Effect of Nb element on the solidification microstructures and high-temperature tensile properties of nickel-based eutectic composite

Nickel-based eutectic composite is considered a promising high-temperature structural material due to its intrinsic stability of the eutectic microstructure at elevated temperatures and its composite reinforcement of strength by carbide fibers and γ′ precipitations. To optimize the microstructure and enhance the mechanical properties of nickel-based eutectic composite, the effect of Nb, a key element promoting the formation of NbC fibers and γ′ phase, is investigated on solidification microstructure and tensile properties. The 3D reconstruction by X-ray imager indicates that NbC fibers are the branch arms growing from the top corners of NbC blocks and continuous in 3D space. The NbC fibers and γ/γ′ phases have an orientation relationship of [100]_γ/γ′||[100]_NbC and (002)_γ/γ′||(002)_NbC. As the Nb content increases from 3 to 5.6 wt%, the stability of planar interface growth during directional solidification decreases, resulting in the formation of long strip-shaped NbC fibers in the composites with 3 and 4.3 wt% Nb, and skeleton-like NbC in the composite with 5.6 wt% Nb. The volume fraction of NbC fibers and γ′ phase respectively decreases and increases with increasing Nb content, due to compositional re-distribution induced by varying Nb content. The composite with 3 wt% Nb exhibits the best tensile properties at 900 °C, mainly attributed to its good microstructures consisting of Nb fibers with high volume fraction and slender transverse area and γ′ precipitates with small size. Additionally, the fracture morphologies and fracture mechanism are discussed. These findings provide valuable insights for the composition design and performance optimization of nickel-based eutectic composites.