Avoiding Microstructural Instabilities in High Tungsten Containing Superalloys by Adjusting Al Content

The effects of Al contents on the microstructures and mechanical properties of superalloy with high tungsten content were studied. During solidification, Al was found to be enriched in the residual liquid, which led to the transformation of eutectic morphology from reticular to bulk-like. When the Al content increased to 8 wt%, beta-NiAl phase formed in the inter-dendritic region, Thermo-Calc calculations confirms such the experimental phenomenon. The absolute value of the lattice misfit between the gamma/gamma' phases and the elastic strain energy increases, then the size of the gamma' phase increases when the Al concentration rises from 7 wt% to 8 wt%. When the Al content increased to 7 wt%, the tungsten-rich alpha-W phase is precipitated and the volume fraction is 1.20%. The volume fraction of the alpha-W phase increases to 7.84% when the Al content reached 8 wt%, which is consistent with the Thermo-Calc calculation results. After the room temperature tensile and stress rupture life test at 975 degrees C / 235 MPa, it revealed that the precipitation of hard and brittle alpha-W and beta-NiAl phases with high Al content, which resulted in the mechanical properties decreased due to the crack initiation at multiple locations and then interconnected. The higher density of gamma' phase with 6 wt% Al makes it difficult for dislocations to bypass. The site of crack initiation is decreased due to the absense of brittle phase. The stress rupture life can reach 49.21 h of the alloy.