Phase transformation behavior and wire drawing properties of Ti-Ni-Mo shape memory alloys

Transformation behavior and wire drawing properties of Ti-Ni-Mo shape memory alloys have been investigated by means of differential scanning calorimetry (DSC) measurements, X-ray diffraction, electron microscopy, tensile tests and wire drawing tests. Mo addition to a Ti-Ni binary alloy induced the R phase transformation, and consequently Ti-Ni-Mo alloys showed two stage transformation, i.e., from the B2(cubic) parent phase to the R(rhombohedral) phase, and then from the R phase to B19′(monoclinic) phase. In the thermo-mechanically treated 51Ti-48.3Ni-0.7Mo alloy, reverse transformation temperature, Af, kept constant, irrespective of thermo-mechanical treatment conditions, while it changed in the thermo-mechanically treated 51Ti-49Ni and 51Ti-48.5Ni-0.5Mo alloys. Mo addition to Ti-Ni binary alloy decreased wire drawing stress. Wire drawing stress decreased with raising intermediate annealing temperature monotonously when the annealing treatment was made in vacuum. When the annealing treatment was made in air, however, it decreased with raising annealing temperature up to 923 K, and then increased. Optimum intermediate annealing temperature of Ti-Ni-Mo alloys for wire drawing was 823 K, above which a thick oxide film which reduced the drawability of the alloys was formed on the surface of alloy wires.