Study of three-phase equilibrium in the Nb-rich corner of Nb-Si-Cr system

The full understanding of the Nb-Si-Cr ternary system is important for the development of Nb silicide based composites, which show great potential for high temperature applications. There was, however, disparity in experimental observations of phase equilibrium in the vicinity of the Nb-corner. Two kinds of three-phase equilibrium, Nb-ss + C14 + alpha Nb5Si3 and Nb-ss + CrNbSi + alpha Nb5Si3, in the Nb corner of the Nb-Si-Cr system have been reported in the literature. This work aims to clarify the three-phase equilibrium near the Nb-corner, by studying phase equilibrium in the Nb-18Si-15Cr ingot. Such a composition was chosen with the assistance of CALPHAD calculations to avoid unnecessary load of work. The alloy ingot was prepared by clean melting followed by heat treatment at 1000 and 1500 degrees C. The C14 Laves phase formed in all the samples and was stable at both temperatures. The results confirmed that Si has the effect of stabilising the C14 Laves phase down to at least 1000 degrees C. The three-phase equilibrium Nb-ss + C14 + alpha Nb5Si3, instead of Nb-ss + CrNbSi + alpha Nb5Si3, was observed in this work. The current work demonstrates that ingot metallurgy is necessary to check the reliability of the information obtained about the ternary and higher-order phase diagrams, especially for the regions where the exact phase boundaries are in question. The investigation of the selected alloy was greatly helpful to clarify the confusion of the three-phase field near the Nb corner in the Nb-Si-Cr ternary system. The work confirmed the CALPHAD prediction of phase equilibrium near the Nb corner. showing the power to combine phase diagram predictions with experimental work for cost effective alloy development. (c) 2006 Elsevier Ltd. All rights reserved.