Crystallization and grain refinement of Ti-30Ni-20Cu (at%) alloy ribbons prepared by melt spinning

Abstract

Crystallization behavior of amorphous Ti-30Ni-20Cu (at%) alloy ribbons and martensitic transformation behavior, shape memory effect and superelasticity of crystallized ribbons were investigated by means of X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, thermal cycling tests under constant load and tensile tests. Crystallization occurred in the sequence of amorphous-B2-Ti(Ni,Cu)(2) and the activation energy for crystallization of the B2 phase was 198.8 kJ/mol. Average grain size of the sample annealed in two-step, i.e., annealing at 773 K for 40 s followed by annealing at 748 K for 3.6 ks was 0.25 mu m, which was very small comparing with the sample annealed in one-step, i.e., annealing at 823 K for 3.6 ks (1.20 mu m). Grain refinement reduced the heat of transformation (Delta H) associated with the B2-819 transformation and increased the critical stress for slip deformation causing the perfect shape memory recovery under the applied stress of 400 MPa. (C) 2012 Elsevier B.V. All rights reserved.