Flow stress characteristics and microstructural evolution of cast superalloy 625 during hot deformation

Abstract The study is aimed to analyze the macro- and microstructural changes of homogenized cast alloy 625 during uniaxial hot compression testing at various strain rates ( e ˙ = 0.01–10s−1) in a temperature range of 1050–1200 °C. A Gleeble-3800 simulator was used for conducting the tests. The peak stress analysis shows that the average activation energy of 520 kJ is required to accelerate dynamic softening over hardening in the studied alloy, where recrystallization is the rate defining process. The flow stress behavior of the alloy is studied using a physical model, and further finite element method is used as a tool to map the strain distribution. At intermediate e ˙ , especially at 1s−1, an unusual flow saturation is observed without revealing any peak stress. The macrographs of these samples indicate that the applied plastic strain is localized in small regions that are occupied with fine recrystallized grains, thereby leaving large undeformed zones. Detailed characterization of the simulated samples by SEM, EBSD and TEM indicate that at intermediate strain rates of 0.1 and 1s−1, the recrystallization rate is controlled by the formation of ∑3 twins at the interface of migrating high angle boundaries. The generation of recrystallization twins continuously changes the direction and velocity of a moving boundary by forming a new dynamically recrystallized grain. It is also observed that several recrystallization fronts are created due to the presence of carbide particles in the starting material through particle stimulated nucleation. The formation of the δ-phase by incipient melting/dissolution of carbide particles is observed for the sample deformed at 1200 °C with 10s−1. The results were discussed in terms of different softening mechanisms, such as dynamic recovery and recrystallization during the hot deformation of the studied alloy 625.