Effects of phosphorus on creep properties of nickel-iron base superalloy

Abstract

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.