M.Tech. / A mixture of HfC and TiC powders and a (Hf,Ti)C powder have been hot pressed with 4wt% Ni. In the absence of Ni the hot pressed temperature was 2000 °C and in the presence of Ni 1650 °C. The pressure of 30 MPa was applied in both cases. The starting powders were substoichiometric, as deduced from XRD spectra analyses, and the (Hf,Ti)C powders consisted of a range of compositions, as indicated by the width of the XRD peaks. In the absence of Ni the powders sintered without the formation of a liquid phase. In the case of HfC and TiC mixture, high- energy dry milled HfC + TiC +C black powder sintering occurred with simultaneous formation of (Hf,Ti)C solid solution. On account of mutual solid solubility of two carbides vacancy interdiffusion controlled the solution as well as the sintering process, assisted by the high concentration of vacancies in the starting powders. In the case of (Hf,Ti)C powder diffusion was also the controlling process because t he solid solution was not h omogeneous a nd the system t ended to homogenization, as shown by t he narrowing of t he XRD peaks after sintering. Since the diffusion of HfC into TiC did not occur at the same rate as the diffusion of TiC into HfC (as expected, on account of the different melting points of the two materials) diffusional porosity was observed in some of the (Hf, Ti) C grains. Grain growth was substantial but uniform. In the presence of Ni, sintering occurred with the formation of a liquid phase. The volume fraction of the liquid phase formed was sufficient to yield a low porosity. Grain growth was less than in the case of material sintered without Ni, probably just on account of lower sintering temperature. In the case of high- energy dry milled the reduction in particle size was observed.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:9812 |
| Date | 10 September 2012 |
| Creators | Maseko, Emily Tholakele |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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