Effect of δ phase on size effect in microtensile deformation of a nickel-based superalloy

The plastic deformation behavior in microforming is difficult to predict due to the size effect. Studying the effect of delta phase on size effect in microtensile deformation of a nickel-based superalloy is of great significance because the size effect in microforming of superalloys has been less studied. In this paper, various solution heat treatments and aging heat treatments of a nickel-based superalloy were performed to obtain various grain sizes and delta phases. The microtensile tests were carried out to investigate the effect of the delta phase on the size effect and plastic deformation behavior. The results show that the average length and diameter of the delta phase significantly increase with the grain size and aging time. Additionally, the volume fraction of the delta phase significantly increases with the aging time. The delta phase plays a strengthening role in the mechanical properties during the microtensile deformation. The interaction of the grain orientation and delta phase resultes in a "smaller is stronger" phenomenon when 1 to 2 grains occur across the specimen thickness. A new size effect constitutive model is established by considering grain boundary strengthening, solution strengthening and precipitation strengthening. The calculated flow stress values agree well with the experimental values. Fracture morphology analysis shows that the low stress triaxiality leads to ductile fracture with micropore formation, and high stress triaxiality results in intergranular fracture near the crack tip region.