Phase transitions in reactive formation of Ti 5 Si 3 /TiAl in situ composites
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INTRODUCTION
THE TiAl-based composites have been widely investigated due to their potential use in high-temperature applications such as aerospace engines. Major focus was on particulate-reinforced TiAl composites due to the availability of multitude of cost-effective techniques.[1,2,3] Both the external addition and internal formation (in situ) of second phases, such as ceramic and/or ductile phases in TiAl matrix that are widely used[4,5,6] for producing two-phase materials, have been used to improve the room-temperature ductility and high-temperature properties of TiAl-based materials. But in situ formation is more appealing in TiAl composite syntheses[7,8] because of the attainment of many beneficial effects in properties,[9,10] which arise mostly from the development of a “tolerable” interfacial structure in the composite.[11] Significant efforts toward microstructural control of TiAl alloys have been made via the use of a composite route, by which the microstructures can be expected to be designed (or fine-tuned) as in the case of other material.[12–15] Mechanical alloying (MA) has been used to prepare TiAl compounds either fully stable or fully metastable or inbetween.[16–19] To maintain a metastable state, however, the processing temperature should be somewhat lower than usual.[20,21] The benefits of instability of metastable phases as well as the ability of maintaining them in stability are to be Y.J. DU, formerly with the Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong, is with the Department of Metallurgy and Materials Engineering, Michigan Technological University, Houghton, MI 49931. K.P. RAO, Associate Professor, Department of Manufacturing Engineering and Engineering Management, and J.C.Y. CHUNG, Assistant Professor, Department of Physics and Materials Science, are with the City University of Hong Kong, Kowloon, Hong Kong. X.D. HAN, formerly with the Department of Physics and Materials Science, City University of Hong Kong, is with the Department of Materials Engineering, University of Pittsburgh, Pittsburgh, PA 15260. Manuscript submitted January 21, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
proven. It is of practical interest to know when the metastable phases are unstable and will transform into useful stable phases with energy release when heated to a higher temperature.[22,23] Such an “unstable environment” can be realized by putting the metastable phases into a reactive system in which an exothermic reaction would occur. For the latter case, however, the efforts are on the utility of keeping metastable phases in stability to develop structures for improving the toughness of materials, for example, amorphous reinforcements in a composite.[24,25] Elemental Ti and Al (in powder form) will react to form TiAl compound (Ti3Al or TiAl or TiAl3) if the ignition temperature is reached[26] This reaction is exothermal and proceeds in a combustion mode with tremendous heat release within a short time (normally a few seconds). The combustion temperatu
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