ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2026 20 compendia. Later Pratt & Whitney (P&W) developed PWA1472, which also had 6% Nb and 2% Ti but with chromium lowered to 12% to eliminate Laves phase. This alloy has been successfully used as castings in P&W engines[9]. More focused science-based investigations were undertaken in the 1980s. One approach employed a partial substitution of tantalum for niobium; because tantalum partitions to and stabilizes the gamma double prime phase and is less segregation prone[10]. Tantalum also suppresses delta phase making grain size control more difficult. The price of tantalum is five times that of niobium making this alloy commercially unattractive. Another approach taken by Tien and others investigated the manipulation of Al/Ti and Al+Ti/Nb ratios to improve the stability of gamma double prime[11]. It was known from Eiselstein’s early work that both Al and Ti preferentially stabilize gamma prime. But closely controlled experiments demonstrated that within limits improved gamma double prime stability could be achieved. These positive results unfortunately produced only incremental improvement but paved the way for more successful alloy developments. Aircraft engine technical leaders preached the need for an alloy with 718 strength and welding characteristics with additional 100°F (55°C) temperature capability[4,9]. Virtually the entire supply chain took a crack at this problem. René 220, developed by Chang at GE Corporate R&D in 1989, was the first serious candidate. In this alloy, iron was replaced by cobalt and 3% tantalum was added[12]. The new alloy had higher tensile and stress-rupture strength than alloy 718 and met the goal of +100°F temperature capability. The substitution of cobalt for iron and increase in nickel suppressed Laves in the as-cast microstructure, thereby improving castability and weldability. René 220 was successfully inserted into aircraft engines in structural castings. The alloy was not successful as a wrought alloy due to segregation in large ingots and poor hold time crack growth. Loria suggested revised Al-Ti-Nb-Ta ratios to achieve a better balance of gamma prime and gamma double prime, but this avenue was not pursued[13]. The goal for a wrought alloy was finally achieved about a decade later with the introduction of Allvac 718Plus by ATI[14]. This alloy has equal amounts of iron and cobalt, higher Al/Ti ratio and a small addition of tungsten and no tantalum. In wrought form Allvac 718Plus maintains the mechanical pro- turbine rotor remains the only significant use for alloy 706, but the hundreds of heavy parts manufactured annually puts it high in the rank of superalloy volume. Efforts to modify alloy 706 have had no commercial success to date, in part due to the formation of Laves phase and the reduced stability of gamma double prime. Most proposed modifications fall in the composition gap between alloys 706 and 718. ENHANCED TEMPERATURE CAPABILITY The desire of aircraft engine designers to extend temperature capability began shortly after the alloy was introduced. Special Metals Corp. developed an alloy called 630 that had 6.5% Nb. It was extensively evaluated by Standard Pressed Steel Co. for aerospace fasteners under a NASA contract. The alloy did have higher room temperature strength than alloy 718, but it exhibited notch brittleness, presumably due to excessive formation of lamellar delta phase. In addition, it did not have superior high temperature strength[8]. The alloy was never commercially produced but it still appears in some superalloy Workers assemble a gas turbine, blade by blade. Courtesy of Dreamstime.com. Sample fastener used in aerospace engines. Courtesy of Howmet Aerospace. Close-up of gas turbine blades. Courtesy of GE Vernova.
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