Effect of Chromium on Microstructure and Mechanical Properties of Nb–Si Hypoeutectic and Eutectic Alloys

摘要

The niobium silicide based alloys have been found to be the best alternative over Ni-based superalloys as they provide the balance between both ductility and fracture toughness at low temperature and strength and creep resistance at elevated temperature. The concept of making Nb-silicide based alloys is to form hard silicide phase which is thermodynamically stable and imparts adequate strength to the material at high temperature, within the primary ductile solid solution phase `of niobium. However, the main concern of using these alloys at high temperature is their low oxidation resistance and strength at high temperature. To overcome these limitations, alloying elements can be added, which will improve mechanical properties as well as high temperature oxidation resistance of the alloys. Our present investigation is to study the effect of chromium on the microstructure of Nb–Si hypoeutectic and eutectic alloys. Cylindrical rods of pre-set composition are synthesized by vacuum-arc-melting-cum-suction casting using argon atmosphere. The binary hypoeutectic alloys show the presence of primary (Nb)ss dendrites and lamellar eutectic between (Nb)ss and Nb3Si whereas primary Nb3Si along with eutectic (Nb)ss/Nb3Si are observed in the eutectic binary alloy. In the hypoeutectic Nb–Si–Cr ternary alloys, primary dendritic (Nb)ss, silicide (Nb3Si and/β-Nb5Si3), Laves (Cr2Nb)(depends on chromium concentration) and their eutectic phases are observed whereas Laves phase is found in the microstructure of eutectic Nb–Si–Cr ternary alloys even at very low concentration of Cr (2 at.%). At room temperature, compressive properties of these rods have been reported.