Development and production of ductile TiZrNbHfTa refractory high entropy alloy system for extreme environments

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2022-1-13

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İroç, Lütfi Koray

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High entropy alloy (HEA) is a trending material class that was discovered in the early 2000s. By definition, it consists of a single or dual phase by combining 5-13 elements with a 5-35% atomic ratio. They exhibit extraordinary properties, such as structural, mechanical, corrosive and thermal. Moreover, this field gives an opportunity to combine infinite number of elements with infinite compositions. These properties and opportunities make them candidates for various extreme application areas, which will grow further in the future. Among the HEAs, Refractory High Entropy Alloys (RHEAs) are considered as future materials for high-temperature and nuclear applications due to their thermal stability and high-temperature mechanical properties. This study focuses on improving structural and mechanical properties and understanding the high-temperature and irradiated characteristics of RHEAs. By using CALPHAD modeling and thermophysical parameter optimization, two alloys were designed as oxygen-doped and undoped compositions. These alloys were produced by vacuum arc melting (VAM), and the structural characterization was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It has been found that both alloys consist of a single solid solution with BCC structure without any ordered phase, as designed. Besides, micro-indentation was performed to understand their hardness. It has been observed that oxygen-doped and undoped alloys exhibit hardness values of 440HV and 321HV, respectively and oxygen-doped alloy exhibited 1240 MPa compressive yield strength with a ductile behavior. The high-temperature behavior of promising oxygen-doped alloy is investigated by differential scanning calorimetry (DSC), in-situ XRD and TEM analyses. The results revealed that oxygen-doped RHEA contains a single BCC structure above 1000 °C without any metallic oxide. Also, oxygen doping does not make any significant structural or morphological difference compared to undoped alloy, similar to the simulation results. Eventually, considering the room temperature structure and mechanical improvements of oxygen-doping, the results are encouraging for the high-temperature applications of oxygen-doped RHEAs. Finally, to understand the radiation resistance, the oxygen-doped alloy is irradiated at three dosages (3, 10 and 30 dpa) and two temperatures (room temperature and 450 °C). Radiation effects are investigated using transmission electron microscopy (TEM) and nanoindentation. Mechanical and dislocation loops analyses revealed that there is no phase transformation, structural change, void formation as well as low hardening for all conditions. The observed high resistance under radiation makes oxygen-doped RHEA a good candidate for nuclear applications.

Subject Keywords

Refractory High Entropy Alloys (RHEA), Oxygen-doping, Alloy Design, Thermal Stability, Radiation Resistance

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https://hdl.handle.net/11511/95251

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Graduate School of Natural and Applied Sciences, Thesis

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Citation Formats

L. K. İroç, “Development and production of ductile TiZrNbHfTa refractory high entropy alloy system for extreme environments,” M.S. - Master of Science, Middle East Technical University, 2022.