Two-stage annealing treatment to uniformly refine the microstructure, tailor δ precipitates and improve tensile properties of Inconel 718 alloy

Abstract

In order to further improve the tensile properties of Inconel 718 alloy, a novel two-stage annealing method was employed, which involved cold rolling, primary annealing, secondary annealing and aging treatment. The evolution of microstructure and precipitates were analyzed by OM (optical microscope), EBSD (electron backscattered diffraction) and EPMA (electron probe microanalyzer). For the conventional single-stage annealing method, the cold-rolled sample was directly annealed at 950 ℃ for 1 h (0–950 sample). The recrystallization behavior competed with the precipitation of δ phase during annealing, as a result, clustered needle-like δ phases precipitated along a specific direction. Besides, the bimodal distribution of grain size indicated the microstructure of 0–950 sample was not uniform. However, for the two-stage annealed sample, the primary annealing at 1010 ℃ for 45 s lowered the recrystallization barrier, causing the recrystallization behavior prior to the precipitation of δ phase during subsequent secondary annealing at 950 ℃ for 1 h (45s–950 sample). As a result, numerous granular δ particles precipitated along the fresh grain boundaries and uniformly refined grains were obtained in the 45s–950 sample. The 45s–950 sample after aging treatment had an excellent combination of tensile strength (∼1140 MPa) and total elongation (∼25 %) when deformed at 650 ℃. For the room temperature properties, the ultimate strength was about 1490 MPa and the corresponding elongation was ∼ 20 %. The results showed that the appropriate two-stage annealing can improve the microstructure uniformity, refine the grains and tailor the precipitates, all of which were contributors to the enhanced mechanical performance.