In SiC/Ti composites, the physical-chemical behaviour at the interface at fabrication temperatures, is important. Ideally, only a thin bonding film is desired after fabrication with zero growth during the actual service conditions. In this study, two mechanisms which lower the growth rate of silicides about the SiC/Ti interface were investigated. Planar composites that incorporated Mo₅Si₃ or Al films, were compared with the uncoated SiC/Ti standards to investigate the "barrier" and "rejection" mechanisms respectively. Also, SiC/Ti₃Al/Ti composites were used to study the effectiveness of the two mechanisms combined, with the Ti₃Al acting as a barrier and with Ti(Al) retarding the reaction by rejection. Samples were vacuum-annealed at 875°C for various times and examined by X-ray diffraction techniques. The results from the SiC/Al/Ti and SiC/Ti₃Al/Ti composites indicated that silicide formation would be retarded in the presence of an Al saturated Ti matrix. Also, the formation of a Ti₃Al layer near the SiC/Ti interface serves as an effective barrier to diffusion of Si atoms. In the SiC/Mo₅Si₃/Ti composites however, the dissociation of the Mo₅Si₃ phase formed during sample preparation results in additional silicide formation at the interfaces. If stoichiometric Mo₅Si₃ films are sputter-deposited, they could be used as effective barrier.
The inter-diffusion of Al films, sputter deposited on Ti substrates, at elevated temperatures have been analyzed. Samples were vacuum annealed at 635°C and 900°C for various times and then examined using X-ray diffraction. In each case, intensity bands from alpha-Ti and diffraction lines from an intermetallic compound have been observed. For subsequent annealing times, the integrated intensities from the compound decreased slightly, indicating that the compound partially decomposed and released Al into the alpha-Ti lattice. This resulted in an alpha-Ti diffracted intensity band that results from a range of compositions. Intensity bands from Ti(101) were used to obtain composition profiles for Al in alpha-Ti, by using X-ray diffraction techniques and computer simulations. In each case, mass conservation of Al atoms was used to determine the interface motion. It was assumed that the Al profiles extend continuously into the_disordered Ti₃Al and Ti₂Al composition regions. This assumption was verified by preparing a Ti₂Al (disordered) powder standard and obtaining accurate lattice parameters. An iterative solution was used to determine composition dependent diffusion coefficients of one-dimensional zones, for two-phase systems. Diffusion zones extending from a few tenths to several microns have been examined. Al diffusion coefficients in alpha-Ti and the Ti₃Al phase have been determined. Activation energies were calculated from the diffusion coefficients at 635°C and 900°C and compared with those obtained from a melting point correlation. Lower activation energies from the present investigation indicated that there was some grain boundary diffusion at these intermediate temperatures. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/109938 |
| Date | January 1980 |
| Creators | Rao, Venkatraman B. |
| Contributors | Materials Engineering Science |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | xii, 300 pages, 2 unnumbered leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 07086259 |
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