L1₂-strengthened multicomponent Co-Al-Nb-based alloys with high strength and matrix-confined stacking-fault-mediated plasticity

This study presents the alloy development of a new class of L1₂-strengthened Co-Al-Nb-based alloys with high γ′-solvus temperatures together with superb strengths at both ambient and elevated temperatures. The L1₂-Co₃(Al, Nb) phase was found to be in equilibrium with the γ-Co matrix and the B2-CoAl phase in the ternary Co-10Al-3Nb alloy after an isothermal aging at 700 °C; however, it transformed into the Laves phase as the aging temperature increased to 800 °C. Alloying additions of Ni helped to suppress the B2 phase formation, resulting in a clean γ-γ′ dual-phase microstructure. Ti and Ta elements further stabilized the L1₂ structure and increased the γ′-solvus temperature to 1150 °C without inducing the formation of any other deleterious intermetallic phases. The newly developed Co-Al-Nb-Ni-Ti-Ta multicomponent Co-rich alloy has demonstrated outstanding yield strengths at both ambient and elevated temperatures, reaching 1023 ± 27 MPa at 25 °C and 897 ± 53 MPa at 700 °C, respectively. Furthermore, electron microscopy analyses uncovered unique deformation substructures, in which plasticity is predominantly carried out via nanoscale matrix-channel-confined stacking faults. As determined by the first-principle calculations, the absence of particle shearing upon deformation at ambient temperature is ascribed to the ultrahigh planar fault energies of the multicomponent γ′ precipitates. High-density superlattice-stacking-fault shearing and their interactions are responsible for the yield anomaly at 700 °C. These findings not only provide the fundamental understanding of the deformation behavior of the L1₂-strengthened alloys, but also demonstrate the great potential for developing next-generation high-temperature structural materials based on the multicomponent Co-rich alloy systems.