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Expanding upon the effort to redesign Alloy 718 in order to provide microstructural and mechanical stability beyond 65O”C, six modified compositions have been studied after a precipitation hardening treatment and then after aging for 534 hr at 730°C (1350°F). The variations in Al, Ti and Nb content provided an (Al+Ti)/Nb ratio between 0.66 and 0.92, an Al./Ti ratio between 0.88 and 1.69 and total hardener (Al+Ti+Nb) content between 5.80 and 6.80 at pet; two of the alloys also contained 0.76 or 1.77 at pet W as a solid solution strengthening element. Even though the rate of y ’ coarsening was faster after the aging treatment, the coarsening rate of y ” and the decline in strength occurred more slowly in the four alloys with a higher (Al+Ti)/Nb ratio, and with less transformation to 6 phase, than was the case in the two lower (Al+Ti)/Nb ratio alloys. The two alloys having the highest (Al+Ti)/Nb and Al/Ti ratios and a W addition provided increasing strength and a slower rate of y ” coarsening. Hardness and tensile strength after the conventional heat treatment exceeded the published results for high strength processed 718 and, after 534 hours at 730°C and testing at room temperature, these mechanical properties were retained to the highest degree in these same alloys. Under stress rupture conditions at 700°C (1300”F), these alloys survived from 35 to 53 hours under a stress of 92.5 ksi whereas conventional 718 has a published rupture life of 18 hours when stressed at 85 ksi. At this stress level, one of the alloys having both ratios and total hardening element content on the high side of the range survived for 74.3 hours. These preliminary results suggest that the basic composition of Alloy 718 can be modified to a minor degree in order to improve temperature capability beyond 650°C. Superalloy 718--Metallurgy and Applications Edited by E.A. Loria The Minerals, Metals & Materials Society, 1989

Improving Thermal Stability of Alloy 718 via Small Modifications in Composition