Effects of Co addition on microstructure and mechanical properties of NiTiNb shape memory alloys

Effects of Co addition on the microstructure, phase transformation behavior, shape memory effect and mechanical properties of the wide hysteresis NiTiNb alloy were investigated. The results show that the addition of Co not only restrains the appearance of brittle (Ti, Nb)₂Ni phase effectively, but also decreases markedly the martensite transformation temperature (M_s) in NiTiNb alloys. The value of wide hysteresis becomes narrower from 119.3 to 111°C with the addition of Co element, but still remains enough high level. Compared with the NiTiNb alloys, the yield stress, the elongation, the recovery stress and the maximum recovery strain of NiTiNbCo alloys are increased from 320 MPa, 20%, 434 MPa and 5.3% to 460 MPa, 24.5%, 486 MPa and 7.8%, respectively. Therefore, alloying is an effective method to improve the yield stress and recovery stress.