DENSE COAL-BASED ALLOYS WITH IMPROVED DUCTILITY - SOLID-STATE SYNTHESIS AND MICROSTRUCTURE CONTROL

Dense single- and multiphase B-2 CoAl-based intermetallics were synthe sized from fine elemental Co-Al blends by a completely solid-state pro cessing route at temperatures as low as 800 degrees C. To ensure full density of the final product, a moderately high external pressure (les s than or equal to 500 MPa) was applied during the solid-state reactiv e synthesis. Microstructure and mechanical properties of the materials obtained were investigated employing X-ray diffraction (XRD), scannin g electron microscopy, transmission electron microscopy, and microhard ness and compression testing. To improve the room-temperature ductilit y, an attempt was made to control the microstructure of hyperstoichiom etric CoAl alloys via solution treatment with subsequent aging. At the early stages of aging, fine Widmanstatten precipitates of Co were for med; the coarsening of Co dispersions during long or higher temperatur e anneals resulted in the pronounced compressive ductility of the over aged samples. In addition to general precipitation, cellular precipita tion was observed in CoAl alloys with a high Co content. The relativel y coarse equilibrium cellular precipitates were found to rapidly overg row the fine metastable Widmanstatten Co plates, causing rapid overagi ng of these alloys. The lattice parameter of hyperstoichiometric CoAl was found to decrease linearly with increasing Co content.