtuse. The outer interface of the butterfly wings and the variant combinations of the butterfly wing pair are also investigated. The orientation of the outer interface is determined depending on the substructures, and a strong variant selection in the butterfly wing pair is observed despite the variety of substructures.