Microstructural investigation of high-velocity oxygen-fuel (HVOF) sprayed AlCoCrFeNi + 12 wt% CoCrFeNi composite coatings

Gas atomization produces predominantly spherical and chemically homogeneous powders with finer grains than those typically obtained through bulk preparation methods. This refined powder quality plays a crucial role in alloy development, where incorporating the larger atomic-sized element Al into high-plasticity FCC-structured CoCrFeNi medium-entropy alloys (MEAs) causes the formation of a harder AlCoCrFeNi high-entropy alloy (HEA) phase exhibiting a BCC/B2 crystal structure and reduced localized plasticity. Using the blend of gas atomized AlCoCrFeNi and CoCrFeNi powders in an 88:12 weight ratio, this microstructure-oriented study explores the potential to improve the balance between strength and localized plastic deformation in a composite coating deposited via high-velocity oxygen-fuel (HVOF) spraying. The resulting microstructure primarily consists of unmelted splats (USs), semi-molten splats (SMSs), and fully molten splats (FMSs). Relatively coarse, non-recrystallized, equiaxed dendritic grains are observed in USs and the central regions of SMSs, while dynamic recrystallization in the outer SMS layers leads to (ultra-)fine equiaxed/ribbon-like grains. In contrast, FMSs display fine, fully-(or near-)columnar grains formed through directional re-solidification. In addition to spinel-type in-flight oxides (IFOs) and Al-rich post-impact oxides (PIOs), minimal porosity is present. Consequently, the BCC/B2 AlCoCrFeNi and FCC CoCrFeNi splats, together with reinforcing oxides, indicate that the composite coating may achieve a more favorable combination of strength and localized plasticity than the individual AlCoCrFeNi or CoCrFeNi coatings. Multiple strengthening mechanisms, including BCC/FCC solid solution strengthening, order (B2-type) strengthening, and grain refinement, along with deformation mechanisms like both deformation and annealing twins, contribute to its superiority relative to laser cladded coatings of a similar composition. • AlCoCrFeNi/CoCrFeNi (88:12) blend provides a balance of strength and plasticity. • Al addition increases hardness in BCC/B2 AlCoCrFeNi splats. • Deformation twins appear in unmelted FCC CoCrFeNi splats. • Annealing twins form in dynamically recrystallized semi-molten FCC CoCrFeNi splats.