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A detailed and systematic microstructural characterization has been carried out on a Ti-47Al-2Cr-2Nb (at. pct) intermetallic alloy processed by powder metallurgy (PM). Heat-treatment parameters such as isothermal temperature, holding time, and cooling rate were varied in order to produce a series of near-c, duplex, and fully lamellar microstructures. These were then quantitatively analyzed in terms of grain size, surface fraction, lamellar spacing, second-phase spatial distribution, and serrated grain boundary morphology. Owing to these extensive quantitative image analyses, several unusual microstructural features occurring in this wellknown TiAl-based alloy were identified and assessed. First, a dissolution of the smallest c grains was emphasized in subtransus conditions as the isothermal temperature or holding time was increased. Second, the competition that occurs between the a Þ a + c transformation and the direct-ordering a Þ a 2 reaction upon cooling from above the a-transus temperature is mainly governed by the reduction in chemical free energy. Third, new grains were found to nucleate upon cooling, which is presumably induced by a minimization of interfacial energy at prior a grain boundaries. Finally, new c grains were formed as a result of the coarsening of primary c lamellae under furnace-cooled (FC) conditions.

decreases from 2000 to 1300 C. Sufficient hydrocarbon radical density is produced through the interaction of atomic hydrogen with methane, and methane itself is incompletely dissociated and atomic hydrogen concentration is low at a lower-than-normal filament temperature, which results in a low growth rate. It is anticipated that the growth rate could be improved by introducing diluent gases. Such experiments were not within the scope of the research reported in this communication and will be addressed in future experiments.In summary, the first diamond deposition using ultralow filament temperatures has been achieved. The lowest filament temperature for the deposition of diamond films is about 1300 C.