Primary hot working characteristics of an as-cast and homogenized nickel base superalloy DMR-742 in the sub and super-solvus temperature regime

Abstract

Primary deformation behaviour of a medium alloyed nickel base superalloy DMR-742 in the as-cast and homogenized condition (furnace cooled) was evaluated in the sub-solvus and super-solvus temperature regimes. The flow stress in the sub-solvus regime was found to be highly sensitive to temperature. The contribution of back stress to the flow stress sensitivity was estimated using the microstructure dependent analytical equations and compared with the back stress estimated using experimental steady state flow stress. It was established that Orowan looping or by-passing mechanism contributes to the back stress resulting in high flow stress sensitivity in the sub-solvus regime. In the super-solvus regime, the material was relatively temperature insensitive and exhibited discontinuous dynamic recrystallization (DDRX) with a characteristic power dissipation value of 38–44% in three different temperature-strain rate regimes. Disregarding the industrially non-viable low strain rate regime, the recrystallization kinetics in the moderate and high strain rate regime was estimated using grain size factor compensated grain orientation spread maps. A higher rate of recrystallization observed in the material deformed at high temperature-strain rate was attributed to relatively strong dislocation interactions and weak recovery. The true stress exponent, activation volume and energy estimated using back stress and elastic modulus compensated flow stress revealed that diffusion controlled climb and cross-slip as the rate controlling deformation mechanism. Side-pressing under high secondary tensile stress conditions was used to evaluate the workability and validate the experimental findings.