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Titanium alloys fabricated by wire and arc additive manufacturing (WAAM) are prone to produce defects with coarse intracrystalline α lamellae, continuous grain boundary of α phase (GB-α), and chemical segregation. The synergistic combination of the factors leads to high strength at the expense of plasticity. Adding activating flux of CaF2 during WAAM can provide an effective way to achieve a remarkable strength-ductility combination of Ti–6Al–4V alloy. Adding CaF2 produced refined intragranular α lamellae, discontinuous GB-α with dowel-like α lamellae (DL-α, primary α phase penetrated through the GB-α and embedded within adjacent β grains), as well as achieved concentration modulations. The tailored microstructure induced multistage strain hardening behavior (sequentially activated multiple strain hardening) during tensile plastic deformation, which allows WAAM-fabricated Ti–6Al–4V alloy a high tensile strength with good plasticity. The good strength-ductility balance is mainly attributed to the utilization of multiple strengthening mechanisms in titanium alloys. This study provides a novel method to improve the mechanical properties of WAAM-fabricated titanium alloys.

Phase decomposition is commonly observed in Fe-Cu alloy and Cu-HEAs that has been proved to harmful for the mechanical properties. Here we found that the phase decomposition based on the immiscibility between Cu and Fe plays an active role in the mechanical properties of a carbon-doped face-centered-cubic (FCC) CuFeMnNi high-entropy alloy (HEA) while maintaining low raw-material costs, by employing only hot-forging and annealing. Upon annealing at 900 °C for 120 min, the annealed HEA exhibits dual-level heterogeneous microstructures, i.e., (i) the heterogeneous grain distribution composed of fine grains (≤ 10 µm) within the Fe-rich FCC 1 phase and coarse grains (>10 µm) in the Cu-rich FCC 2 phase, and (ii) special duplex phases characterized by the transitional region (∼ 1 µm in thickness) at the junction of the above dual phases. Moreover, metastable FeMn-carbides in the FCC 1 phase, chemical medium-range order, nano-twins, and 9R-martensite in the transitional region, as well as nanosized Cu-precipitates in the whole microstructure are detected. Compared with C-free CuFeMnNi HEAs, the present alloy exhibits satisfactory ultimate tensile strength around 800 ± 10 MPa and excellent ductility of 56 ± 1.14%. Sustainable strain hardening is achieved by sequentially triggering multistage strain-hardening mechanisms including hetero-deformation-induced hardening, precipitation hardening, and hardening jointly contributed by microbands, twins and dislocation cells. These findings open a new insight for regulating phase decomposition in Fe-Cu alloy and Cu-HEAs. [Display omitted] • The innovative dual-level heterogeneous microstructures were created in the annealed carbon-doped CuFeMnNi HEA. • Multiple micron-grade carbides and various nanostructures were induced in the annealed C-doped HEA. • Compared with CuFeMnNi HEAs, the annealed C-doped HEA exhibits satisfactory ultimate tensile strength and superior ductility. • Multiple hardening mechanisms were sequentially stimulated in the annealed HEA. [ABSTRACT FROM AUTHOR]