Cobalt Effect on Creep Behavior of Nickel Solid Solution Alloys

To understand the role of cobalt in nickel-base superalloys, multicomponent nickel solid solutions with various amounts of cobalt were studied. Alloys A, B, C and D which contain cobalt at 30, 17, 8 and 0, respectively, and constant concentrations of Cr(25), Mo(5), Al(0.5), Ti(0.5), Nb(0.4), Hf(0.1), all in weight percent, were produced by casting. Compressive creep tests of these alloys were conducted at temperatures between 650°C and 810°C. Table 1 shows the minimum creep rates at 704°C at a stress of 120% of their respective yield strengths. It can be seen that the alloy without cobalt (Alloy D) creeps at a rate 20 times faster than Alloy A with 30 wt.% cobalt and the values for Alloys B and C with intermediate cobalt contents fall between Alloys A and D. Thin foil studies have revealed that the differences can be attributed, in part, to (i) change of stacking fault energy with cobalt, (ii) precipitation of fine gamma prime particles during creep.