Effect of direct ageing on the microstructure, tensile properties and fracture behavior of a Re-bearing polycrystalline nickel-based superalloy

The effect of post-forging heat treatment on the microstructure and tensile properties of the heavily alloyed nickel-based superalloy SDZhS-15 (Ni-37(Co,Cr,Mo,W,Re)-12.5(Al,Ti,Nb,Ta)-0.17(B,C,La,Y,Ce), wt.%)) intended for disc applications at temperatures up to 850 degrees C was studied. The as-cast superalloy was subjected to homogenization, subsolvus hot forging, and various heat treatments, including either solution treatment at subsolvus temperature and ageing or direct ageing. The tensile tests showed that the strength/ductility combination at T = 20-850 degrees C was appreciably better in the fine-grained condition. SEM study revealed that the fraction of intergranular brittle cleavage fracture increased with increasing the test temperature and at transition from the fine-grained condition obtained via direct ageing to the coarse-grained one obtained via solution heat treatment and ageing.