Deformation behaviour of an as-cast nickel base superalloy during primary hot working above and below the gamma prime solvus

Abstract

Primary hot working behaviour of an as-cast nickel base superalloy has been evaluated by carrying out isothermal hot compression tests both above and below the gamma prime solvus. The flow (yield σ0, peak σp, steady state σss) stress of the material was found to be highly temperature sensitive when deformed below the gamma prime solvus. The coherent gamma prime precipitates present in the material were found to be sheared by the moving dislocations in this regime. The material was relatively insensitive to temperature changes above the gamma prime solvus and exhibits discontinuous dynamic recrystallization (DDRX) when deformed between 1100 °C – 1140 °C and 6 × 10−3 to 10−2/sec with a characteristic efficiency value of 48% as defined by dynamic materials modeling. The kinetics of DDRX was evaluated by carrying out hot compression tests up to a strain of 140% under peak efficiency conditions. The friction corrected flow curve was used to estimate the critical strain (εc) for DDRX initiation and the saturation stress (σsat) for dynamic recovery. The fraction recrystallized (XDDRX) estimated using the flow curve analysis was found to correlate well with the quantitative microstructural observations. The activation energy map developed using the back stress corrected flow stress and the true stress exponent revealed that glide along with diffusion controlled climb as the rate controlling mechanism. Side-pressing test was used to validate the experimental findings and also to establish a suitable thermo-mechanical processing schedule to achieve an order of grain refinement. The results obtained are presented and discussed here.