Creep properties of modeled nickel-iron base superalloy with phosphorus (P) content of 8 to 450 ppm by mass have been investigated at 973 K under a stress of 333 MPa. The P-doped alloys exhibit many grain-boundary precipitates in which niobium and phosphorus are enriched after a certain heat treatment condition prior to the creep test. An alloy with P of 130 ppm shows a maximum value of grain-boundary coverage by the precipitates (designated as ρ ) of 0.56. The time to rupture and minimum creep rate correspond well to the value of ρ . The rupture life was extended by up to 30 times and the minimum creep rate was decreased by more than two orders of magnitude in the alloy with P of 130 ppm, with respect to those in an alloy with P of 8 ppm where ρ =0. The grain-boundary precipitates remarkably delay the onset of accelerating stage, thereby leading to the longer rupture life. These results strongly suggest that optimization of the ρ value is a key to improve the creep properties of the alloys.
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