Abstract: This study aims to elucidate the multiple strengthening mechanisms provided by the heterostructure formed during the solidification and cooling of a novel, compositionally complex Ni-based filler metal alloy (Inconel 680) produced by wire arc additive manufacturing (WAAM), which underlie their remarkable mechanical properties. This material is designed to offer high strength, ductility, toughness and corrosion resistance for various applications, including 3D printing and welding of high-strength materials. Through in-depth microstructural characterisation using advanced techniques, we provide detailed information about the structure and chemistry at the nanoscale. Our findings reveal that the most significant solid-solution strengthening elements segregate strongly during the solidification process. The interdendritic volume is supersaturated in Nb and Ti. The combination of thermodynamic conditions, fast cooling rate and multiple heating cycles during deposition leads to the precipitation of γ″-Ni₃Nb nanoprecipitates in the interdendritic volume. This heterogeneous microstructure consists of a solid-solution γ-fcc Ni matrix single phase in the dendrite cores and a dual-phase microstructure in the interdendritic region formed by γ-fcc plus 23 % of γ″-Ni₃Nb phase. The resulting heterogeneous microstructure provides a high yield strength (624.5 MPa measured and 651.9 MPa calculated), outperforming traditional solid-solution strengthened Ni-based alloys by approximately 200 MPa, without compromising ductility. This research provides crucial insights for advancing high-performance materials designed for additive manufacturing or welding techniques. It emphasises the importance of fostering heterogeneous microstructures that effectively mitigate the strength-ductility trade-off, thereby enhancing material properties.

Key words: Ni-based alloy, Heterogeneous structure, γ″-Ni₃Nb phase, Mechanical properties, Microstructure