Improving Thermal Stability of Alloy 718 via Small Modifications in Composition

Abstract

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.

Creep Behavior of Compact γ′-γ″ Coprecipitation Strengthened IN718-Variant Superalloy

“…Moreover, reports suggest that in similar γ -γ precipitate containing alloys, a slower cooling rate generally increases high temperature strength as well as rupture life [31,32] However, creep behavior of the developed alloy contradicts these reports. In fact, all the reports on creep behavior suggest that air-cooled IN706 and IN718-type alloys containing compact coprecipitates are always outperformed by conventional IN706 or 718, respectively [26,[32][33][34][35][36]. This is consistent with our results, where the aged IN718-27 in both AC and FC conditions rupture at much lower strains than air-cooled and aged IN718 at 1200 • F (649 • C).…”

The Effect of Nb, Ti, Al on Precipitation and Strengthening Behavior of 718 Type Superalloys

“…al [2] studied the precipitation behavior of γ″ and γ′ and intended to improve the structure stability of Alloy 718 at high temperature. In recent twenty years many researchers such as J. K. Tien [3,4] , Encai Guo [5,6] , Keh-Minn Chang [7] , Zhuangqi Hu and Wenru Sun [8] studied the stability of Alloy 718 and try to develop modified 718 Alloys to be used beyond 650 . Recently Wei-Di Cao and R. Kennedy has developed a modified 718 Alloy to be used at 700 designated as Allvac ® 718 Plus T M [9] .…”

Research on Inconel 718 Type Alloys with Improvement of Temperature Capability

“…Numerous attempts have been made over the past several decades to improve temperature capability of Inconel 718, typically through chemistry modifications involving adjustment the content of Al, Ti and Nb and minor elements such as P and B. Cozar and Pineau firstly developed the compact morphology of γ″ and γ′ by adjustment Nb, Ti and Al content and their ratios, and found this special compact microstructure characterized with superior structure stability than that of the separated precipitation of γ″ and γ′ [3,4]. Subsequently, Tien, Guo, Radavich and Chang et al adjusted the Al/Ti ratio and (Al+Ti)/Nb ratio based on conventional Inconel 718 alloy, and the experimental results indicate that higher Al/Ti ratio and (Al+Ti)/Nb ratio can remarkably improve the structure stability of 718 alloy [5][6][7][8][9][10][11][12][13][14][15]. However, these experimental results mainly concentrate on the importance of γ″ and γ′ stability during long time aging and neglect other phases precipitation behavior.…”