Dynamic Precipitation During Forging of a γ–γ′ Nickel-Based Superalloy
Résumé
Hot compression and creep tests under different conditions were performed on a typical γ–γ′ polycrystalline nickel base superalloy below the γ′-phase solvus temperature, to shed light on the mechanisms of γ′ precipitation during deformation and the driving forces behind these mechanisms. Scanning Electron Microscopy (SEM) and image analysis were employed to characterize the different populations of γ′ precipitates and their evolutions. The γ′-phase area fractions observed after hot compression are systematically higher than the fraction prior to deformation within the investigated range of thermomechanical conditions. Two new populations of precipitates formed during deformation are identified and characterized. These are interpreted to result from a dynamic precipitation process, the hypothesis of their formation during the quenching step being very unlikely with regards to their size and amount. The fraction of γ′-phase produced during deformation increases with increasing deformation magnitude, decreasing deformation temperature and for slower strain rates. Finally, a scenario for dynamic γ′ precipitate nucleation and growth is proposed to explain the experimental observations and the influence of dislocation density.