Effect of ultrasonic surface rolling extrusion on microstructural evolution and wear resistance of laser-clad CoCrFeMnNi high-entropy alloy with low stacking fault energy

Abstract

Ultrasonic surface rolling extrusion (USRE) was employed to strengthen face-centered cubic (FCC) phase CoCrFeMnNi high-entropy alloy (HEA) coatings prepared by laser cladding. The influence of USRE's static load on the microstructure, mechanical properties, and wear resistance of the coatings was examined to elucidate the grain refinement mechanism. The findings indicate that USRE preserves the phase composition of the CoCrFeMnNi HEA coatings. After USRE at loads of 100 N, 200 N, and 300 N, the measured thicknesses of the severe plastic deformation layers are 2.51 μm, 3.03 μm, and 3.45 μm, respectively. The process triggers dislocation multiplication, which accumulates at the boundaries of Mn-rich regions, forming dislocation cells and evolving into subgrains, thereby refining the grain structure under severe plastic deformation. Statistical analysis reveals that the dislocation density within the grains is 7×10¹⁴ m⁻², while at the grain boundaries, the dislocation density is 4×10¹⁵ m⁻². The average grain size has been refined from 97.7 μm to 34.7 μm. Both microhardness and wear resistance escalate with increased static load, with the maximum microhardness of 347 HV and the minimum wear rate of 2.6×10^-5 mm³·N^-1·m^-1 achieved at a 300 N static load. This hardness enhancement is due to the combined effects of grain refinement, dislocation strengthening, and sub-grain formation. The wear mechanisms for the USRE-treated CoCrFeMnNi HEA coatings include adhesive wear, abrasive wear, and oxidative wear. While adhesion and abrasion diminish with higher static pressures, oxidative wear intensifies.