Supersolvus Recrystallization and Grain Growth Kinetics for the Fine Tuning of Grain Size in VDM Alloy 780 Forgings

Abstract

VDM Alloy 780 is a new polycrystalline nickel-based superalloy developed for aeronautical applications. In most of the targeted applications, grain size after forging must be precisely controlled to meet the targeted mechanical properties and in-service life requirements. Grain size in forgings is the direct consequence of the recrystallization and grain growth kinetics which are addressed in this paper at high temperatures, above the solvus temperature of γ′ and η/δ phases. The dynamic and post-dynamic recrystallization kinetics as well as the grain growth kinetics of VDM Alloy 780 are detailed over a range of thermomechanical conditions. Dynamic recrystallization appears to be limited, with only 30 pct recrystallized at quite high strain of 1.7 applied at 1050 °C and 0.01 s−1 for instance, but this is compensated by fast post-dynamic evolution. Within the investigated thermomechanical range, recrystallization is completed with 5 minutes of post-deformation hold in VDM Alloy 780 independent of the prior strain, strain rate and dynamic recrystallization fraction. For a strain as low as 0.08, an isothermal annealing of 30 minutes at 1050 °C generates a homogenous and fully recrystallized microstructure. Capillarity driven grain growth following recrystallization is also relatively slow, for instance an exposure at 1050 °C (50 °C above the solvus temperature) for 2 hours results in an increase in average grain size from 20 to 70 μm. This opens the possibility to fine tune the grain sizes by subsequent heat treatments within a time scale that is compatible with industrial conditions. The high cobalt content (25 pct) is suspected to play a role in the control of microstructure evolution kinetics. It is noteworthy that VDM Alloy 780 is shown here to not undergo the heterogeneous grain growth phenomenon reported in low strain regions for other nickel-based superalloys, which is also an asset for applications requiring strict control of grain sizes and grain size distributions.