Morphology and Crystallography of β′ Martensite in TiNi Alloys

Martensitic transfromation in TiNi alloys of the near-equiatomic composition has been studied by utilizing electron microscopy and diffraction under various thermal treatments. It has been observed that the martensitic transformation is associated with the variants of the product. Two orientations of the martensitic phase were observed as adjacent regions in quenched alloys. These orientations were forming in such a manner that they bore a common (111) plane and their b and c axes were interchanged. The slow cooled samples and those annealed at intermediate temperatures (650°C) showed (111) twinning in addition to the variants of the same transformation. The morphology of the (111) twinned product appeared as alternating platelets forming as adjacent regions. The samples that were aged at low temperatures (200°C) appeared to show internal twinning on (010) plane. The formation of these variants is considered in relation to the existing theories of the martensitic transformation. A possible mechanism of the shape memory phenomenon is also discussed.