Microstructures and Mechanical Properties of Additive Manufactured Niobium Silicide Alloys

Nb-Si alloys, noted for their high-temperature strength, may outperform nickel-based superalloys in demanding applications. However, further research is needed to optimize their composition and understand the impact of alloying elements on their microstructure and properties. This project seeks to study the fracture mechanisms of Nb-Si alloys, the effect of alloying elements on their properties, and analyze materials from the Laser Metal Deposition (LMD) 3D printing method, considering the impact of post-processing via Hot Isostatic Pressing (HIP) and Heat Treatment (HT).A greater amount of γ silicide improves fracture toughness, which is promoted by Ti, Mo, and B, and stabilized by Zr and V. Fracture toughness relates to Geometrically Necessary Dislocation (GND) density and dislocation distribution. Y stabilizes α and β silicide phases but may reduce toughness. Ta encourages α silicide formation and increases hardness over the base alloy, with its atomic size enhancing silicide grain resistance. LMD can result in material voids, which can be eliminated by methods like HIP and HT.The materials display a hypereutectic structure with primary γ-silicide shapes affected by the thermal gradients of 3D printing. These gradients promote grain growth aligned with heat flow, creating a dominant γ-silicide orientation. Post-processing did not alter the bulky silicide structure, likely due to insufficient temperature and duration. Higher γ-silicide volumes enhance fracture toughness because of their favourable Bulk to shear modulus (B/G) ratio, which enhances resistance to cracks.Dominance of Ti in the composition favours Nb5Si3 formation over Nb3Si. Besides the Niobium solid solution matrix and silicide, other orthorhombic or trigonal phases arise from residual liquid elements post-formation. This may be caused by powder segregation before 3D printing, where minor elements cluster. Shorter transport and feeding distances can reduce this effect. Divorced/anomalous eutectic microstructures arise due to cooling rates, alloy composition, impurities, and melt superheat.