Roles of Al on microstructures, mechanical properties and oxidation resistances of FCC AlxCoCrNiFe high entropy alloy coatings prepared by laser directed energy deposition

Abstract

In this work, FCC Al_xCoCrFeNi high entropy alloys (HEAs) coatings were by laser directed energy deposition (L-DED). Single FCC microstructures were formed in all the coatings. With the addition of Al, the microstructures transformed from columnar crystals to equiaxed crystals and grain refinement was obtained, owing to the transformation starting from FCC to B2. Sub-grains with the sizes of 5∼10 μm can be found due to the cyclic thermal activation effect. Besides, a comparative number of dislocations and stacking faults were found inside the sub-grains. The addition of Al played the dual effect of enhancing strength and plasticity. Multiple mechanisms such as fine-grain strengthening, dislocation strengthening and solid solution strengthening together contributed to the strength and plasticity in this work. Al0.3 coating retained its original characteristics exposured at 850 °C for 30 h, signaling excellent oxidation resistance to high-temperature. The anti-oxidation effect of Al was much stronger than that of Cr. Al was required to reach a sufficient content to generate a dense full-covered protective film of Al₂O₃, which hindered the oxidation flaking of other alloying elements.