Crystallography of the B2 → R → B19′ phase transformations in NiTi

The crystallography of B2 to R-phase and the subsequent R-phase to B19′ martensite transitions was analysed in NiTi alloys. Using the energy minimization theory of martensitic transformations, the habit plane and twin plane normals were established for the R-phase, B19′ martensite variants as a function of rhombohedral angle α. Two types of twins, classified as {1 0 0}〈0 1 1〉 and {1 1 0}〈0 0 1〉 compound twins could form in the R-phase. There are 12 R-phase variants with {1 1 0}〈0 0 1〉 compound twins when α<90°. When α>90° there are 24 R-phase variants with {1 0 0}〈0 1 1〉 compound twins and 12 variants with {1 1 0}〈0 0 1〉 compound twins, respectively. In total, eight possible twins can be formed in B19′ martensite. In the single step B2 to B19′ martensitic transformation, only four twin types are qualified as lattice invariant shear (LIS); while all the eight twin types are qualified as LIS in R-phase to B19′ transition the {1 0 0}〈0 1 1〉 compound, {1 1 0}〈0 0 1〉 compound and {11̄0}〈1 1 2.7008〉 Type I twins have been observed in the experiments. The results indicate that the premartensitic R-phase transition plays an important role in understanding the phase transformations in NiTi shape memory alloys.