Investigation on microstructural and microhardness evolution of GH4698 superalloy under transiently varying strain rates

Abstract

The high-temperature deformation behavior of GH4698 superalloy was explored via isothermal compressive tests under transiently varying strain rates. The impacts of loading types and deformation temperature on flow stress, microstructural evolution, and microhardness were explored, and the underlying nucleation mechanisms for dynamic recrystallization (DRX) were elucidated. Experimental results indicated that the flow stress decreased when the strain rate decreased transiently. Specifically, under loading type I (interrupted strain = 0.11), the flow stress was higher than that under loading type II (interrupted strain = 0.36). Moreover, the flow stress decreased with increase in deformation temperature. When the initial interrupted strain was high, a high degree of DRX was observed in the deformed microstructure. The DRX fraction of types I and II are 48 % and 61 %, respectively. Higher deformation temperatures accelerated the growth of DRX grains, and at a deformation temperature of 1100 °C, DRX fraction reaches 87 %. The microhardness of the DRX grains was higher than that of the deformed grains. At higher deformation temperatures, the γ′ phase dissolved, and the DRX grains grew, resulting in smaller microhardness of the DRX grains than that at lower deformation temperatures. However, microhardness was not sensitive to the loading type (i.e., the value of interrupted strain). Furthermore, discontinuous dynamic recrystallization (DDRX), activated by the initial bulging of grain boundaries, served as the fundamental mechanism for DRX nucleation. Similarly, continuous dynamic recrystallization (CDRX), characterized by the formation of subgrains through dislocation cell structures within individual grains, as well as the γ′ phase and twin boundaries, also played crucial roles in the nucleation of DRX during transiently variation states conditions.