Effects Of High Temperature Heat Treatments On The Microstructure Of An Nb Silicide Based Ultrahigh Temperature Alloy

Three Nb-Si based ultrahigh temperature alloy ingots with different nominal compositions were prepared by vacuum consumable arc-melting. Ultrahigh-temperature high vacuum heat-treatment furnace was employed to conduct heat treatments respectively at 1300, 1350, 1400, 1450, 1500 and 1550℃ for 24h, and also at 1450℃ for 12, 50 and 100h respectively. An optical microscope (OM), a scanning electron microscopy (SEM) equipped with an energy dispersive spectroscope (EDS), an X-ray diffraction analyzer (XRD) and a transmission electron microscope (TEM) were employed to analyze the characteristic of arc-melted microstructure, the constituent phases and their compositions. The microstructural evolution upon heat treatments was also evaluated.The microstructures of the three ingots were all fine and uniform, and the primary phases in ingots 2^# and 3^# were niobium solid solution (Nbss), but that in ingot 1^# was big silicide blocks. Large amounts of eutectic colonies existed in the microstructure of the three ingots. The microstructures of the three Nb-Si based ultrahigh temperature alloys were all composed of Nbss and silicides of M₅Si₃ type. The silicide in ingot 1^# was hexagonal (Nb,Ti)₅Si₃, and that in ingot 2^# was β(Nb,Ti)₅Si₃. Nb contents both in the (Nb,Ti)ss and (Nb,Ti)₅Si₃ were higher than Ti contents in ingot 1^#; but in ingots 2^# and 3^#, the results were reverse. The contents of Nb+Ti in (Nb,Ti)ss were about 86at%, while in the silicide the contents were approximately 56at% in ingots 1^# or 2^# .After heat treatments at different temperatures or for different prolonging time, sharp interfaces and corners of the primary big (Nb,Ti)₅Si₃ blocks gradually changed into blunted and curved ones, and some of the primary big (Nb,Ti)₅Si₃ blocks broke into small ones. The morphology of (Nb,Ti)₅Si₃ in the eutectic colonies changed from slices into particles. With the increase of heat treatment temperatures or time, the boundaries of hexagonal blocks and strips of silicide became round and smooth. The amounts of retained primary big silicide blocks and eutectic colonies decreased with the increasing heat treatment temperatures or time. After heat treatments at 1550℃/24h or 1450℃/100h, the retained eutectic colonies were few. The microstructure was still composed of (Nb,Ti)ss and (Nb,Ti)₅Si₃ with hexagonal crystal structure after heat treatments. Compared with the ;arc-melted ones, the lattice...