Composition-driven interface engineering in diffusion-bonded PM superalloys: γ′-forming element content controls microstructural evolution and fracture behaviour

The influence of γ′-forming elements (Al, Ti, Ta) on interfacial microstructure and mechanical properties of diffusion-bonded powder metallurgy Ni-based superalloys were investigated using alloys with γ′ volume fractions of 37%, 45% and 55%. Low γ′ content induced a wide (~50 μm) γ′-depleted zone with curved recrystallised grains, while medium and high contents formed a narrow (~5 μm) stacked γ′ zone without element depletion. All joints followed hetero-epitaxial recrystallisation to form γ-encapsulated γ′ grains. Excess Ta in the high-γ′ alloy precipitated nanoscale Ta-rich MC carbides (~35 nm) and causes local Ta depletion. Tensile tests revealed a composition-dependent fracture transition: low γ′ alloy fractured at base metal with 31.7% elongation, high γ′ alloy failed at the interface with reduced elongation (18.5%) despite the highest strength. The alloy with 45% γ′ exhibited optimal strength-ductility synergy. This study demonstrates that synergistic design of base alloy composition and interlayer is crucial for high-quality diffusion-bonded joints, offering references for the manufacturing of aero-engine turbine components.