Transformation behavior in Ti₅₀Ni_50−xCu_x (at%) alloys has been investigated by carrying out X-ray diffraction in the temperature range between 343 and 83 K, and the following was found. In the alloys with Cu-content less than 5 at%, B2→B19′ transformation takes place. In the 7.5 at%Cu alloy, both B2→B19′ and B2→B19→B19′ transformations take place. In the alloys with Cu-content between 10 and 15 at%, B2→B19→B19′ transformations take place. The B19→B19′ transformation is not complete even at 83 K in alloys with Cu-content more than 10 at%, while complete in the 7.5 at%Cu alloy. In the 20 at%Cu alloy, only B2→B19 transformation takes place. This transformation behavior is consistent with that obtained from electrical resistivity and DSC measurements.
Lattice parameters of the parent and martensite phases were obtained from X-ray diffraction and the lattice deformation was calculated by using the parameters. In relation to the calculated lattice deformation, the transformation elongation and hysteresis observed in thermal cycling tests under a constant load were examined, and the following was found. Cu-content dependence of the transformation elongation originates from that of the lattice deformation. The decrease in transformation hysteresis with increasing Cu-content originates from a decrease in friction against the movement of the interface between the parent and martensite phases.