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The hot deformation characteristics of as-cast nickel-base superalloy GH742y after hot isostatic pressing (HIP)(hereafter referred to as-cast alloy) have been investigated by hot compression tests in the temperature range of 1050 to 1140℃, strain rate range of 0.01 s-1 to 10 s-1 and strain range of 35% to 50% by means of Gleeble-3500 thermal mechanical simulator.The results show that the as-cast alloy exhibits the poor deformability, and shows wedge-shaped cracking beyond the strain of 35%.At strain rates less than 1.0 s-1,the stress-strain curves exhibit nearly steady-state behavior, while at strain rate of 10 s-1, a yield drop and serrated yielding occur.The activation energy values developed on the basis of the experimental data are divided into three domains.The first domain appears at lower strain rate (≤1.0 s-1) and lower temperature (≤1080℃), with the lowest mean value of activation energy about 261.4 kJ/mol.The second domain appears at the same strain rate as the first domain, but higher temperature (>1080℃), with the intermediate mean value of activation energy about 328.8 kJ/mol.The third domain appears at higher strain rate (10 s-1) and temperature range of 1050 to 1140℃, with the largest mean value of activation energy about 605.05 kJ/mol.Three different constitutive equations are established in corresponding to domains.Microstructural observations in the third domain reveal non-uniform dynamic recrystallization (DRX) of homogeneous γ phase, which leads to the poor deformability and the highest Q value.In contrast, microstructures in the first domain show fully DRX of homogeneous γ phase, leading to the better deformability and the lowest Q value.It is noted that the grain size increases with the increment of strain rate or temperature.These results suggest that bulk metal working of this material may be carried out in the first domain where fully DRX of γ homogeneous occurred.