Dense CoAl-based alloys with improved ductility: Solid-state synthesis and microstructure control

Dense single- and multiphase B-2 CoAl-based intermetallics were synthesized from fine elemental Co-Al blends by a completely solid-state processing route at temperatures as low as 800 degrees C. To ensure full density of the final product, a moderately high external pressure (less than or equal to 500 MPa) was applied during the solid-state reactive synthesis. Microstructure and mechanical properties of the materials obtained were investigated employing X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and microhardness and compression testing. To improve the room-temperature ductility, an attempt was made to control the microstructure of hyperstoichiometric CoAl alloys via solution treatment with subsequent aging. At the early stages of aging, fine Widmanstatten precipitates of Co were formed; the coarsening of Co dispersions during long or higher temperature anneals resulted in the pronounced compressive ductility of the overaged samples. In addition to general precipitation, cellular precipitation was observed in CoAl alloys with a high Co content. The relatively coarse equilibrium cellular precipitates were found to rapidly overgrow the fine metastable Widmanstatten Co plates, causing rapid overaging of these alloys. The lattice parameter of hyperstoichiometric CoAl was found to decrease linearly with increasing Co content.