Effect of Grain-boundary Phases on the Tensile and Compressive Creep Behavior of a Ni-Co Based Superalloy with High W Content

The effects of intergranular W- rich compounds ( W6C/W12C and mu phases) on the creep properties of a. '-precipitation- strengthened, wrought Ni-Co-based superalloy were examined. Two microstructures with different intergranular precipitates (but the same volume fraction and size of intragranular. ' precipitates) were prepared by adjusting aging conditions, leading to two distinct grain- boundary precipitation microstructures, with grain-boundaries exhibiting: (i) W-rich compounds (W6C/W12C and mu), or (ii) W-free compounds (Cr23C6 and. ' phases, as in conventional Ni-based alloys). In compressive creep tests, lower minimum creep rate and longer 1% creep life were observed for alloys with the second grain-boundary microstructure (with Cr23C6 and. ') than for the first grain-boundary microstructure (with W6C/W12C and mu). On the other hand, in tensile creep tests, longer creep rupture life was observed for alloys with the first grain-boundary microstructure than for the second, especially at higher stresses. Based on the grain boundary precipitation strengthening mechanism, it is suggested that the difference in compressive minimum creep rate between both microstructures can be explained by the difference in grain-boundary coverage. In tensile creep tests, the creep rate due to cavity and crack growth appears to be the dominant factor in the overall creep rate at higher stresses, consistent with Stress Assisted Grain-Boundary Oxidation (SAGBO). It appears that the grain-boundary precipitation of W6C/W12C and mu phases is more effective in suppressing SAGBO than grain-boundary precipitation of Cr23C6 and gamma '.