Date of Award
Doctor of Philosophy
Niobium silicide-based composites are promising materials for high-temperature structural applications, such as gas turbines, but their implementation is hindered by the lack of effective processing methods. Self-propagating high-temperature synThesis (SHS) is an attractive method for fabrication of advanced materials. However, it is difficult to perform SHS of niobium silicides because the melting point of Nb is higher than the combustion temperature, which greatly inhibits Nb-Si reaction. To ignite an Nb/Si mixture and obtain Nb5Si3, the desired phase in niobium silicides, either preheating or mechanical activation of the mixture is necessary. This problem becomes even more critical in the SHS of Nb5Si3/Nb composites, where the combustion temperature is lower than in the stoichiometric Nb/Si mixture. Niobium silicide-based alloys also contain various additives, the most important of which is titanium.
The present work focused on the mechanically activated SHS of Nb5Si3, Nb5Si3/Nb, and Nb5Si3/Ti5Si3/Nb materials. Thermodynamic calculations helped design the experiments and understand the results. Milling of Nb/Si mixtures (23 â?? 37.5 at% Si) in a planetary ball mill enabled SHS. The effects of the milling time, pellet diameter, and mixture ratio on the process characteristics and product composition were investigated. Combustion of the stoichiometric Nb/Si mixture produced primarily Î± and Î³ phases of Nb5Si3. With adding more Nb, the content of the Î³ phase, which has poor mechanical properties, dramatically decreased. The addition of Ti, which melts at a relatively low temperature and readily reacts with Si, enabled combustion of mixtures with Si concentration as low as 20 at%. Such mixtures reacted in the spin combustion mode, producing Î±-Nb5Si3/Ti5Si3/Nb composites.
Received from ProQuest
Trevino, Reina, "Mechanically Activated Combustion SynThesis Of Niobium Silicide Based Composites" (2020). Open Access Theses & Dissertations. 3127.