| Description |
x, 64 leaves : illustrations ; 29 cm |
| Summary |
"A powder metallurgy route based on hot isostatic pressing (HIPing) of tape-cast monotapes was investigated for the direct fabrication of dense thin sheets of gamma titanium aluminide ([gamma]-TiAl). Sheets of [gamma]-TiAl can be produced by rolling cast ingots or powder metallurgy plates but the process requires considerable thermomechanical treatment, making it expensive. In the present work, [gamma]-TiAl powder was tape-cast to form a sheet (thickness = 500-700 [mu]m). Insertion of the tape-cast sheet into a HIP can, binder removal in situ by thermal decomposition, HIPing for 15 min at 1100°C and 130 MPa, and decanning by a dissolution and oxidation process produced a dense sheet with a thickness of 250-300 [mu]m. The research involved process optimization, chemical analysis, hardness testing, microstructural analysis, and microstructural manipulation by thermal annealing. The carbon content in the dense sheet was 0.13 wt.%, which was only 0.035 wt.% higher than that of the starting powder, but the oxygen content (0.44 wt.%) was significantly higher, presumably introduced during the oxidation stage of decanning. The hardness measured using Vickers microindentation technique was 384 ̲+ 9 HV. Polarized light microscopy revealed that the dense sheet consisted of a fine-grained microstructure (average grain size ̃ 3 [mu]m) but also contains a few isolated larger grains (̃ 20 [mu]m). Microstructure analysis showed that the primarily metastable microstructure of the rapidly solidified starting powder transformed to the equilibrium near-[gamma] microstructure during HIPing. The fine-grained microstructure of the HIPed sheet seems to have resulted from the fine, uniform structure of the atomized powder. Thermal annealing at 1175 and 1250°C for up to 1 hour, in flowing argon, produced limited grain growth but no discernable change in the near-[gamma] microstructure. As-HIPed sheets heated to 1325 and 1375°C had a duplex microstructure with lamellar grains (̃ 25 [mu]m) near the surface. The surface effects were likely caused by oxidation. Removal of the oxidized surface layer (̃ 25-50 [mu]m thick) is likely to result in a sheet with uniform microstructure"--Abstract, leaf iii. |
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