Research on Inconel 718 Type Alloys with Improvement of Temperature Capability

Abstract

Inconel 718 was developed by International Nickel Company in 1962, has become the most widely used superalloy in the world. The alloy's popularity is due to its excellent combination of mechanical properties, moderate price and good processability. However, its maximum use temperature is restricted to 650ºC because the main strengthening γ″ phase responsible for the alloy's outstanding properties rapidly overages. There has been a substantial amount of work by numerous investigations over the years to increase the temperature capability of Inconel 718. Xie and Dong et al. has systematically studied the effect of variation of alloying elements including Al, Ti, Nb, P, B, S and Mg etc. content on structural stability and mechanical properties by experiments and thermodynamic calculations.This attempt was made to improve the temperature capability of Inconel 718 via controlling the morphology of γ″ and γ′ phase and increase their solvus temperature by variation of Al, Ti and Nb content, and W addition for solid solution strengthening. The phases precipitation behaviors, long time microstructure stability and mechanical properties of modified 718 type alloys have been systematically studied. The results indicate that compact morphology of γ″ and γ′ has been observed in grains. In addition, a new stable globular phase with higher solvus temperature mainly precipitated at grain boundaries, different to the δ phase in conventional 718 alloy, has been observed in modified 718 type alloys (this phase is temporarily named δ′′ phase for description convenience). After 680ºC aging for 1,000h, the combination of δ′′ granular phase at grain boundaries and compact morphology precipitation of γ″ and γ′ phase in grains characterize with superior thermal stability. Preliminary mechanical properties test results show that the alloy with this kind of microstructure can provide better stress rupture properties and fatigue resistance than that of conventional Inconel 718. It appears to provide a new approach for improvement of temperature capability of Inconel 718 and the new designed 718 type alloy can be used at 680ºC or higher temperatures.