Unraveling role of Co addition in microstructure and mechanical properties of biomedical Ti-Nb based shape memory alloy

In this paper, the effects of Co addition on the microstructural evolution and mechanical properties of Ti-Nb based shape memory alloys were systematically studied. The results showed that the addition of Co into Ti-Nb shape memory alloy can tailor its average bond length of atoms and average d orbital energy level, further changing the phase constituents. With increasing Co content from 0.5 at.% to 5.0 at.%, the evolution of phase composition for Ti-Nb based shape memory alloys was as follows: α´´ + β → β + ω → β → α´´ + β → β. In contrast, the volume fraction of α´´ martensite phase in Ti-Nb shape memory alloys with 2.0 at.% Co was obviously higher than that in Ti-Nb shape memory alloys with 0.5 at.%. Moreover, the self-accommodation configuration with a V-shaped and triangular martensite variants were typical morphologies in Ti-Nb-Co shape memory alloy in order to the minimum energy theory. On the basis of solid solution strengthening, the yield strength and maximum tensile strength continuously increased, as the Co content is increased from 0.5 at.% to 3.0 at.%. However, the reduction of yield strength and maximum tensile strength was observed in Ti-Nb-Co shape memory alloy with 5.0 at.% Co owing to the formation of metastable β parent phase. Meanwhile, the largest value of micro-hardness (421 HV) can be optimized by tailoring 1.0 at.% Co as a result of solid solution strengthening and precipitation strengthening.