Effect of Hf on the lattice invariant shear and self-accommodation of martensite in Ti–Ni–Hf alloys

Shape-memory effect and superelasticity are strongly affected by the microstructure of thermoelastic martensite. The transformation strain originating from the change of shape is accommodated through combination of crystallographically equivalent multiple habit plane variants (HPVs) (i.e., self-accommodation). We investigated the microstructure and crystallography of martensite variants in Ti50−xNi50Hfx alloys by electron backscatter diffraction analysis and transmission electron microscopy observation. The crystal structures of martensite in Ti50−xNi50Hfx alloys substituted with Hf at concentrations less than 25 at% and greater than 35 at% are the B19` and B33 structures, respectively. In the Ti25Ni50Hf25 alloy, the lattice invariant shear (LIS) is the {001}B19` compound twin; a pair of HPVs with plate- and polygonal-like morphologies were bounded by the {011}B19` Type I twin formed around each of the <100> B2 axes of the B2 parent phase. In the Ti15Ni50Hf35 alloy, no twin exists in the martensite variants with plate- and polygonal-like morphologies, providing that the LIS is (010)B33 basal stacking faults. A pair of HPVs were bounded by the {021}B33 compound twin formed around each of the <100> B2 axes of the B2 parent phase. The boundaries connected with the HPVs with different habit planes were the {121}B33 planes of the B33 structure, which correspond to the {110}B2 planes of the B2 phase. The minimum self-accommodation unit necessary to relax the strain energy due to martensitic transformation (MT) in both alloys is a pair of HPVs. As the MT proceeds, each HPV pair should contact and impinge on the {110}B2 plane, resulting in a mosaic-like microstructure. Figure a, b Inverse pole figure maps and c, d Bright-field images and e, fSchematic illustration of lattice invariant shear and minimum selfaccommodationunit: a, c, e, Ti25Ni50Hf25 alloy. b, d, f Ti15Ni50Hf35 alloy