tailieunhanh - Mass Transfer in Multiphase Systems and its Applications Part 10

Tham khảo tài liệu 'mass transfer in multiphase systems and its applications part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Control of Polymorphism and Mass-transfer in Al2O3 Scale Formed by Oxidation of Alumina-Forming Alloys 349 Time min Po2 Pa 10-14 10 105 10 a-Al2O3 e a-Al2O3 Co Ni Al Cr 2O4 e a-Al2O3 Co Ni Al Cr 2O4 200 1 a-Al2O3 Co Ni Al Cr 2O4 ị 600 1 ị a-Al2O3 Co Ni Al Cr 2O4 Table 1. Crystalline phases in the oxide scales. Surface Cross section Fig. 3. SEM micrographs of the surfaces and cross-sections of the samples oxidized at 1323 K for 600 min under PO2 of 10-14 10 and 105 Pa. The SEM micrographs of the surfaces and cross-sections of the samples oxidized at 1323 K for 600 min under PO2 of 10-14 10 and 105 Pa respectively are shown in Fig. 3. The surface 350 Mass Transfer in Multiphase Systems and its Applications of the oxide scale formed under a PO2 of 10-14 Pa is relatively smooth and its thickness is about 1 micrometer. On the other hand the higher the PO2 for the oxidation is the larger the oxide crystals are which are exposed on the scales increasing the density of surface irregularities. The scale thickness increases with an increase in PO2 for the oxidation the scale thickness for oxidation under a PO2 of 105 Pa is at least twice that under a PO2 of 10-14 Pa. Some of the crystals grown on the oxide scales under the higher PO2 are considered to be Co Ni Al Cr 2O4 as shown in Fig. 1 and Table 1. It is well known that the morphology of theta-Al2O3 consists of blade-like crystals known as whiskers . In addition when theta-Al2O3 survives for a long time at high temperatures this oxide crystal grows outward about an order of magnitude faster than alpha-Al2O3 Tolpygo et al. 2000 . Therefore since the theta-phase exists longer under a higher PO2 the oxide has longer whiskers than those transformed earlier resulting in the formation of an oxide scale with a rougher surface. Figure 4 shows the SIMS depth profiles of selected elements through the CoNiCrAlY coats of the samples oxidized at 1323 K for 600 min under PO2 of 10-14 and 105 Pa respectively. For the oxidation under a .

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