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

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 gamma ' and gamma '' during ageing. Lower GND density at high-temperature forging leads to increased formation of gamma '' 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 similar to 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.

Automated summary

This study reveals the influence of geometrically necessary dislocations (GNDs) generated during hot deformation on the age-hardening performance of Alloy 718 (A718). Lower GND density during high-temperature forging results in increased precipitation of gamma '' monoliths and a two-fold hardness increase after aging. At all selected forging temperatures, higher GND density at an intermediate strain rate generates lower fractions of duplex co-precipitates, minimizing the age-hardening increment. Understanding the inverse correlation between GND density and age-hardening efficiency is crucial for designing A718 for next-generation aerospace applications.