Experimental and computational assessment of high-temperature properties in ti-based high entropy alloys

During the last two decades, high entropy alloys (HEAs) have gained noticeable attention due to their outstanding properties and potential for various applications. Within the prospective applications, due to their high melting point and high thermal stability, refractory high entropy alloys (RHEAs) are considered an alternative to Ni-based superalloys for high-temperature applications. Therefore, in order to shed light on the high-temperature properties of RHEAs, two aspects of their properties were investigated. After a brief introduction to the concepts discussed in this thesis, the oxidation behavior of four different HEAs was investigated via static oxidation experiments utilizing a variety of surface and structural characterization methods such as X-ray photon spectrometry (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM). The results suggested that three different oxidation mechanisms in these alloys are dependent on the composition, temperature, and crystal structure of the HEAs. In addition, each element's effect on these alloys' oxidation behavior was discussed. In the second part of the thesis, using atomistic simulations, the effect of the formation of chemical short-range order (CSRO) on mechanical properties was investigated in two different compositions of RHEAs. The results showed that the CSRO formation in these alloys can increase the materials' strength while maintaining the HEAs' ductility. In addition, the effect of CSROs on deformation-induced phase transformation (DIPT) was captured via atomistic simulations. To validate these results, an extensive amount of in-situ compression experiments under SEM were performed and these results validated the formation of HCP phases due to DIPT. Overall, the results and discussions in this thesis provide a better view of the high-temperature properties of the HEAs with the aim of enabling their applications at high temperatures.