Unveiling the microstructure-stress triaxiality-fracture toughness relation in ductile Nb/stiffening Nb₅Si₃ biphase alloys prepared using spark plasma sintering

Nb-12Si alloys were fabricated via spark plasma sintering using Nb and Nb₅Si₃ powders with systematically varied sizes. When coarse Nb₅Si₃ powders (73.7 μm and 39.6 μm) were employed with different Nb powders, a microstructure consisting of a Nb matrix containing Nb₅Si₃ islands was obtained. Reducing the Nb powder size from 75.5 μm to 4.5 μm resulted in a substantial refinement of the Nb grain size from 72.1 μm down to 7.9 μm. Conversely, when fine Nb₅Si₃ powder (2.9 μm) was utilized, the microstructure inverted to form a continuous Nb₅Si₃ matrix with isolated Nb islands. The alloy prepared with 4.5 μm Nb powder and 73.7 μm Nb₅Si₃ powder demonstrated significantly enhanced fracture toughness of 14.37 ± 0.55 MPa·m^1/2, which is markedly superior to that of the alloy made with 75.5 μm Nb powder and 73.7 μm Nb₅Si₃ powder. (10.68 ± 0.37 MPa·m^1/2). Finite element method simulations further elucidated that reducing the Nb grain size effectively lowers the stress triaxiality (TRIAX) imposed by the constraining Nb₅Si₃ phase. This reduction in TRIAX promotes higher plastic strain and activates the a/2 <1 1 1>{1 1 0} slip systems in the Nb phase. Consequently, the fracture mode transitions from brittle {0 0 1} cleavage to ductile dimple-tear failure, thereby substantially improving the overall toughness of the Nb-12Si alloy. This study establishes a critical microstructure-TRIAX-toughness relationship and identifies TRIAX control as a fundamental principle for designing advanced toughened Nb-Si based superalloys.