Microstructure and mechanical properties of a severely cold-rolled and annealed dual-phase compositionally complex alloy (CCA) with an exceptionally deformable Laves phase

Abstract

A novel CCA was designed by substituting Nb in (FCC + C14 Laves) CoCrFeNi_2.1(Nb)_0.2 CCA by (Hf + Nb + Ta). The (HfNbTa)_0.2 CCA was homogenized, heavily cold-rolled, and isothermally annealed at 800 °C and 1000 °C for different time intervals. The (HfNbTa)_0.2 alloy revealed the presence of a Hf and Ni enriched cubic C15 Laves phase. The considerations of site occupancy behavior, formation energy, and highly off-stoichiometric composition stabilized the (Hf, Ni) rich cubic C15 Laves phase. In contrast to the brittle hexagonal C14 Laves phase in (Nb)_0.2 CCA, the C15 Laves phase in (HfNbTa)_0.2 CCA showed exceptional deformability owing to the high propensity for nano-twin formation. Meanwhile, the FCC matrix developed a deformation-induced nano-lamellar structure with a spacing of ∼45 nm. Annealing resulted in ultrafine recrystallized FCC matrix and precipitation of DO₁₉ structured ε nano-precipitates. The isothermal grain growth kinetics revealed a high grain growth exponent (n) ∼7, which confirmed a Zener-drag mediated process due to the ε nano-precipitates. The Hall-Petch analysis of the hardness data showed relatively high friction stress originating from the dissolution of Hf, Nb, and Ta in the FCC matrix. A high Hall-Petch coefficient indicated increased shear stress for plastic flow across the boundaries, resulting from the elongated Laves phase at the boundaries. The highly deformable Laves phase, ultrafine grain size, and ε nano-precipitates resulted in high yield strength (∼975 MPa) and superior ductility (∼16 %) in the (HfNbTa)_0.2 CCA, even surpassing the (Nb)_0.2 CCA. It was envisaged that strong yet deformable Laves phases could pave the pathway for developing Laves phase-based CCAs for advanced structural applications.