Two-stage transformation and reverse process in a Ti-Ni-Cu alloys

The martensitic transformation and dynamic recovery process of a Ti 49 Ni 41 Cu 10 alloy have been investigated by means of electrical resistivity, differential scanning calorimetry (DSC), X-ray diffraction, TEM observation, measurements of resonance frequency of transverse vibration, recovery strain and stress. The alloy undergoes a thermoelastic martensitic transformation in two stages: B2 → B19 → B19'. Thecubic B2 → orthorhombic B19 transformation has a significant transformation heat effect with clear and sharp DSC peaks in a narrow temperature range and is accompanied with a pronounced softening of the elastic constant, but its resistivity change is minor. The orthorhombic B19 → monoclinic B19', transformation has a small heat effect with diffuse DSC peaks over a wide temperature range and an obvious resistivity change. TEM observations reveal the twin substructure of the B19' martensite in contrast to the defect-free B19 martensite. The difference in transformation behaviour of the B2 → B19 and the B19 → B19' might be related to the different lattice type and substructure of the martensite. The two-stage character is also shown in the recovery strain and stress curves. The B19 → B2 reverse transformation has a higher velocity of the strain recovery and a larger recovery stress than the B19' → B19 transformation