The crystallography of the austenite-cementite transition

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Abstract
The similarity between {113} planes in austenite and (001) planes in cementite remarked by Mehl et al. (1933) is used as a basis for deriving a crystallographic model giving the relative movements of Fe atoms during the phase transition. Two slightly different correspondences are obtained. The cementite unit cell dimensions, a=4°42 Å, b=5·09 Å c=6·74 Å, obtained from the γ phase lattice parameter, agree well with those experimentally obtained by Lipson and Petch (1940): a=4·52 Å, b=5·08 Å, c=6·73 Å. It is shown that the BM and WLR theories of martensite crystallography cannot be applied in this case, as the assumptions underlying these theories are very probably not fulfilled. Limited scatter regions for the orientation relations are derived from the correspondence relations by a method based on the assumption that during an affine lattice transition one unspecified corresponding plane remains unrotated, and in this plane one crystallographic direction. The orientation relations observed by Pitsch (1963) for Fe3C platelets precipitated in Fe-Mn-C γ phase are in good agreement with the scatter regions predicted for one of the two correspondences. Finally six separate secondary correspondences for ferrite and cementite are derived from the two austenite-cementite correspondences and the three variations of the Bain correspondence. One of these secondary correspondence relationships, based on the one austenite-cementite correspondence that is confirmed by experiment, is identical to the primary ferrite-cementite correspondence derived earlier by Andrews (1963), which explains why the assumption of an intermediate ferrite phase may correctly give the crystallography of the austenite-cementite transition. It is now clear that the assumption does not represent a necessary condition, as had been suggested. It is shown that the method employed here for relating orientation to correspondence relationships correctly predicts the observed orientation relations for cementite precipitated in the α phase from the primary correspondence relations given by Andrews.