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

Abstract

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 °C for 100 h) condition have been investigated. Microstructural studies have shown the presence of primary niobium-rich solid solution (Nbss), and eutectic comprising Nbss+(β or α) (Nb, × )5Si3, where × = Mo, Ti, or Fe. Additionally, β-Tiss 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 Nbss, and phase volume fractions, is found to peak in Ti-containg alloy (~150 GPa), with further increment on annealing (~162 GPa) due to β → α (Nb, × )5Si3 transformation. Whereas microhardness of Nbss (6–7 Hv) is influenced by solute content, that of eutectic (16–17 GPa) is also affected by (Nb, × )5Si3 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 (~22 MPa√m) in the Ti-containing alloy, with toughening contributions from bridging and arrest of cracks by the ductile Nbss phase.