Effect of heat treatments on In-Situ Al 2 O 3 /TiAl 3 composites produced from squeeze casting of TiO 2 /A356 composites
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INTERMETALLIC compounds have been considered for a wide range of high temperatures, and much research effort has been devoted to their development in the last ten years. The Ti-aluminides are prime candidates for hightemperature applications such as aircraft turbine engines, the space shuttle, and others.[1] The ordered intermetallic compounds Ti3Al(␣2), TiAl(␥), and TiAl3() have an attractive combination of properties such as a low density, high elastic modulus, high melting temperature, and good oxidation resistance.[2,3] However, these materials have poor creep resistance and poor room-temperature toughness. Intermetallic matrix composites (IMCs) may provide the right combination of high-temperature strength, creep resistance, and environmental stability with adequate ambient-temperature ductility and low density. DiPietro et al.[4] have reported on the compressive properties of TiB2 particulate composites with TiAl3 as a matrix. Below 600 ⬚C, the strength-to-density ratio has been reported to be approximately 4 times that for superalloys. These results imply that TiAl3 is worthy of more extensive studies as a composite matrix. Currently, a variety of methods have been applied to the fabrication of IMCs. Particulate-reinforced composites have been produced by powder mixing and consolidation,[5] liquid/particle cospraying,[6] and in-situ precipitation.[7] For example, Peng et al.,[8] using Al, TiO2, and B powders, produced TiAl3-Al2O3-TiB2 composites by the XD process CHIA-WEN HSU, formerly Graduate Student, Institute of Materials Science and Engineering, National Chiao Tung University, is Senior Process Engineer, UMC, Hsinchu, Taiwan 30077, Republic of China. CHUENGUANG CHAO, Professor, is with the Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan 30050, Republic of China. Manuscript submitted April 28, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
(The XD process is a liquid-solid reaction method of producing ceramic reinforcing in-situ particulate in a matrix). Welham[9] reported that TiO2 and milled aluminum powder showed complete reaction to Al2O3 and TiAl3 after heating to 600 ⬚C, whereas the unmilled powder reaction was incomplete after 1 hour at 1200 ⬚C. Some researchers produced TiAl3/Al2O3 composites by squeeze casting, using TiO2 powders as the reinforcement, molten pure aluminum as the matrix, and a process of reaction.[10] Two kinds of reaction methods were utilized. One method was to produce reaction squeeze-cast products through higher-temperature conditions of the squeeze-casting process.[11] The hardness of such samples was HV 300 to 1050. However, it is difficult to make a uniform fine structure with the simple combination of aluminum and TiO2. The other method involved producing nonreacted (TiO2/Al) composites at lower-temperature conditions of the squeeze-casting process, and then, through subsequent heat treatments, allowing the reaction of TiO2 and Al to occur. Isao Tsuchitori et al.[12] suggested that the hardness of a TiO2/Al composite i
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