Effect of Intergranular Carbides on Creep Strength in Nickel-Based Heat-Resistant Alloys

Creep behaviors and microstructures for two Ni-based heat-resistant alloys with different carbon contents were investigated. The chemical compositions of the alloys were Ni-20Cr-15Co-6Mo-1Ti-2Al-2Nb-0.004 and 0.021C (mass%). The 0.004C and 0.021C alloys are referred to as the low-and high-C alloys, respectively. After solid-solution treatment at 1373 K for 1 h and isothermal annealing at 1023 K for 32 h, fine Ni3Al (gamma') particles were formed in the grain interior of both alloys. The average diameter and number density of gamma' particles were similar in both alloys. M23C6 carbides were formed on grain boundaries after the isothermal annealing. Coverage ratios with the carbides in the high-C alloy were higher than that in the low-C alloys. Creep tests were performed at 1123 K and 130 MPa. The rupture time for the high-C alloy was longer than that for the low-C alloy, though both minimum creep rates were similar. In the high-C alloy, the creep strain was stored uniformly in the grain interior and the formation of a precipitate-free zone during the creep deformation was suppressed. Therefore, intergranular carbides with a high coverage ratio decreased the creep rate in the acceleration region.