Effect of titanium,niobium and zirconium on creep rupture strength and ductility of cobalt base superalloys

An attempt has been made to develop a cobalt base casting superalloy (30Cr-10Ni-7W-Co) having high creep rupture strength and ductility for first stage nozzles of gas turbines. In cobalt base superalloys, there was found to exist a close correlation between the creep rupture strength and MC type carbide forming elements such as Ti, Nb and Zr. In cobalt base alloys with 0.25 wt pct C, precipitation and coarsening of carbides can be reduced by addition of Ti, Nb and Zr. Therefore, by adding the optimum amount of Ti, Nb and Zr, precipitation of carbides in the alloy reaches such an amount as to give the highest creep rupture strength. Excess addition of Ti, Nb and Zr does not improve the creep rupture strength. By adding C, creep rupture strength of the cobalt alloy with Ti, Nb and Zr can be improved and becomes the highest at 0.40 wt pct. C. According to the experimental results, the creep rupture strength becomes the highest at a value of (Ti + Nb + Zr)/C (atomic ratio) of about 0.3. Contrary to the expectation, it was found in this experiment that the ductility in creep rupture tests increases with increasing carbon content up to 0.6 wt pct.