Microstructural control of Nb-Si alloy for large Nb grain formation through eutectic and eutectoid reactions

摘要

Alloys based on Nb-silicides exhibit superior high temperature strength than the commercial nickel based superalloys. In order to improve the low-temperature ductility by microstructural control, authors have attempted to understand the nature of the eutectic and eutectoid reactions in Nb-rich portion of the Nb-Si binary system, especially the crystallographic orientation relationship among the phases using FESEM-EBSD. During the eutectic reaction it was found that Nb rods of about 1 mu m in diameter having a certain crystallographic orientation disperse in Nb_3Si matrix in a eutectic cell. Then, during the following heat treatment Nb_3Si decomposes into a lamellar structure composed of Nb and alpha-Nb_5Si_3 in a manner of eutectoid reaction. It is observed that eutectoid Nb plates tend to nucleate at the eutectic Nb rod because both of them have a similar crystallographic orientation, resulting in a network of Nb phase with fine alpha-Nb_5Si_3 dispersoids. The Nb network may act as a large Nb grain which is expected to show better mechanical properties at both low and high temperatures. Both the reaction kinetics of the eutectoid decomposition of Nb_3Si and the tendency of the Nb network formation are significantly affected by doping elements, which are attributed to the control of the interfacial energies among phases.