Secondary γ′ evolution and relationship to hot deformation behavior of a supersolvus-treated superalloy with high precipitate volume fraction

This paper presents results from a research initiative aimed at the evolution of secondary gamma' and its effects on hot deformation behavior by using compression tests of a supersolvus-treated superalloy at 1080-1200 degrees C and 0.001-1 s(-1). Secondary gamma' in deformed regions and in dynamic recrystallization (DRX) regions have different evolution behaviors during hot working. In deformed regions precipitates have a noteworthy coarsening tendency depending on strain rates. At high strain rates, e.g. at 1 s(-1), gamma' is elongated along strain direction by plastic deformation. Whereas, directional coarsening takes place at low strain rates, e.g. 0.001 s(-1), owing to the severe plastic deformation and the high concentration level of gamma'-forming element induced by supersolvus treatment. And thus secondary gamma' evolution in deformed regions can be attributed to two competing factors: (a) plastic flow and (b) directional coarsening induced by diffusion. In contrast, gamma' in DRX regions always dissolves following grain boundary migration and almost re-precipitates at boundaries. Only a small number of re-precipitated gamma' can be found in the interior of new grains. Abnormal phenomenon of sharp stress decline appears at low strain rates and low deformation temperatures, which is related to DRX, gamma' directional coarsening and gamma' dissolution.