Shape-memory effect and pseudoelasticity associated with the R-phase transition in Ti-50·5 at.% Ni single crystals

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Abstract
The rhombohedral phase (R-phase) transition and associated deformation behaviour in Ti-50·5 at.% Ni single crystals have been investigated systematically by tensile testing age-treated specimens in various orientations over a wide temperature range. Shape-memory effects and pseudoelasticity were observed in the first stage as well as in the second stage of the stress-strain curves; here we attempt to clarify the characteristics of the transition and deformation behaviour in the first-stage yielding. The strain in the first stage shows strong temperature and orientation dependences as follows: (1) the strain increases with decreasing temperature below the TR point, which is the critical temperature for the R-phase transition; (2) the strain is largest in the [111]B2 orientation and smallest in the [001]B2 orientation. Both the temperature and orientation dependences show excellent quantitative agreement with the calculated results based on the lattice distortion associated with the R-phase transition. Deformation was observed to proceed by {011}R and/or {001}R twin boundary movements. On the basis of these results, the deformation behaviour associated with the R-phase transition is quantitatively characterized.