Revisiting the Crystallography of {225} γ Martensite: How EBSD Can Help to Solve Long-Standing Controversy.

Explaining the crystallography of iron alloys martensite with a {225}_γ habit plane remains a challenging task within the phenomenological theory of martensite crystallography. The purpose of this study is to re-examine the martensite formed in a Fe-8Cr-1.1C alloy using EBSD, which has a better angular resolution than the conventional transmission electron diffraction techniques previously used. The results show that the single morphological plates, which hold a near {225}_γ habit plane, are bivariant composites made up of two twin-related variants. It is shown that a {113}_γ plane is systematically parallel to one of the three common 112 α planes between the two twin-related crystals. This observation suggests that the lattice invariant strain of transformation occurs through a dislocation glide on the {113}_γ 〈110〉_γ system, rather than through twinning as is commonly accepted. Based on this assumption, the predictions of Bowles and Mackenzie's original theory are in good agreement with the crystallographic features of {225}_γ martensite. Unexpectedly, it is the high shear solution of the theory that gives the most accurate experimental predictions. [ABSTRACT FROM AUTHOR]