Investigation of grain refinement during a rejuvenation heat treatment of wrought Alloy 718

The grain refinement resulting from a rejuvenation heat treatment (RHT) of Alloy 718 was investigated by using both metallographic analysis and orientation imaging microscopy (OIM). A step heat treatment was used to show that a phase transformation induced recrystallization process did not occur and supported static recrystallization as the method by which grain refinement occurred. Prior deformed grains in the wrought Alloy 718 microstructure were subject to recrystallization during RHT once the intragranular δ-phase pinning grain boundaries was dissolved. The coexistence of fully recrystallized, substructured and deformed grains in the rejuvenated microstructure revealed by OIM showed that an inhomogeneous recrystallization process occurred during RHT. Both the orientation relationship of grains and the behavior of grain boundaries involved in the recrystallization process were differentiated based on OIM characterization.