Tuning strength-ductility combination on two-phase FeCrAlTix high entropy alloy coating through grain refinement induced the generation of a second phase

Developing a high entropy alloy (HEA) coating with high ductility and high strength has been a tremendous challenge since the inherent solid solution strengthening usually compromises its ductility. Herein, we present a fine-grained strengthening approach through the generation of a second phase induced by the short-range ordering (SRO) characteristics of HEA, which leads to the greatly enhanced ductility property of the FeCrAlTi _x HEA coating that surmounts the strength-ductility trade-off. The quantitative microstructure- and composition-property relationships are revealed. The prepared FeCrAlTi _x HEA coating comprises a biphasic structure of a BCC parent phase and a B2 second phase anchored on BCC, which exhibits a bio-like dendritic skeleton microframework. The introduced Ti acts not only as a strengthening element to enhance solid solution strengthening, but also as an SRO promoter to generate the well-grain-refining agent B2 phase for fine grain strengthening, and further serves as a promoter to trigger the formation of a bio-dendrite-like structure. When x = 0.75, these three factors are balanced, which optimizes the strength and ductility of HEA coating, with hardness, elastic modulus, ultimate tensile strength, uniform elongation, and coefficient of friction of 910.7 HV, 278.2 GPa, 1061.9 MPa, 25.9 %, and 0.477, respectively.