On the crystallography of martensite the “{225}” transformation in alloys of iron

Abstract

For an accurate study of the martensite transformation in iron alloys it is necessary that the lattice orientation relationship be obtained from a single plate of martensite whose habit plane direction cosines are already known. Heretofore such observations have not been made for the {225} A , type martensite in iron alloys, but such an analysis is reported herein for an alloy, Fe-7.90 wt.% Cr-1.11 wt.% C, for which a habit plane approximately described by {449} A was observed. The theories of Bowles-Mackenzie and Wechsler-Lieberman-Read, which differ in the interpretation of the {225} transformation, were examined with these new experimental results. The WLR theory is in substantial agreement with the results, provided that the lattice invariant shear is assumed on the {011} plane of the martensite. The BM theory, allowing an isotropic interface dilation, provides an equally good description if the shear occurs on a {112} M , plane. In each case, the predicted shear direction is near the expected close-packed 〈111〉 M direction. Some microscopic observations suggest that the interface plane may not be one of zero net distortion, as supposed by WLR and that a dilation in the interface appears reasonable. The significance of observed martensite side plates is discussed.