Influence of Ti and Zr alloying elements on microstructure and micromechanical properties of near-eutectic Nb-18.7Si alloy

The present study investigates the effect of addition of Ti and Zr alloying elements on the microstructure, phase formation and micro mechanical properties of the phases of a near eutectic Nb-18.7Si cast alloy. The alloys were chracterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalyzer (EPMA), electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). Both the alloys Nb-18.7Si and Nb-18.7Si-5Ti exhibited a two phase microstructure consisting of Nbss and Nb3Si. The Nb3Si is not a stable phase and require further long term heat treatments to form stable and high creep resistant α–Nb5Si3 phase. Interestingly, the alloys added with Zr (Nb-18.7Si-5Zr), and Ti, Zr (Nb-18.7Si-5Ti-5Zr) exhibited Nbss, α–Nb5Si3 phases. This indicates that the addition of Zr alone or together with Ti resulted in the formation of equilibrium α–Nb5Si3 phase in the as cast condition itself. The formation of such phases in these alloys is beneficial in improving the properties and being cost effective. The volume fraction of the Nbss (~22–23%) is almost same in Nb-18.7Si and Nb-18.7Si-5Ti alloys. Whereas, the volume fraction is increased more than double (~53–55%) in Nb-18.7Si-5Zr and Nb-18.7Si-5Ti-5Zr alloys. The coarsening of the Nbss phase is also observed with alloying and is more prominant in Nb-18.7Si-5Zr and Nb-18.7Si-5Ti-5Zr alloys. The nano indentation results showed that the hardness of Nbss phase increased with Ti and decreased with Zr additions. Overall, a drastic increase in the elastic modulus of the silicide phase is observed in Nb-18.7Si-5Zr and Nb-18.7Si-5Ti-5Zr alloys as compared to alloys Nb-18.7Si, Nb-18.7Si-5Ti and is due to the formation of α–Nb5Si3 phase in the former alloys compared to Nb3Si phase in the later alloys. The higher volume fraction of Nbss phase, coarsening and decrease in hardness of Nbss in Nb-18.7Si-5Zr and Nb-18.7Si-5Ti-5Zr alloys resulted in improvement in fracture toughness (10.5 ± 0.35 MPa. m1/2). The formation of equilibrium phase α–Nb5Si3 directly in the cast condition is a significant breakthrough since it eliminates the need for long term vacuum heat treatment at higher temperatures.