Microstructural evolution and mechanical properties of as-cast and annealed Nb-Si-Mo based hypoeutectic alloys with quaternary additions of Ti or Fe

The effect of quaternary addition of 20 at% Ti and 4 at% Fe on microstructural evolution, dynamically measured Young's modulus and Poisson's ratio, hardness, indentation fracture toughness (IFT), and compressive strengths of arc-melted hypoeutectic Nb-12Si-5Mo in as-cast or annealed (1500 degrees C for 100 h) condition have been investigated. Microstructural studies have shown the presence of primary niobium-rich solid solution (Nb-ss), and eutectic comprising Nb-ss+(beta or alpha) (Nb, x)(5)Si-3, where x = Mo, Ti, or Fe. Additionally, beta-Ti-ss and Nb4FeSi have formed in Ti and Fe containing alloys, respectively. Annealing has led to conversion of partially lamellar to non-lamellar eutectic morphology, besides compositional alterations in phases. The dynamic Young's modulus being influenced by Mo-content of Nb-ss, and phase volume fractions, is found to peak in Ti-containg alloy (similar to 150 GPa), with further increment on annealing (similar to 162 GPa) due to beta -> alpha (Nb, x)(5)Si-3 transformation. Whereas micro-hardness of Nb-ss (6-7 Hv) is influenced by solute content, that of eutectic (16-17 GPa) is also affected by (Nb, x)(5)Si-3 content. Further, both hardness and compressive strength being influenced by phase volume fraction, and eutectic morphology, have decreased in Nb-12Si-5Mo and Nb-12Si-5Mo-20Ti alloys, but increased marginally in Nb-12Si-5Mo-4Fe alloy on annealing. The IFT appears to be the highest (similar to 22 MPa root m) in the Ti-containing alloy, with toughening contributions from bridging and arrest of cracks by the ductile Nb-ss phase.