Thermal Stability of Alloys 718 and Allvac 718-ER

Abstract

Thermal stability of ~l l \~a c \ l l o~ 7 1 8-~~' and alloy 718 was evaluated by the change in microstructure and mechanical properties after long time thermal exposure. The tnajjor microstructural changes due to thermal exposure included-J" and y' particle gro~b-th. formation of a .J"-depletion zone. and occurrence of large. block) particles resulting from y" to 6 phase transformation. All three changes were quite similar in both alloys, indicating that P and B did not hake a noticeable effect on microstructural degradation in alloy 718. Tensile strength at room and elevated temperatures dropped 10-17% after thermal exposure in both alloys. Stress rupture life was reduced by nearly 90%, but creep resistance dropped by more than two orders of magnitude. The degradation of creep rupture properties in terms of percentage reduction was significantly greater in alloy 7 18-ER, and most of the improvement from P and B modification Lvas lost after thermal exposure. The strengthening effect of P and B modification was also a function of test temperature. It n a s concluded that P and B did not increase the thernlal stability of allo) 718 at 704OC x. . 1000 hrs. exposure and that the loss of rupture and creep properties was caused mainly by :/ dissolution and the formation of a y"-depleted zone at grain boundaries. The effect of' P and B on rupture and creep properties of alloy 7 18 appears to be closely associated with y" precipitates. 71 8-ER is a registered trademark of AT1 Properties. Inc. Supi.r;lllo>s 718. 675. 706 and Various Deri\ativcs Edited b E.A. Lorin 111s (Ihc: Xlinzrols. I\li.tnls Kr Materials Socist)) 2001

Evolution of secondary phases in alloy ATI 718Plus®during processing

“…iii. Fully heat-treated disc, subject to the standard heat treatment recommended by ATI Allvac [19]:…”

“…A small amount of rod-shaped η particles, homogeneously distributed over most grain boundaries, is recommended to prevent notch sensitivity at high temperature and improve resistance to intergranular crack propagation [19]. In addition, the pinning effect of the grain boundary precipitates leads to a better control of the grain structure during hot working operations (when these take place below the relevant solvus temperature), essential to meet the grain size target of ASTM 10-14 usually preferred in disc components for static strength and resistance to fatigue crack initiation [20].…”

Allvac 718Plus, Superalloy for the Next Forty Years

“…The alloy composition reflects a continuation of work performed at ATI Allvac in the early 1990's directed at improving the creep and rupture properties of alloy 718 by purposeful additions of P and B [15][16][17][18][19][20][21]. The resulting improvements were equivalent to about a 20°C increase in temperature capability.…”

The Effects of Solutionizing and Aging Temperature on the Microstructure of Allvac 718plus

“…A precipitate free zone (PFZ) was observed to surround the δ phase near the grain boundaries. [23][24][25]. Figure 9 shows the 1000°C solutionized microstructure where the δ phase precipitates decorated the grain boundaries.…”