Microstructures and shape memory characteristics of Ti-25Ni-25Cu(at.%) alloy ribbons

Ti-25Ni-25Cu(at.%) alloy ribbons have been fabricated by melt spinning with a linear velocity of 51.0 m s(-1) and then their microstructures and shape memory characteristics were investigated by means of optical microscopy, transmission electron microscopy, x-ray diffraction, thermal cycling tests under constant load and tensile tests. As-spun ribbon made at the melt spinning temperature of 1803 K was amorphous; below this it was a mixture of amorphous and crystal Structure. The crystallization temperature was measured to be about 727 K, irrespective of the melt spinning temperature. The activation energy for crystallization obtained from the Kissinger plot was 136.6 +/- 33.41 kJ mol(-1). Annealing the as-spun ribbons with the melt spinning temperature of 1803 K at 738 K for 0.6 ks, they were crystallized with the average grain size of 130 nm. The B2-B19 transformation occurred in the ribbons and the B 19 martensites with a single-pair variant were found in most of the grains. The transformation eloneation and thermal hysteresis were found to be 2.6% and 5 K under the applied stress of 231 MPa, respectively. The ribbons showed a perfect superelasticity and their stress hysteresis was as small as 30 MPa. The small hysteresis was attributed to very small grain size and single-pair martensite variants formed in the small grain.