On the control of nanoprecipitation in directly aged Alloy 718 via hot deformation parameters

Abstract

Alloy 718 remains the material of choice for critical parts in next-generation aircraft engines. The direct ageing (DA) route is known to significantly change the final multi-scale microstructure, achieving superior yield strength. This includes changes in the dislocation density and co-precipitation behaviour, although a systematic study of the influence of forging temperature and strain rate is currently unavailable. Here, we elucidate how geometrically necessary dislocations (GNDs) generated during hot deformation impact the morphology and co-precipitation of γ′ and γ′′ during ageing. Lower GND density at high-temperature forging leads to increased formation of γ′′ monoliths and up to two-fold hardness increments after ageing. For all selected forging temperatures, the high GND density at the intermediate strain rate of 1 s − 1 generates ∼8% lower fractions of duplet co-precipitates, minimizing DA hardness increments. Understanding the inverse correlation between GND density and DA efficiency is essential for designing Alloy 718 for next-generation aerospace applications.