Effect of Mo on microstructure and mechanical behaviour of as-cast Nbss–Nb5Si3 in situ composites

The effect of Mo on the structure–property relationships of arc-melted cast in situ composites of Nb solid solution (Nbss) and α-(Nb,Mo)5Si3, with hypereutectic Nb–19.1Si–5.2Mo (A) and Nb–17.9Si–26.3Mo (B), and hypoeutectic Nb–12.8Si–4.1Mo (C) and Nb–12.3Si–14.8Mo (D) compositions has been studied. The influence of Mo concentration on lattice constants and microhardness of the constituent phases has been analyzed. The morphology, volume fraction and size distribution of the primary phase and the eutectic have been critically examined and related to the elastic modulus, hardness and fracture toughness of the composites. Fracture toughness values, determined through three-point bend and indentation tests, increase with higher amount of coarse eutectic but decrease with increasing Mo content. Indentation cracking exhibits R-curve type behaviour, promoted through bridging or arrest of cracks by the coarse Nbss in the eutectic. The study suggests promise for near-eutectic compositions of Nb–Si–Mo ternary system with low concentration of Mo for structural applications.