Effects of B4C addition on microstructure and mechanical properties of Nb-16Si-20Ti alloys fabricated by laser melting deposition

Practical applications for NbSi alloys encountered difficulties because of their low room temperature fracture toughness and machinability. To achieve rapid alloy formation and enhance its fracture toughness, this study explored the effect of B₄C additions on the microstructure and properties of Nb-16Si-20Ti alloy formed by Laser Melting Deposition (LMD). The results showed that the relative densities of LMDed alloys were all more than 97.0 % compared with arc melting alloys. The LMDed 0B₄C alloy consisted of Nbss, Nb₃Si, and Tiss phases. With the addition of B₄C increased from 0 to 0.50 at. %, Nb₃Si disappeared, a eutectic transformation reaction (Nb₃Si → Nbss+ α-Nb₅Si₃) occurred, resulting in an increase in the volume fraction of Nbss from 39.5 % to 50.3 %. Further increased the B₄C addition to 1.0 at. %, the microstructure transitioned from a Nbss+ α-Nb₅Si₃ eutectic structure (without B₄C addition) to a fine primary α-Nb₅Si₃ and (Nbss+α-Nb₅Si₃) hypereutectic structure (with volume fraction of all α-Nb₅Si₃ phases up to 41.4 %). The LMDed 1.0B₄C exhibited the highest fracture toughness of 10.7 MPa·m^1/2 among all alloys due to the refinement of the microstructure and the isotropic of the stresses in the equiaxed crystal Nbss. Meanwhile, its compressive strength at room temperature reached a maximum of 2203.3 MPa, a result attributed to the grain refinement strengthening, solid solution strengthening effect of carbon and boron in Nb₅Si₃, and the increase in silicide content (with a total volume fraction of silicide phases at 51.6 %).