Preparation Of Nb/Nb₅Si₃ In-situ Composites By Microwave Activated Combustion Synthesis

Microwave activated combustion synthesis has caused widespread concern in recent years due to different process of combustion reaction. Compared with conventional ignited SHS reactions, in microwave activated combustion process the ignition occurres at the centre of the body and combustion wavefront propagates radially outwards. This leads to different microstructures and more pure products owing to dissimilar time-temperature and spatial temperature profiles. Currently, reports relevant to preparing Nb/Nb₅Si₃ composites adopting microwave heating are not found at home and abroad. Therefore fabricating Nb/Nb₅Si₃ composites by microwave activated combustion synthesis (MACS) technology from elemental powder compacts is researched in this study. The exsiting problems in the experimental process, such as oxidation and cracking of samples and limit of temperature rising, were resolved successfully through the optimized microwave sintering parameters. Based on that the effects of process parameters were investigated respectively such as forming pressure, milling time, sintering temperature, holding time and components factor included Si, Al, Mo content on the Nb-Si alloy microstructure, density and bending strength. The samples were analyzed with optical microscope (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), electron probe microanalysis (EPMA) and other modern analytical tools.The result showed that :(1) By optimizing the microwave sintering process, the sample embedded in powder with the ratio of 40%Al₂O₃, 58%SiC, 2%C (wt. %), which could effectively solve exsiting problems in the experimental process, such as oxidation and cracking of samples and limit of temperature rising.(2) Process parameters included the forming pressure, milling time, sintering temperature, holding time had a great impact on the microstructure and properties of Nb-Si alloy. With the increse of milling time, the main phase of sintering products transformed fromβ-Nb₅Si₃ to Nb0.81Si0.19, and the the microstructure was more uniform and fine. As the sintering temperature increaseed, the intensity ofα-Nb₅Si₃ peaks decreased, and the intensity ofβ-Nb₅Si₃ diffraction peaks increased. The sample achieved the most uniform microstructure with the sintering temperature of 1450℃and the dwell time of 30 minutes.(3) Components factor included Si, Al, Mo content can affected the microstructure and properties of Nb-Si alloy. In terms of microstructure, rising of sintering temperature and increasing content of Si resulted in transforming of Nb₅Si₃ fromα- Nb₅Si₃ toβ- Nb₅Si₃, but the form of Nb₅Si₃ was mainly depending on the Si content in the alloy composition.The addition of alloying elements Al, Mo can affect the phase composition and microstructure of Nb-Si based composites. The addition of alloying elements Al and Mo can improved the stability ofβ-Nb₅Si₃ and removeed metastable phase Nb3Si. Nb particles surrounded byβ-Nb₅Si₃ were observed in the microstructures of in-situ composites Nb/Nb₅Si₃ with addition of Al, otherwise Nbss/β-Nb₅Si₃ lamellar eutectic were observed with additon of Mo. In terms of properties,the bending strength was decreased with the icrease of Si contents, and firstly increased and then decreased with the increase of Al and Mo content, the alloy achieved the highest bending strength with the Al,Mo content of 2at.% and 10at.% respectively.