ADVANCED MATERIALS & PROCESSES | MAY 2026 1 7 nology. By 1965, the alloy 718 composition and heat treatment had been defined in commercial and society spe- cifications. The nominal composition of alloy 718 and other alloys mentioned in this article are listed in Table 1. Before the microstructure of alloy 718 was fully characterized, the strengthening mechanism for nickel- base superalloys was understood to be the precipitation of an ordered cubic phase with a nominal formula of Ni3(Ti,Al) that was designated gamma prime. Strengthening was based on moderate coherency strain, dislocation looping and cutting to form stacking faults[7]. This phase has a wide solubility for transition elements that affect its shear strength and interface properties and compositions have been tailored for optimum performance for specific engine components. Alloying with substantial amounts of cobalt and refractory metals further enhanced strength and temperature capability of these alloys. Alloy 718 was much stronger than the legacy superalloys at low temperatures but overaged and was replaced by the incoherent acicular delta phase above 1200°F. Although alloy 718 also contains some gamma prime, another phase appeared to be responsible for the higher yield strength. The actual strengthening mechanism was not unambiguously defined until the late 1960s when it was shown to be a tetra- gonal phase based on Ni3Nb that was designated gamma double prime[8]. The intense coherency strain generated by this phase accounted for its unique strengthening ability. Another feature of the gamma double prime phase is its slow kinetics of formation, which accounts for the improved forming, heat treatment, and weldability of alloy 718 and its derivatives. OPPORTUNITIES FOR COMMERCIALIZATION When alloy 718 was introduced, Huntington Alloys was still using process equipment dating from the early 1920s. The first commercial scale heats were melted in air-induction furnaces, air-cast into 500-lb (227 kg) ingots, hammer forged and/or rolled to plate, sheet, and bar, and heat treated on hand-serviced mills and furnaces[9]. Hot working and heat treatment temperatures were loosely defined, providing inconsistent control of microstructure. During the period between the patent application and first engine insertions in 1965, the ingot manufacturing practice matured, particularly through innovation by Latrobe Steel and Special Metals[10]. Vacuum induction melting was introduced for improved cleanliness. Secondary melting processes such as vacuum arc remelting for rounds and electroslag remelting for slabs produced solid, more chemically homogeneous ingots for hot working. Press forging and hot rolling on reversing mills with mechanical handling were introduced. Forging and heat treatment temperatures were defined in commercial and society specifications. Inco developed an air melted argon cast version of the alloy at its Bayonne New Jersey foundry for investment cast components. The first commercial vacuum melted castings were produced in 1965 by Precision Castparts Corp.[11] Many processing innovations have since been made to control segregation, generate a uniform fine grain microstructure, and improve consistency of properties, but the 718 forged or cast product of 1965 would be acceptable today for many non-critical applications. Several novel aircraft engine programs were underway during the alloy 718 incubation period. These applications provided opportunities to evaluate alloy 718 in structural components where the poor weldability of Waspaloy or René 41 compromised manufacturing reliability. For General The secret and later cancelled GE XNJ-140E nuclear power plant combined with a GE X211 turbine engine also contained a prototype alloy 718 compressor frame. Courtesy of GE Aviation Evendale. North American Aviation XB70 Valkyrie Mach 3+ bomber. GE J93 engine program provided valuable technical data for fabrication of alloy 718 turbine frame. Courtesy of NASA.
RkJQdWJsaXNoZXIy MTYyMzk3NQ==