Microstructure, dislocation and high-temperature oxidation behavior of as-cladding NiCoFeCrMo high-entropy alloy coatings: Laser cladding vs vacuum cladding

Abstract

NiCoFeCrMo high-entropy alloy (HEA) coatings were fabricated using laser cladding (LC) and vacuum cladding (VC), and their microstructures, dislocation characteristics, and high-temperature oxidation behavior at 600 °C were systematically compared. The LC coating exhibits a dendritic single-phase FCC solid solution, whereas the VC coating presents an FCC+ σ eutectic structure. The former displays greater lattice distortion, higher dislocation density, lower screw density, and more stacking faults than the latter, resulting in superior hardness, plastic deformation resistance, and dynamic recrystallization. The oxidation rate constant (k) and oxidation rate of the LC coating are 1.4247 × 10⁻³ and 2.19 × 10⁻³ mg·cm⁻²·h⁻¹, while those of VC coating are 1.5633 × 10⁻³ and 2.47 × 10⁻³ mg·cm⁻²·h⁻¹, respectively, indicating that the LC coating exhibits better high-temperature oxidation resistance. The LC coating is covered by a double-layer oxide film composed solely of Cr₂O₃, whereas the VC coating develops a loose oxide layer containing Cr₂O₃ and (Cr,Fe)₂O₃. Due to its higher lattice stress, dislocation density, screw density, and stacking faults, the LC coating exhibits greater lattice distortion, high-temperature toughness, and plastic resistance compared to the VC coating. High-temperature oxidation behavior is influenced by element diffusion, oxygen permeation, oxide formation, and internal stress. The continuous formation of dense oxides and the inhibition of oxygen permeation provide the LC coating with excellent oxidation resistance. In contrast, the VC coating′s elevated oxygen permeability, formation of loose oxides, and degradation of the σ phase reduced its protective efficacy.