Effect of cobalt addition on thermal cycling behaviour of Ti50Ni(50-x)cox shape memory alloys

The effect of adding Co on the temperature cycling behaviour of ternary Ti50Ni(50-x)Cox (x = 1, 2, 3) alloys was experimentally studied in this work. The alloys were prepared using a vacuum induction furnace, followed by subjecting them to homogenization, hot-rolling and annealing processes. The alloys were subjected to thermal cycling experiments in a nitrogen atmosphere by differential scanning calorimetry under stress-free conditions between their transformation temperatures. The results indicate that adding Co to NiTi alloys decreases their transition temperatures, improves the thermal cycling stability apart from suppressing the R-phase formation on cooling during cycling. The changes are due to the addition of Co introducing solid solution strengthening and generation of dislocations during cyclic phase transformations, as confirmed by the hardness test results and TEM micrographs, respectively.