Interdiffusion in the MgO-Al 2 O 3 spinel with or without some dopants
- PDF / 1,582,785 Bytes
- 10 Pages / 598 x 778 pts Page_size
- 13 Downloads / 174 Views
I.
INTRODUCTION
ALUMINA-BASED ceramic matrix composites (CMCs), commonly synthesized by the directional oxidation of aluminum-magnesium alloys, are attractive candidates for applications in cutting tools, heat exchangers, engine components, marine hardware, and pumps, v] Characterization of the oxide product in various stages of oxidation revealed the presence of pure MgO, and pure A1203 and spinel in different regions. Interdiffusion of MgO and A120 3 is necessary for the formation of the spinel.tl.2] Unlike the diffusion of very high-purity single crystals that were used for previous interdiffusion studies t31 in the growth of alumina-based CMC by directional oxidation, polycrystalline AI203 and MgO are involved, and materials other than AI203 and MgO are present in the product oxide layer. Much work has been donet4-~0]on the reaction mechanism between MgO and A1203. The counterdiffusion mechanism of Mg2+ and AP+ ions through the spinel product layer was confirmed using the inert marker experiment.t4J Rossi and Fulratht6] found that spinel formed in two product layers, one from each oxide. Nakano et aLtl~] used the radioactive atoms (180) to study the mechanisms of solid state reactions in the system MgO-A1203. They concluded that oxygen is essentially immobile in spinel. Interdiffusion in polycrystalline MgO and A1203 was studied by Mackenzie and Ryant~2] in the temperature range 1573 to 1723 K. The diffusion coefficients were increased by applying external direct current electric fields,t121 The interdiffusion coefficients of AI3+ions in spinel and single crystal MgO were determined by Whitney and Stubicant3~ in the temperature range of 1833 to 2173 K. They found that the interdiffusion coefficients in spinel were nearly independent of composition at 1873 K. The values increased slightly as the
P. ZHANG, Postdoctoral Research Fellow, and S. SEETHARAMAN, Professor, are with the Division of Theoretical Metallurgy, Royal Institute of Technology, S-100 44 Stockholm, Sweden. T. DEBROY, Professor of Materials Science and Engineering, Metals Science and Engineering Program, Penn State, University Park, PA 16802. This article is based on a presentation made in the "In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics" symposium, held February 12-16, 1995, at the TMS Annual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees). METALLURGICAL AND MATERIALSTRANSACTIONS A
composition changed from MgO-rich spinel to stoichiometric spinel, and then decreased beyond that of stoichiometric spinel. Yamaguchi et al.[13] carried out the interdiffusion experiment by defect spinel couples. However, their trends of how the diffusion coefficient varies with composition are different from of Stubican et aL E141 The plausibility of using spinel-based compounds as practical high-temperature materials depends on the high-temperature stabilities of these compounds. Although several investigations have been conducted to determine the low-t
Data Loading...
