Discontinuous Dynamic Recrystallization Mechanism and Twinning Evolution during Hot Deformation of Incoloy 825
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JMEPEG https://doi.org/10.1007/s11665-020-05093-1
Discontinuous Dynamic Recrystallization Mechanism and Twinning Evolution during Hot Deformation of Incoloy 825 XiTing Zhong, LinKe Huang, and Feng Liu (Submitted March 4, 2020; in revised form August 24, 2020) Discontinuous dynamic recrystallization (DDRX), which involves multiple nucleation and grain growth processes, plays a crucial role in grain refinement; however, the underlying mechanism and the significant role of twin boundaries (TBs) remain poorly understood. Here, the evolution of characteristic microstructures and the fraction and density of TBs under different deformation conditions (i.e., 1050– 1200 °C, 0.001–1 s21) in Incoloy 825 was investigated through a thermomechanical simulator, electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). The first strand of recrystallized grains nucleated along the original grain boundaries (GBs), separated by the newly formed random high-angle grain boundaries (HAGBs), which were transformed from low-angle grain boundaries (LAGBs) and pre-existing TBs. Subsequently, straight TBs forming in the new grains increased the misorientation angle, promoting the migration of stagnated grain boundaries, and parts of newly generated twinning chains at the front of the recrystallization zone converted into random HAGBs, providing sites for the following layer of nucleation. Moreover, triple junctions between the recrystallized and deformed grains also served as potential nucleation sites when the LAGBs in the large misorientation gradient were transformed into random HAGBs. Quantitative relationships between the recrystallized grain size and TB density were obtained. Keywords
dynamic recrystallization, hot deformation, Incoloy 825, microstructure evolution, twin boundary evolution
1. Introduction Hot working accompanied by dynamic recrystallization (DRX) is widely used to prepare final or intermediate products in various metals and alloys due to its effective softening and significant control of the microstructure, which improves the formability and mechanical properties of the material (Ref 1-7). Generally, the mechanisms responsible for the formation of multiscale structures can be classified as discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) (Ref 1, 2). For low to medium stacking-fault energy (SFE) alloys that activate dislocation slip and twinning simultaneously (Ref 8, 9), DDRX is the main mechanism during hot deformation and has two obvious steps, i.e., nucleation and grain growth. As the deformed microstructure is replaced by recrystallized grains in an alternating manner of nucleation and grain growth, a typical necklace microstructure XiTing Zhong and LinKe Huang, State Key Lab of Solidification Processing, Northwestern Polytechnical University, XiÕan 710072 Shaanxi, PeopleÕs Republic of China; and Feng Liu, State Key Lab of Solidification Processing, Northwestern Polytechnical University, XiÕan 710072 Shaanxi, PeopleÕs Republic of China; and Analyt
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