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

Inconel 718 has been heat treated to produce materials with microstructures that have the same distribution of gamma''-strengthening precipitates within the grains but with grain boundaries that are either clean or have a nearly uniform distribution of 5 mu delta-Ni3Nb precipitates. Creep tests on specimens with these two types of microstructures have been conducted in the temperature range of 600-650-degrees-C at constant applied stresses in the range of 745-820 MPa. It is observed that the presence of precipitates at grain boundaries increases the creep rate and its stress exponent. It is suggested that the presence of precipitates on grain boundaries changes the creep mechanism from volume-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.