MICROSTRUCTURE EVOLUTION OF LAVES PHASE Cr2Nb/Cr ALLOYS PREPARED BY ARC MELTING

The Cr-12%Nb (atomic fraction) and Cr-20%Nb alloys prepared by vacuum non-consumable arc melting were investigated to understand the microstructure evolution of the Laves phase Cr2Nb/Cr alloys. The solidified microstructures including the phase formation and competitive growth were studied using OM, XRD as well as SEM with an EDS. The results showed that different microstructure morphologies were observed in the Cr-12%Nb and Cr-20%Nb alloys. For the Cr-12%Nb alloy, divorced eutectics were grown at the bottom part of the ingot. Primary Cr dendrites and coupled eutectic (Cr2Nb+Cr) appeared in the middle and upper part of the ingot. For the Cr-20%Nb alloy, cellular eutectics of which the lamellar spacing was about 0.3 mu m were found at the bottom of the ingot. With the decrease in cooling rate in the middle and upper part of the ingot, plate primary Cr2Nb dendrites and petal-like Cr2Nb dendrites were developed. Besides, using the TMK model for rapid eutectic solidification and the BCT dendritic growth model, the interface growth temperatures between the coupled eutectic (Cr2Nb+Cr) and primary Cr2Nb phase were computed and compared with the experimental results. Based on the maximum interface growth temperature criteria, the appearance of multiple solidified microstructures of the Cr-20%Nb alloy could be explained successfully.