Microstructure and phase stability analysis of laser cladding CoCrCu_0.4FeNi high entropy alloy coatings

To improve the hardness and wear resistance of parts, the Co, Cr, Cu, and Ni elemental metal powders were used to prepare the CoCrCu_0.4FeNi high entropy alloy coatings on Q235 steel substrate by laser cladding. The microstructure of the high entropy alloy coatings was analyzed by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD), and the microhardness of the coatings was measured. The lattice and elastic constants of each phase in the coatings were calculated by the first principle. The results show that, the coatings show the good metallurgical bond with the substrate without the macro-cracks and pores. The microstructure of the coatings is mainly composed of dendrite and interdendrite. The dendrite is one kind of face-centered cubic phase (FCC1), rich Cu and poor Cr, the interdendrite is another kind of face-centered cubic phase (FCC2), rich Cr and poor Cu. The thickness of the coatings is about 1.50~1.98 mm, and the size of the coating dendrite is about 7.9~10.4 μm. The microhardness of the coatings is about HV_0.2 170~230, about 1.7 times that of the substrate, and the hardness of the coatings gradually decreases with the increase of the distance from the coating surface. In addition, the lower the laser power, the higher the scanning speed, the finer the dendrite, the stronger the effect of fine grain strengthening, and the higher the hardness of the coating. The error between the calculated lattice constant and the experimental value of face-centered cubic (FCC) phases in the coatings is 1.33%~2.60%. The formation heat of FCC phases is negative, and the elastic constants C₁₁, C₁₂, and C₄₄ meet the mechanical stability constraints of the cubic high entropy alloy, showing the FCC phases are stable. The FCC phases at the dendrite and interdendrite are generally present as the characteristics of toughness according to the ratio of shear modulus to bulk modulus (G/B)＜0.57 and Poisson's ratio (ν)＞0.26. From the lower part to the upper part of the coatings, the calculated elastic modulus and hardness increase gradually, which is consistent with the variation law of the experimental data.