The effect of Mo on microstructure and mechanical properties of Nb-22Ti-16Si alloy additively manufactured via laser directed energy deposition

Alloying process and processing optimization are most effective methods to modify the microstructure and mechanical properties of Nb-Si based alloys. In this work, Nb-22Ti-16Si-xMo(x = 5–20 at%) alloys were successfully fabricated by a laser directed energy deposition additive manufacture with elemental powder mixtures. The effect of Mo on the microstructure, micro-hardness, nano-hardness and fracture toughness of Nb-22Ti-16Si-xMo alloy were investigated. The results indicated that the addition of Mo in Nb-22Ti-16Si alloy is promoting the formation of Ti(Nb)₅Si₃ and β-Nb₅Si₃ phases and inhibiting the formation of Nb₃Si phase. Mo was preferentially enriched in the niobium solid solution (Nbss) phase. The volume fraction of Nbss phase first increases then slightly decrease, while Ti(Nb)₅Si₃ phase was increased with Mo content, especially Nbss matrix also become more continuous. The solidification path of Nb-22Ti-16Si-xMo alloys show a certain deviate from the prediction by the Scheil solidification models. Mo alloying in Nb-22Ti-16Si alloy also lead to the increase in the young's modulus and nano-hardness of Nbss and β-Nb₅Si₃ phases. The fracture toughness of Nb-22Ti-16Si-xMo(x = 5–20 at%) alloys first increases to 12.2 MPa m^1/2 when Mo content increases to 10 at%, then decreases with further increases in Mo content, while their micro-hardness shows the opposite trend.