Crystallography of the B2→R→B 19′ 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, B 19' martensite variants as a function of rhombohedral angle alpha. 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 alpha < 90degrees. When alpha > 90degrees 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 B 19' 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 {1 1 0} <1 1 2.7008sigma> 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. (C) 2004 Elsevier B.V. All rights reserved.