The performance of polycrystalline Ni-based superalloys is largely determined by the fraction and size of theintermetallic hardening phases, such as the γ′ phase. Understanding the evolution of the γ′ precipitate size andvolume fraction during the complex thermomechanical process chain is essential for optimizing the mechanicalproperties of polycrystalline Ni-based superalloys. In this study, the high temporal resolution of in situ synchrotronX-ray diffraction at elevated temperatures was used to monitor γ′ evolution in VDM® Alloy 780 duringcooling and subsequent aging heat treatments in real time. To simulate the industrial forging process, the alloywas initially subjected to compressive deformation at 1000 ◦C. In the first part of the study, immediately after hotforming, three different cooling rates (10 ◦C/min, 100 ◦C/min, and 1000 ◦C/min) were applied to analyse the insitu evolution of γ′ precipitation. The resulting γ′ phase fraction was highest at the slowest cooling rate, yet eventhe fastest cooling rate did not completely suppress precipitation. In the second part of the study, the agehardeningresponse of the alloy was examined at holding temperatures of 720 ◦C and 800 ◦C for five hourseach. The evolution of the γ′ volume fraction, precipitate size, and lattice parameter was monitored in situ byXRD. After aging, γ′ volume fractions of 15 % and 19 % were obtained, with average precipitate sizes of 9 nm and18 nm, respectively. The prior deformation enhanced γ′ formation at 720 ◦C compared to literature data.Complementary Transmission Electron Microscopy (TEM) measurements confirmed precipitate sizes consistentwith the diffraction data.
Fritton et al. (Wed,) studied this question.