Preparation Of Niobium-Based Ultrahigh-Temperature Alloy Ingot & Selection Of Crucibles For Directional Solidification

Niobium-based alloys have high melting points, relatively low densities and excellent mechanical properties including high temperature strength, room temperature fracture toughness, so they have been the focus of recent studies. They have high potential for the future aerospace applications.The composition systems of niobium based alloys have been designed in this paper. The ingots were produced by vacuum self-consumable arc melting. The arc-melted microstructure was examined using an optical microscope (OM), scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscope (EDX). The crucibles for directional solidification were selected and examined through experiments.The results of SEM indicated that the microstructure was uniform and consisted of large amounts of eutectic microstructures. X-ray diffraction analysis showed that this arc-melted microstructure consisted of niobium solid solution (Nbss) and niobium silicides (Nb₅Si₃/Nb₃Si). Nb₅Si₃ has an ordered structure. The analysis of selected area electron diffraction pattern revealed that there were certain orientation relationships between Nb₅Si₃ and Nbss, That was [010] // [135].The samples of niobium based alloys were remelted in graphite crucibles, graphite-based SiC coated crucibles, graphite-based Y₂O₃ coated crucibles and BN crucibles. For the graphite crucibles, the surface of the sample has a thickness of 50μm reaction layer, and the resultant production were mainly the carbides of titanium and niobium. The cross-like carbides are also found inside the alloy bars. Because the reaction or decomposition of SiC coating, the reaction and production was nearly the same as that is the pure graphite crucibles. Because the preparation technology of Y2O3 coating was not well developed, the Y2O3 coating had not prevented the crucible from reaction with the melted alloy. The reaction of the Niobium-based alloy was much more severe when re-melted in a BN crucible, and there were many erose holes in the out region of the alloy bars. The thickness of the reaction layer is about 60μm, and there were many cavities found inside the alloy. The reaction product in the BN crucible is mainly the nitride of the titanium and niobium.