The crystal chemistry of martensite in NiTiHf shape memory alloys

The crystal structure of martensite in NiTiHf shape memory alloys of compositional formulas Ni50Ti50−xHfx, Ti50Ni50−xHfx, and Ni(100−x)/2Ti(100−x)/2Hfx (5 ≤ x ≤ 20) were studied. The monoclinic lattice parameters and angle were changed by introduction of larger Hf atoms into the NiTi crystal. The changes were related not only to larger atomic radius of Hf but also to its effect on bond lengths. At the same Hf content, Ti-rich alloys exhibited larger cell volumes than Ni-rich alloys. The equiatomic alloys showed smaller unit cell volumes than the other two groups possibly due to more isotropic bonding. Valence electron per atom (ev/a) and valence electron concentration (ev/et) of the alloys were found to influence the atomic bond lengths and unit cell dimensions. From the crystal structure refinement, Hf site preference and occupancy were determined. The similar tendency of Hf towards occupancy in Ni and Ti sites, results in its preference for available sites in the structure.