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

Abstract

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 ~1745 MPa, ultimate stress of ~2622 MPa, and a maximum strain at failure of ~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 ~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 ( ~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.