Boride-alumina composites:
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Boride-Alumina Composites: Synthesis and Fabrication
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Within the last two to three decades a large number of areas have developed where refractory borides or composites may be advantageously utilized. A number of borides and/or boron compounds, e.g., B4C and TiB2, have been used to manufacture lightweight protective armor, u} Borides have also been used extensively in nuclear applications, 12] because of their favorable neutron absorption cross section. Borides of Ti, Zr, Hf, and their composites are refractory, hard, highly electrically conducting, corrosion resistant, and wetted by molten metals. Based on these characteristics, they are candidate materials for cathodes for molten salt electrolysis. 13-6]The borides and composites are also candidates for cutting tools and applications where the materials' corrosion and abrasion resistance are important, such as filters and separators for molten metals and fused salts. The lack of widespread use of the borides and composites is undoubtedly related to the costs associated with production of the raw powders and the fabrication of the monolithic and composite components. Conventional refractory ceramic or composite fabrication techniques generally involve at least two processing steps. First, powders (and whiskers) of proper chemistry, phase, and physical characteristics are produced and then the powder is consolidated at elevated temperatures in controlled atmospheres and pressures. Simultaneous synthesis and densification may be used as an alternative. The processing technique, termed selfpropagating high-temperature synthesis (SHS), which has been investigated in the Soviet Union since 1967 may be used for synthesis of these materials, t7,8,9] Recently, significant interest in this technique has developed in this country and in Japan. t'~ The SHS reactions can be used for simultaneous synthesis and fabrication of ceramic parts. These reactions t' ]] involve (1) displacement reactions in the solid state in which the anion associated with one reactant is transferred to another metallic reactant or (2) elemental reactions in which two elements react to form a compound. We have explored combinations of these two types of reactions. Boride-alumina composites have been fabricated using inexpensive raw materials by utilizing reactions of the following types: TiO2 + B203 + 10/3A1 ~ TiB2 + 5/3A1203
[1]
ZrO2 + B203 + 10/3AI---> ZrB2 + 5/3A1203
[2]
S.P. RAY, Senior Scientific Associate, is with the Advanced Ceramic Center/Chemical Systems Division, ALCOA Technical Center, ALCOA Center, PA 15069-0001. This paper is based on a presentation made in the symposium "Reaction Synthesis of Materials" presented during the TMS Annual Meeting, New Orleans, LA, February 17-21, 1991, under the auspices of the TMS Powder Metallurgy Committee. METALLURGICAL TRANSACTIONS A
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