ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2026 18 Before the ink dried on the alloy 718 patent, the International Nickel Co. (Inco) and others sought to leverage the intense niobium-enabled precipitation reaction to create unique alloy capabilities. Three threads of investigation emerged that are still being actively pursued today. In addition to composition optimization within the limits of UNS N07718 to enhance specific properties as discussed in Part II of this series, there has been a continued effort to reduce cost or strategic element content of the base alloy. At the urging of aircraft engine manu- facturers, new variants designed to raise the upper use temperature of the alloy were sought. In a third thread of alloy development, the gamma double prime age-hardening system was applied to other nickel alloy bases that were selected for unique physical or corrosion properties. BACKGROUND This article describes the development and use of these so-called “718 derivatives.” The subject was of such interest that all the conferences following the inaugural 1989 Superalloy 718 Conference have included the term “derivatives” in the event name. The properties of alloy 718 are based on a delicate balance of three compounds having the nominal stoichiometric ratio Ni3X. Gamma double prime stabilized by niobium has a tetragonal structure forming disk shaped particles that create a high level of coherency strain. This phase was proposed by Cometto and Raymond and then confirmed by Paulonis, Oblak, ALLOY 718: PART III DERIVATIVE ALLOYS This final installment in a three-part series reviews how modifications to the composition of 718 led to new alloys with increased capabilities and applications. John deBarbadillo, FASM,* Barboursville, West Virginia *Member of ASM International and Duvall[1]. Gamma prime is stabilized by aluminum and titanium and forms spherical or cubic particles and is the primary strengthening phase for most superalloys. Delta phase, which is strictly Ni3Nb, has an orthorhombic structure and forms spherical or lamellar particles of little strengthening value but essential for grain size control. These phases are stable over limited ranges of nickel, chromium, cobalt, and iron, sometimes forming complex particles[2]. The story of alloy 718 derivatives is one of balancing formation of these phases to obtain desired properties. First, it must be acknowledged that the most commercially successful alloy 718 derivative is alloy 625, a nominally non-aging, lower niobium content alloy termed by Herbert Eiselstein, FASM, as a “matrix-stiffened alloy.” This initial derivative was known primarily for its formability, weldability, and resistance to corrosion in seawater. The development of alloy 625 immediately followed alloy 718 as described by Eiselstein and Inconel 625 billets. Courtesy of Alloys International Inc. Tillack at the second Superalloy 718 Conference[3]. The alloy does experience modest and very sluggish age-hardening during prolonged exposure at temperatures above 1200°F (649°C). The associated embrittlement limits its use in this temperature range. The properties and applications for alloy 625 are widely known and will not be further discussed here. Alloy 625 has achieved significant volume second only to alloy 718 among nickel-base alloys. The nominal composition of alloys 625, 718, and other alloys mentioned in this article are contained in Table 1. LOWER COST ALLOYS The decade of the 1970s was marked by concern over potential disruption in the supply of the strategic elements cobalt, chromium, tungsten, niobium, and nickel, none of which were produced in quantity in the United States. The cost and availability of cobalt was said to have accelerated the acceptance of alloy 718 since unlike most superalloys it is cobalt-free[4]. Approaches to reduce the niobium content through increased aluminum and titanium were not successful in maintaining the high strength property[5]. Eiselstein lowered the nickel and niobium content to reduce cost, improve machinability, and minimize risk of solidification segregation. The new
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