Microstructure and martensitic transformation and mechanical properties of cast Ni rich NiTiCo shape memory alloys

The influence of Co additions on the microstructure, second phase precipitates, phase transformation and mechanical properties of cast Ni51-xTi49Cox (x=0, 0.5, 1.5 and 4 at-%) shape memory alloys was investigated. At the expense of Ni, Co added to NiTi alloy significantly increases the martensitic transformation temperature. The matrix phase in the microstructure of Ni51Ti49Co0 alloy is the austenite phase (B2) in addition to martensite phase (B19') and precipitates of NiTi intermetallic compounds. However, the parent phase in the other three alloys, Ni50.5Ti49Co0.5, Ni49.5Ti49Co1.5 and Ni47Ti49Co4, is martensite. Ti2Ni phase was found in the microstructures of the all investigated alloys; however, Ni3Ti2 phase precipitated only in the NiTi alloy with 0 at-% Co. The volume fraction of Ti2Ni phase decreased by the additions of 0.5 and 1.5 at-% Co, while it is slightly increased with 4 at-% Co. The hardness value of NiTi alloy is affected by Co additions.