Alloy 718: Part III, Derivative Alloys

Abstract

This final installment of a three-part series examines how modifications to alloy 718's composition produced a family of "718 derivatives" pursued along three development paths: cost/strategic-element reduction, enhanced temperature capability, and application of the gamma double prime hardening system to other nickel alloy bases. The article traces the commercially dominant lower-cost alloy 625, the large-volume but developmentally stagnant alloy 706, and a series of high-temperature variants (630, PWA1472, René 220, and ultimately the successful wrought alloy 718Plus, plus the newer VDM 780) developed to meet aircraft engine manufacturers' demand for +100 °F capability beyond 718. It also covers special-purpose derivatives for oil and gas service (625 Plus, 725, 625M, and newer lower-nickel alloys 945/945X/AD955) and the unique low-thermal-expansion "Invar effect" family (903, 909, 908) used in aircraft rings, seals, and cryogenic applications. Despite decades of derivative development, alloy 718 remains dominant, exceeding the combined volume of all its derivatives.