Strong yet ductile carbide-strengthened niobium alloy fabricated by laser powder bed fusion

Additive manufacturing (AM) of refractory alloys and high-entropy alloys (RHEAs) are gaining increasing interest due to their superior high-temperature strength and low buy-to-fly ratio. However, most of refractory alloys and RHEAs suffered from room temperature (RT) brittleness and hence severe cracking during AM. Here we report the strong yet ductile carbide-strengthened niobium alloy (so-called Nb521) fabricated by laser powder bed fusion (LPBF). The LPBF Nb521 alloy showcases excellent tensile strength-ductility at RT and outstanding specific yield strength at 1200°C, as compared to other promising niobium alloys and RHEAs. The LPBF Nb521 alloy exhibits hierarchical microstructures at various length scales, including irregular-shaped grains, nano-scale dispersed carbides and carbides-decorated low/high-angle grain boundaries. Transmission electron microscopy investigations revealed that the good tensile ductility originates from movement of unusually predominant mixed dislocations. Density functional theory calculations uncovered potential reasons for the sufficient ductility, i.e., higher Rice ductility parameter and lower unstable stacking fault energies than brittle elements. Further analyses pointed out the reasons of high strength of the LPBF alloy and potential directions for pursuing even higher strength. Consequently, this study not only deepens the understanding of deformation mechanisms of carbide-strengthened niobium alloys, but also provides a reference for the further design of strong yet ductile niobium alloys.