SHAPE MEMORY CHARACTERISTICS AND LATTICE DEFORMATION IN TI-NI-CU ALLOYS

Transformation behavior in Ti-50Ni(50-x)Cu(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.