Metastable phases in sputtered stoichiometric Co₃Al

Co-Al based superalloys have been considered as the promising alternatives to Ni-Al based superalloys. Appropriate alloy design can stabilize a dual phase structure of face-centered cubic (FCC) Co matrix and L1₂ Co₃(Al, X) intermetallics and avoid precipitation of B2 CoAl intermetallics for Co-Al based superalloys. It has been shown that stable Co₃Al single phase compound does not exist. Instead, at such a chemistry, Co-Al binary phase diagram predicts the coexistence of hexagonal close-packed (HCP) Co and B2 CoAl at room temperature. Here we report the synthesis of stoichiometric Co₃Al via magnetron sputtering. Transmission electron microscopy investigations identified metastable phases of Co₃Al, dominated by HCP supersaturation with minor L1₂ intermetallics. Furthermore, the phase transformation from HCP-to-FCC Co₃Al induced by stacking faults in Co/Co₃Al multilayers is also discussed. This investigation provides new insights on the crystal structures of stoichiometric Co₃Al and the design of novel Co-Al based alloy systems.