DEPENDENCE OF CREEP FRACTURE OF INCONEL-718 ON GRAIN-BOUNDARY PRECIPITATES

In this study, Inconel 718 was heat treated to obtain various material s with identical grain size and microstructures but with a different s tare of dispersion of delta precipitates at the GBs. The density of de lta precipitates rho in different materials was from 0% to about 70%. Creep tests were conducted on these materials at 795 MPa and 625 degre es C. It was found that both the rupture rime and total creep strain d ecreased with an increase in the value of rho when its value was below about 45%. However, when the value of rho was above 45%, both the rup ture time and creep strain increased with an increase in rho to a valu e much higher than that observed in the material without delta precipi tates at GBs. SEM observations of the crept samples and their fracture surfaces showed that the presence of delta precipitates at GBs result ed in the formation of creep voids in all specimens. However, the effe ct of creep voids on the final fracture of the material was dependent upon the value of rho. At lower values of rho, creep voids were observ ed to be isolated, and the Fracture may be due to the propagation of w edge cracks initiated at triple points of grain boundaries. At higher values of rho, the probability of wedge crack formation might be reduc ed. The fracture would then be controlled by the growth of cavities co upled with grain boundary sliding, which was observed to be strongly i nfluenced by precipitates at grain boundaries. (C) 1997 Acta Metallurg ica Inc.