On the hot deformation simulator HDS-20 uniaxial compression tests of two nickel superalloys supplied in the molded state were performed. The cylindrical specimens were heated at rate of 10 °C·s-1 directly to the deformation temperature (i.e. from 880 °C to 1,240 °C with dwell time of 180 s after heating) and then were compressed at a constant strain rate of 0.1 s-1, 2 s-1 or 30 s-1 to a true strain of 1.0. Both materials exhibit a similar true stress-true strain curve with a relatively strong peak signalling the initiation of dynamic recrystallization. From the peak stress, the values of hot deformation activation energy were calculated, namely 483 kJ mol-1 for Inconel 600 and 601 kJ mol-1 for Incoloy 825. With the knowledge of these variables, equations describing peak stress could be derived for both superalloys (known sinus hyperbolic relation) with good accuracy. It corresponds to the peak strain (power dependence) depending on the Zener-Hollomon parameter, which represents the temperature-compensated strain rate. These dependencies and experimental data were mathematically and graphically compared for both materials. Peak stress is the amount appropriate for simple and fast prediction of the maximum flow stress due to the deformation parameters, excluding the effect of strain.Keywords: nickel superalloys, activation energy during hot deformation, true strain, true stress
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