Directionally Solidified Microstructure And Properties Of Nb-Si Based In-Situ Composites

Nb-Si based in-situ composites were processed by vacuum consumable arc-melting method. Samples with uniformly oriented microstructure were produced in a high temperature gradient directional solidification apparatus named Electron Beam Floating Zone Melting (EBFZM) furnace. The microstructure characteristics of both arc-melted and directionally solidified (DS) samples were analyzed. The relationship between microstructure and zone melting rate has been investigated. The effect of Zone melting rate on the high temperature tensile properties and the room temperature fracture toughness of the samples was also revealed in this paper. Additionally, the oxidation microstructure and kinetics were analyzed.As revealed by X-ray diffraction, in the as-cast condition, the microstructure consists of Niobium solid solution (Nbss) and Niobium silicide a -Nb5Si3/Nb3Si. While in the Zone melted condition, the microstructure consists of well-oriented primary Nbss dendrites and (Nbss+Nb5Si3/Nb3Si) eutectics. The side branches of primary Nbss dendrites have been constrained and are relatively small in size after zone melting. Furthermore, the (Nbss+Nb5Si3/Nb3Si) eutectics continuously distribute along the longitudinal axis of the bar. The relative contents of primary Nbss dendrites decrease slightly when the zone melting rate becomes higher. In the conditions described above, niobium silicides are mainly of a - Nb5Si3 type.The tensile testing at 1250℃ and the fractography show that all the samples ruptured in a ductile mode. The maximum tensile strength of Zone melted specimens reached to about 85.0MPa, which was 1.6 times higher than that of the as-cast ones. Room temperature fracture toughness was measured by three-point bending test and the KQ value for the DS condition was much higher than that for the as-cast condition. The maximum fracture toughness was 19.7MPam^1/2.It has been found through oxidation experiments at temperatures of both 950℃and 1050℃ that completely different inner and outer oxidation layers form for Nb-Si based in-situ composites: The former has compact microstructure with some needle-like oxide particles in it, and the latter has coarse oxide particles (column-like) that distribute vertically to the interface between the inner and outer oxidation layers. When exposed at 950℃ and 1050℃, the thickness of outer layers increased with oxidation time. The oxides at 1050 ℃ include TiO2, TiO2·Nb2O5 and 2TiO2·5Nb2O5 , While at 950℃, there appears Nb2O5 in addition to the above oxides. In the inner layer, there's some Nb5Si3 that has not been oxidized. Further analysis reveals that the oxidation kinetics of Nb-Si based in-situ composites follow parabolic law when exposed at 950℃ and 1050℃.