A novel strategy for promoting corrosion and wear resistance of Mg-Li alloys: Gradient eutectic high-entropy alloy coating induced by in-situ bidirectional diffusion

In this work, a novel surface strengthening strategy for Mg-Li alloys was proposed, called cold spraying assisted high-speed laser cladding. CuAl9 aluminum bronze coating was firstly deposited on Mg-Li alloy by cold spraying, and then CoCrFe_0.5Ni_1.5Mo_0.1Nb_0.68 eutectic high-entropy alloy (EHEA) coating was prepared on the CuAl9 coating utilizing high-speed laser cladding. A gradient coating consisted of in-situ transition region and EHEA layer formed by bidirectional diffusion. The in-situ transition region was composed of β-Li, Cu₂Mg and Cu₃Al₂ phases. TEM analysis indicated that Cu₂Mg and Cu₃Al₂ phases were well matched with the β-Li matrix phase. The EHEA coating had a nano-lamellar eutectic microstructure with relatively small lamellar-spacing (< 100 nm). Metallurgical bonding interfaces formed between the EHEA coating, transition region and Mg-Li substrate. The evolution mechanism of the coating was revealed from the perspectives of mixing enthalpy, atomic radius difference and laser energy distribution. In 3.5 wt.% NaCl solution, the corrosion potential of the EHEA coating (-24 mV_SHE) was 1345 mV_SHE higher than that of Mg-Li alloy (-1369 mV_SHE), while the corrosion current density of the EHEA coating (3.13 × 10⁻⁷ A·cm⁻²) was almost three orders of magnitude lower than that of Mg-Li alloy (1.25 × 10⁻⁴ A·cm⁻²). The wear rate of Mg-Li alloy (1.11 × 10⁻³ mm³/N·m) was about 36 times higher than that of the EHEA coating (3.05 × 10⁻⁵ mm³/N·m).