Dynamic and Post Dynamic Microstructure Evolutions During Forging of the New Nickel Based Superalloy VDM Alloy 780

Nickel based superalloys are widely used in the aerospace applications owing to their superior mechanical properties, resistance to oxidation at high temperature and ease of manufacturing. With the ever increasing demand of higher service temperatures in the aerospace industry, the use of alloys with improved microstructural stability becomes crucial in alloy design. With the objectives of improved microstructural stability, a new nickel based superalloy VDM Alloy 780 was developed as a high temperature turbine disk alloy for aerospace applications. This new superalloy consists of γ’ precipitates for strengthening in addition to the presence of a second phase to pin the grain boundaries (δ and possibly η phase).

Hot forging processes involve various metallurgical phenomena affecting the microstructure of the material which, in turn decides the final mechanical properties. The present work aims at studying the recrystallization mechanisms and kinetics during hot forging of the VDM Alloy 780. In order to closely relate to the industrial forging conditions, the post dynamic evolution occurring after the end of deformation while the material is still hot is also taken into account. The effect of strain rate on the resulting microstructures immediately after the end of hot compression and after subsequent post dynamic holding is investigated in this work. The results suggest that the effect of strain rate on the kinetics of recrystallization differs in the dynamic and post dynamic regimes and the effect is less pronounced in the latter case.