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 CoCrFe0.5Ni1.5Mo0.1Nb0.68 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 mVSHE) was 1345 mVSHE higher than that of Mg-Li alloy (-1369 mVSHE), 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).