Effect of nickel addition on microstructure and properties of Ti-Co-Ni alloys.

The focus of this study was placed on the effect of nickel addition (5 wt%) on the microstructure, castability, biocorrosion resistance and some other properties of a series of cast high-cobalt (up to 25 wt%) Ti-Co-Ni alloys. Results indicated that melting temperatures of the present alloys were much lower than the melting temperature of pure titanium. Addition of 5 wt% Ni to replace an equivalent amount of Co slightly lowered the eutectoid and melting temperatures. Castability of the alloys was enhanced by the high alloy content and higher mould temperature. Substitution of 5 wt% Ni for Co increased the castability values. The phase transformation of beta-Ti to alpha-Ti was enhanced by nickel addition and higher mould temperature. Microhardness of the alloys increased with cobalt content and decreased with mould temperature. The addition of nickel lowered the hardness of the alloys. Breakdown potentials of the alloys were all higher than 900 mV and the critical anodic current densities were all lower than 8 microA cm-2.