THE EFFECT OF GRAIN-BOUNDARY PRECIPITATES ON THE CREEP-BEHAVIOR OF INCONEL 718

Inconel 718 has been heat treated to produce materials with microstruc tures that have the same distribution of gamma''-strengthening precipi tates within the grains but with grain boundaries that are either clea n or have a nearly uniform distribution of 5 mu delta-Ni3Nb precipitat es. Creep tests on specimens with these two types of microstructures h ave been conducted in the temperature range of 600-650-degrees-C at co nstant applied stresses in the range of 745-820 MPa. It is observed th at the presence of precipitates at grain boundaries increases the cree p rate and its stress exponent. It is suggested that the presence of p recipitates on grain boundaries changes the creep mechanism from volum e-diffusion controlled to grain-boundary controlled. It is likely that the ''constant structure'' creep model proposed by Robinson et al. is applicable to the material with precipitates on the grain boundaries.