Aging Effects on the γ' and γ'' Precipitates of Inconel 718 Superalloy

Abstract

The excellent mechanical properties of Inconel 718 are due to the γ′ and specifically the γ′′ precipitates. Solution heat treatment was performed at 1095 ºC for 1 h followed by air cooling. A δ-phase precipitation heat treatment was made at 955 ºC, 1 h/AC. Finally a double-aging at 720 ºC, 8 h/FC at 57 ºC/h down to 620 ºC, 8 h/AC to precipitate both γ′ and γ′′ phases. The precipitation of both γ′ and γ′′ phases were studied in details after aging treatment by the use of transmission electron microscopy. The spherical precipitates were identified as γ′ phase and the ellipsoidal precipitates were γ′′ phase. At 720 ºC for 8 h both γ′ and γ′′ are growing; the mean long-axis of γ′′ particle is 40 nm and the mean short-axis is 14 nm whereas the average diameter of the spherical γ′ particle is 17 nm. However, at 620 ºC, γ′′ stops growing but γ′ continue to grow and the diameter increases to 21 nm.