| The effects of the fabrication and heat-treatment methods on the microstructures and properties of the Nb-Si system intermetallics have been studied and the optimized Nb-Si intermetallic system as well as the optimized fabrication and processing procedures have been selected. The presence of alloying elements as well as the morphology and microstructure of the compound phases with these elements has been investigated. The effects of the alloying elements on the high-temperature oxidation behavior of the Nb-Si system intermetallics have also been studied. The micro-mechanisms of the high temperature oxidation resistance and room temperature compression rupture behavior of the Nb-Si system intermetallics have also been discussed in this paper. The foundation to develop Nb-Si system intermetallics base high temperature structure materials has also been established in some degree in this paper.Arc melting, powder metallurgy and optical floating zone technology have been used and compared to fabricate the Nb-Si system intermetallics. The results indicate that arc melting is a good method to produce Nb-Si system intermetallics due to its simpler technics, lower cost and compact products. However, powder metallurgy is found to be not suitable to produce the Nb-Si system intermetallics due to its coarse and loose products resulting from the poor molding property of Nb and Si mixed powders. Optical floating zone technology, which is used to fabricate Nb-Si intermetallic composites for the first time, is also found to be a good way to produce Nb-Si system intermetallics because of its compact products and good property despite of its relatively high cost.The results reveal that the microstructure of the Nb-Si system intermetallics consists of Nb and NbaSi phases. Because of unstable microstructure of NfySi phase at high temperature, the equilibrium Nb+NbsSia dual-phase microstructure of the Nb-Si system intermetallics should be acquired by means of heat-treatment. According to the Nb-Si phase diagram, the temperature of 1550癈, which is lower than the eutectoid transformation temperature of NbsSi (1783癈), is chosen for heat-treatment. According to the presentconditions of the furfaces, heat-treatment by stages is first proposed for the Nb-Si system intermetallic composites to keep the furface in good conditions. The results indicate that the equilibrium Nb+NbsSia dual-phase microstructure of the Nb-Si system intermetallics forms gradually via such eutectoid reaction as NbaSi梌Nb+NbsSis with the heat-treatment time. In the Nb-Si system intermetallics after heat-treated at 1550癈 for 100 hours (25hx4), the microstructure of the Nb-Si system intermetallic composites completely becomes equilibrium Nb+NbsSia dual-phase with only a little coarsing grain size. Then the heat-treatment of 1550癈/100h (25h*4) by stages is a good way for the Nb-Si system intermetallics.The microstructures and interface structures of the Nb-Si system intermetallics with/without heat-treatments as well as and microalloying element distribution and the microstructures of the compounds phases of the alloying element have been investigated using scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy and X-ray diffraction technics. The results indicate that the microstructure of the as-cast Nb-lOSi (at.%) alloy fabricated by arc melting consists of continuous NbsSi matrix and dispersive distributed Nb particles. The metastable NbaSi phase is found to have a tetragonal crystal structure with space group P42/n and lattice parameters a=1.021nm, c=0.519nm. Compared with those of the as-cast Nb-lOSi (at.%) alloy, the microstructures of the as-cast Nb-18.7Si (at.%) alloy are found to have increased volume fraction of NbsSi phase due to the increased Si content and more cracks and defects.. The microstructure of single-crystal Nb-18.7Si (at.%) fabricated by optical floating zone consists of continuous NbsSi+NbsSia intermetallics and Nb particles, which indicates that some NbaSi particles have transfor...
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