Mechanical and microstructural properties of a CoCrFe0.75NiMo0.3Nb0.125 high-entropy alloy additively manufactured via cold-spray

We present a combined experimental and computational investigation of the mechanical properties of a CoCrFe0.75NiMo0.3Nb0.125 (composition in molar ratio) high-entropy alloy additively manufactured via cold spray. We find that the sprayed alloy exhibits extraordinary mechanical properties under compression, reaching yield stress of similar to 1745 MPa, ultimate stress of similar to 2622 MPa, and a maximum strain at failure of similar to 9%. These exceptional mechanical properties are the result of four independent hardening mechanisms. First, using ab initio simulations, we find that non-equiatomic compositions increase the enthalpy of mixing, promoting better solubility of solute Mo and Nb atoms while simultaneously preserving the electronegativity of the base alloy. The higher solubility results in solid-solution hardening and nanosized precipitate formation, promoting additional hardening. These effects are confirmed in the experimental characterization of the manufactured HEA, where nanosized precipitates of similar to 226 +/- 65 nm in size are identified. Additional hardening effects are associated with the manufacturing process, where the high velocity impacts of the microparticles promote dynamic recrystallization through dislocation emission and grain refinement. To understand the dynamic recrystallization of particles, high-velocity impact simulations using molecular dynamics are performed. We find that when particles reach a critical impact velocity (similar to 600-800 m. s(-1)), the dislocation density reaches a maximum, and grain refinement is maximized. The decaying wave pressures developed during the impact generate gradual refinement levels, leading to heterogeneous microstructures combining nano and micro grains, which was later confirmed experimentally using electron backscatter diffraction. These subtle atomic and microstructural features result in outstanding experimentally evaluated yield and ultimate stresses compared to other high-entropy alloys with similar compositions. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The combined effects of non-equilibrium compositions and high-velocity impacts during manufacturing result in increased solubility and nano-sized precipitate formation, leading to solid-solution hardening and grain refinement, which contribute to the exceptional mechanical properties observed in the high-entropy alloy.