DEPENDENCE OF CREEP FRACTURE OF INCONEL 718 ON GRAIN BOUNDARY PRECIPITATES

In this study, Inconel 718 was heat treated to obtain various materials with identical grain size and microstructures but with a different state of dispersion of δ precipitates at the GBs. The density of δ precipitates ρ in different materials was from 0% to about 70%. Creep tests were conducted on these materials at 795 MPa and 625°C. It was found that both the rupture time and total creep strain decreased with an increase in the value of ρ when its value was below about 45%. However, when the value of ρ was above 45%, both the rupture time and creep strain increased with an increase in ρ to a value much higher than that observed in the material without δ precipitates at GBs. SEM observations of the crept samples and their fracture surfaces showed that the presence of δ precipitates at GBs resulted in the formation of creep voids in all specimens. However, the effect of creep voids on the final fracture of the material was dependent upon the value of ρ. At lower values of ρ, creep voids were observed to be isolated, and the fracture may be due to the propagation of wedge cracks initiated at triple points of grain boundaries. At higher values of ρ, the probability of wedge crack formation might be reduced. The fracture would then be controlled by the growth of cavities coupled with grain boundary sliding, which was observed to be strongly influenced by precipitates at grain boundaries. © 1997 Acta Metallurgica Inc.