EFFECT OF Cr ADDITION ON TRANSFORMATION AND CYCLIC DEFORMATION CHARACTERISTICS OF Ti-Ni SHAPE MEMORY ALLOY

The effect of Cr addition on the transformation and the cyclic deformation characteristics in 550 degrees C annealed Ti-50.8Ni shape memory alloy (SMA) were investigated by differential scanning calorimetry (DSC) and tensile test. The B2 -> R -> M/M -> B2 type reversiable transformation occurred in Ti-50.8Ni alloy upon cooling/heating. After the addition of 0.3%Cr in Ti-50.8Ni alloy, the transformation type for Ti-50.8Ni-0.3Cr alloy was B2 -> R -> M/M -> R -> B2, and the transformation temperatures decreased significantly. At room temperature, the phase composition was parent phase B2 for both Ti-50.8Ni and Ti-50.8Ni-0.3Cr alloys, while the former showed shape memory effect; the latter showed superelasticity. With increasing cyclic deformation number n, Ti-50.8Ni alloy evolved from shape memory effect to linear superelasticity, and its stress inducing martensitic critical stress and the accumulation residual strain increased rapidly; while Ti-50.8Ni-0.3Cr alloy evolved from incomplete superelastic to complete superelastic, and the stress-strain curve shape was stable. With increasing n, the superelastic residual strain Er decreased and the superelastic strain recovery ratio eta(s). of Ti-50.8Ni alloy increased, while the Er and 71, in Ti-50.8Ni-0.3Cr alloy kept at lower and higher values, respectively. The superelastic of Ti-50.8-0.3Cr alloy was stable.