Synthesis and development of refractory high entropy alloys

High entropy alloys (HEAs) serve as a new metallic alloy system for high-temperature applications in aerospace, nuclear, and chemical process industries due to their attractive properties. This thesis focuses on the synthesis and development of refractory HEAs, and the investigation of their phase stability, the evolution of microstructure, high-temperature behavior, mechanical properties of nonequiatomic Al-Co-Cr-Fe-(Ni-Nb-Ti), and equiatomic Al-Hf-Mo-Nb-Ta-Ti-V alloy systems under as-cast and isothermally annealed conditions. The alloys were predetermined based on the calculated thermodynamic data. The alloys were then were produced by vacuum arc melter using high purity elements. The high entropy alloy systems were investigated using X-ray diffraction (XRD), in-situ X-ray diffraction (XRD), optic microscope (OM), scanning electron microscope (SEM), hardness, compression, and tension tests. The analysis reveals the dependency of mechanical properties and phase formation on Nb and Ti concentration of the Al-Co-Cr-Fe-(Ni-Nb-Ti) alloy systems. The Vickers microhardness tests reveal an increase in hardness with the addition of Nb and Ti. The ductile type of fracture was seen for as-cast Al8Co30Cr18Fe9Ni31Nb4 and Al8Co30Cr18Fe9Ni31Ti4 alloys. The highest toughness value, highest ultimate tensile strength of 844 MPa and highest compressive fracture strain of 29.76% were observed for Al8Co30Cr18Fe9Ni31Ti4 alloy, which has tetrataenite FeNi intermetallic phase distributed in the dendritic matrix. The Al-Hf-Mo-Nb-Ta-Ti-V alloy systems with B2/BCC dual-phase structure indicate good phase stability upon heating. The structure analysis reveals that these systems can be good candidates for refractory HEAs for high-temperature applications.