Microstructure evolution in GH4742 superalloy by combining hot deformation and heat treatment

Abstract

The microstructure evolution of GH4742 superalloy under the coupling action of hot deformation and heat treatment was studied by isothermal compression and then solvus heat treatment (SHT), with particular attention focused on the interaction mechanism between γ′ precipitates and dynamic recrystallization (DRX) grains. The deformation of material below the γ′ solvus temperature (1020–1050 °C) by DRX was retarded because of the strong pinning effect of γ′ precipitates. At a strain of 0.6 and all strain rates (0.01–1 s−1), a necklace-like microstructure formed with fine DRXed grains (∼2 μm) and a small DRXed fraction (<20%). The γ′ precipitates dissolved with the migration of DRX grain boundaries (GBs), and there were no secondary γ′ precipitates within DRXed grains. During sub-solvus heat treatment (sub-SHT) of pre-deformed samples below the γ′ solvus temperature, the nucleation rate of post dynamic recrystallization (PDRX) increased significantly (30–50%), and the recrystallized grains were small (∼10 μm) due to a large amount of energy stored after thermal deformation. Rod-shaped γ′ precipitates appeared on the DRXed GBs, while coarse γ′ precipitates (3–5 μm) appeared on the PDRXed GBs and formed an approximate γ+γ′ duplex structure. When the strain was 0.6, regardless of changes in the strain rate (0.01–1 s−1) and temperature (1020–1140 °C) of pre-deformed samples, the microstructure was comprised of large, equiaxed grains with straight GBs and precipitates (20 nm) formed by cooling during super-solvus treatment (super-SHT).