Crystallographic Study of Diffusion Phase Transformations for Martensites with Complex Structure

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There is a beautiful phenomenological theory for the crystallography of diffusionaless phase transformations called martensitic transformations, which were developed by Wechsler-Lieberman-Read and Bowles-Mackenzie. However, there still remain alloys, for which the crystallography of the transformations are still not yet established. It is the purpose of the present work to study them in detail, and to obtain the unified view for the martensitic transformations. We picked up B2->14M(7R) transformation in Ni-Al alloys and beta_2 (B2)->zeta_2'(trigonal) transformations in a Au-49.5at%Cd alloy as such cases. First, we found that the phenomenological crystallographic theory applies well for the B2->14M transformation. The self-accommodation of the alloy was also studied, and we proposed a new basic morphology called "the parallelogran morphology" instead of the previously proposed "diamond morphology". Furthermore, in Ni-Al-Mn alloys, we found new martensite structures designated as 10M(15M) and 12M(36M), which have not been found in other alloys either. For the latter Au-49.5at%Cd alloy, we determined the structure of the zeta_2' martensite, which have been undetermined for 50years. On the beta_2->zeta_2' transformation, Ledbetter assumed the presence of a lattice invariant shear, while Tadaki et al.deined the presence. In the present study, we found two types of martensite morphologies called roof-type and herring-bone type, in which the former has a lattice invariant shear while the latter does not. Thus, we reconciled the big inconsistency between the two researchers. from the above results, we conclude that the phenomenological crystallographic theory applies well even for the martensites with complex structures, if the lattice invariant shear is chosen correctly.