High entropy alloys (HEAs, also known as complex concentrated alloys), which consist of four or more principal elements in equiatomic or near equiatomic concentrations, have attracted significant attention since first reported due to their novel alloy design concept and excellent performance. HEAs with face-centered cubic (FCC) structures exhibit high ductility and fracture toughness at room and cryogenic temperatures, but their relatively low yield strength seriously limits their practical applications. This thesis focuses on the strengthening of FCC-structured Al_0.3CoCrFeNi-type and CoCrFeNi-type HEAs through various hardening mechanisms. The joining of CoCrFeNi HEA using friction stir welding was also explored.

In the first part of the thesis, enhancement of Al_0.3CoCrFeNi alloy by adding various W content was reported. The effect of W addition on phase constitution, microstructure evolution and mechanical properties of the Al_0.3CoCrFeNi base alloy were systematically evaluated. The addition of W element promoted the structure change of as-cast Al_0.3CoCrFeNiWx HEAs from single FCC to FCC + μ, and the volume fraction of μ phase increased as the W content increased. The yield strength of the as-cast alloys was enhanced with increasing W content, which could be attributed to solid solution and second phase strengthening. Moreover, high temperature oxidation resistance was also studied to have a better understanding of the investigated alloys. Oxidation studies were performed on the HEAs at 800°C for 100 h. Protective external Cr₂O₃ layer and internal Al₂O₃ formed in the investigated alloys. With increasing W content, higher mass gains during oxidation experiments were obtained. This would be explained by the relative decrease of Cr content as well as the selective oxidation of μ phase. The results of our work might provide some clues for the development of new HEAs for high temperature applications.

In the second part of the thesis, a novel heterogeneous structure was designed to strengthen CoCrFeNi-type HEA by adding a minor amount of carbon and thermomechanical processing. Heterogeneous structures were introduced by concurrent recrystallization and carbide precipitation during thermomechanical processing in the carbon-containing CoCrFeNi alloy. The effects of the annealing temperature on microstructure and mechanical properties of the carbon-containing CoCrFeNi alloy were studied. The grains in the annealed alloys were significantly refined compared with that in the as-cast state, and the grain size increased with the increasing annealing temperature on the whole. Heterogeneous structures with tiny grains in strips were also observed in the annealed alloys. Compared with the as-cast condition, the strength was significantly increased along with little decrease in ductility in the annealed alloys. The microstructure-property relationship of the annealed alloys with heterogeneous structures was illustrated. The enhanced strength could be attributed to the combined effects of grain refinement, precipitation strengthening and microstructural heterogeneity. This work demonstrated that the heterogeneity design could be realized by thermomechanical processes, which provided a practical strategy for the strengthening of HEAs with high performance.

The joining of CoCrFeNi HEA by friction stir welding (FSW) under different processing parameters was also conducted. The correlations between the process parameters, structure and mechanical properties of the friction stir welded CoCrFeNi HEA were systematically investigated. Three distinct zones, namely, stir zone (SZ), thermo-mechanically affected zone (TMAZ), and base metal (BM), were identified in the joint. The SZ showed an equiaxed microstructure because of the dynamic recrystallization, while the TMAZ exhibited a mixed microstructure with elongated and equiaxed grains. Under different process parameters, the morphology of the SZ changed from basin shape to ellipticity accompanied with the occurrence of “onion rings” structure. The grain size of the SZ increased with the increasing rotation speeds of the welding tool with other parameters unaltered. The hardness of the SZ in welded samples was higher than that of the base metal due to the dynamic recrystallization induced by the FSW process, which was proved to follow the Hall–Petch relationship. At the optimum welding condition, the tensile strength and ductility of the joints reached 627 MPa and 42%, comparable with the base metal (628 MPa and 63%). The results were analyzed in terms of the heat input, and it turned out that the heat input could be used to predict the reliability of the joints. The successful welding of HEAs may enhance their potential engineering applications.

It is expected that our work could offer important clues to the development of high-performance HEAs for advanced structural applications. Moreover, inspired by the results obtained in our work, the combined utilization of friction stir processing (FSP) and the addition of aluminum/ carbon is proposed to enhance the CoCrFeNi HEA. The preliminary results suggest that high strength would be obtained with the help of the combined strengthening mechanisms. Extended works would be taken in the future.