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%, β-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 γ/γ' phases and the elastic strain energy increases, then the size of the γ' phase increases when the Al concentration rises from 7 wt% to 8 wt%. When the Al content increased to 7 wt%, the tungsten-rich α-W phase is precipitated and the volume fraction is 1.20%. The volume fraction of the α-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 °C / 235 MPa, it revealed that the precipitation of hard and brittle α-W and β-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 γ' 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. •The precipitation temperature of tungsten-rich α-W phase increases with the increase of Al content by Thermo-Calc.•When the Al content reaches 8wt.%, hard and brittle β-NiAl phase is precipitated in the eutectic position.•The content and morphology of tungsten-rich α-W phase with different Al contents were studied by XCT.•Spherical and petal-shaped tungsten-rich α-W phases precipitated when the Al content approaches 7wt.%.