Influence of Phosphorus on Creep Performance of DA Alloy 718

Abstract

Achieving a balance between low cycle fatigue and creep in alloy 718 critical rotating hardware has become challenging as application conditions become more aggressive and geometries more complex. Optimization of both forge practice and of minor chemistry variations are required to meet this property balance. A review of potential factors influencing creep variation in direct age alloy 718 forgings identified phosphorus level as a statistically significant factor over the range of 50 ppm to 125 ppm. Verification of the findings was accomplished using a series of full scale heats with phosphorus levels targeted to a 90 ppm minimum. Data from the 12 full scale VIM heats, including 104 creep tests from multiple forgings, exhibited an average creep life of 111 hours with Q1 and Q3 distributions of 80 hours and 145 hours, respectively, when tested at 1100F/120 KSI and 0.2% strain. This is an approximate 2X increase in mean life from historical levels. The results of the evaluation show that with an optimized well controlled forge process phosphorus levels above 80 ppm, but below 150 ppm, will improve creep performance of DA alloy 718.

Effects of Phosphorus on Creep Properties of Nickel-Iron Base Superalloy

“…They assumed that the decrease of creep rate was due to the solid solution strengthening of phosphorus addition. Heaney et al 11) also confirmed the beneficial effects of phosphorus on the creep properties of Inconel 718 at approximately 593°C (1 100°F), and reported that the optimum phosphorus content is in a range of 80 to 150 ppm. There are several other reports on the influence of phosphorus on the creep properties of Inconel 718, and in most cases phosphorus has a positive effect in increasing the creep rupture life.…”

“…The results obtained in this work basically agree with those reported by Guo et al; 12) the 0.2% proof stress and tensile stress of Inconel 718 at room temperature and at 650°C did not change, and the time to rupture at 650°C, 650 MPa peaked at the phosphorus content of 0.013%. Heaney et al 11) also mentioned that controlled phosphorus addition between 80 and 150 ppm would be effective for the improvement in creep performance of Inconel 718. Although there are some differences in the chemical composition between Inconel 718 and the alloy used in this study, the suitable phosphorus content to improve creep properties is seemed to be equal.…”

Effects of Phosphorus on tensile and stress rupture properties of GH4169 alloy

“…Other researchers consider the effect of P may come from its direct action to grain boundary [3,13]. The segregation of P to grain boundary increases the grain boundary cohesion by impeding dislocation motion, resulting in the improvement of stress rupture life.…”