New insights into the kinetics of dynamic and post-dynamic softening in Alloy 718 engine disks

Abstract

The dynamic and post-dynamic recrystallisation (DRX and PDRX) kinetics of Alloy 718 are known to be dependent on the strain rate, deformation temperature, and the fraction of the δ-phase in the heterogeneous starting materials. Better control of these regimes is required for direct ageing (DA), to unlock superior high-temperature strength in next-generation aeroengine disks. There are gaps in the current understanding of the (post-)dynamic regime and its effect on γʹ and γʹʹ co-precipitation. Our aim is to reveal the critical parameters controlling the grain size and recrystallised fractions after thermo-mechanical processing (TMP) to model the post-dynamic microstructure evolution.Two initial microstructures with different δ-phase fractions are subjected to 1.4 strain with 0.1–10 s−1 strain rates to simulate industrial conditions. A short isothermal ost-deformation annealing is applied to assess the restoration behaviour. DRX is suppressed at strain rates >1 s−1 at 950 °C due to limited time for dynamic restoration, while PDRX become predominant due to higher stored energy. At 1000 °C, a fully recrystallised microstructure is achieved in all conditions. The presence of δ-phase increases DRX and PDRX rates and promotes microstructure stability. Our results are underpinned by models that predict PDRX and precipitation kinetics during DA.

Keywords

• Nickel-based superalloys
• Thermomechanical processing
• Dynamic recrystallisation
• Precipitation
• Strengthening