Non-classical {3 3 4}_β type of twinned α′ martensite in a Ti-5.26 wt.% Cr alloy

Some uncommon crystallographic features of α′ martensite were characterized in the martensitic transformation in a water-quenched Ti-5.26 wt.% Cr alloy. A pair of martensite plates, type K and type M, sit side-by-side and share a {3 3 4}_β habit plane. However, they have different habit planes in terms of the hexagonal close-packed lattice. While the classical phenomenological theory of martensite crystallography (PTMC) is unable to explain the habit plane of type K martensite even though they contain {11̄01̄} internal twins, application of the edge-to-edge matching (E2EM) model succeeded in accounting for both the orientation relationship and the habit plane orientation of the two types of martensite. The elastic strain energy as a function of the thickness of type K martensite plates was calculated, and the result supports the preference of the E2EM model over PTMC in explaining this type of martensite. In addition, a self-accommodation mechanism for strain was proposed to maintain the macroscopic invariant plane, and reduce the overall elastic strain energy of the adjacent pair of martensite plates. The application of the E2EM model plus this self-accommodation mechanism provides a new perspective on understanding martensitic transformations and providing a link between the crystallography of displacive and diffusional transformations.