Effect of Nb and Nb₅Si₃ powder size on microstructure and fracture behavior of an Nb-16Si alloy fabricated by spark plasma sintering

The influence of Nb and Nb₅Si₃ powder sizes (Nb: 83.8, 37.4, 4.9 μm; Nb₅Si₃: 86.4, 43.6, 2.9 μm) on microstructure and fracture behavior of an Nb-16Si alloy fabricated by spark plasma sintering (SPS) was investigated. The results revealed that all as-sintered samples consisted of Nb and Nb₅Si₃ phases, with no new phases formed during SPS. The morphologies of the Nb and Nb ₅Si₃ phases in the as-sintered samples depended on the original Nb and Nb₅Si₃ powder sizes. Large Nb ₅Si₃ powders 86.4 and 43.6 μm in combination with Nb powder of described sizes resulted in an Nb matrix plus Nb₅Si ₃ island biphase microstructure, which supplied excellent fracture toughness. The microstructure made from the finest Nb powder (4.9 μm) and the largest Nb₅Si₃ powder (86.4 μm) had a high fracture toughness of 12.4 MPa m^1/2 through a mixed fracture mechanism of dimple, tear, and cleavage from the small Nb grains. When the Nb ₅Si₃ powder size was decreased to 2.9 μm, the sample tended to form an Nb₅Si₃ matrix plus Nb island biphase microstructure, which exhibited a poor fracture toughness value of between 5.6 and 8.0 MPa m^1/2 due to the crack propagation mainly within the brittle Nb₅Si₃ phase.