Refractory High-entropy Alloy and Nitride Thin Films

Abstract: This thesis focuses on understanding the process-structure-property relation-ships for several refractory-metal-based high-entropy alloys and nitrides synthesized by magnetron sputtering.  The thesis begins with the growth of TiZrNbTaNx understoichiometric nitrides by controlling substrate temperature and nitrogen flow ratio fN. Their effects on microstructure and mechanical, electric, and electrochemical properties were investigated. TiZrNbTaN0.46 deposited at 400 ºC shows a NaCl-type structure with (001) preferred orientation and exhibits the highest corrosion resistance in 0.1 M H2SO4 aqueous solution. A stable passive region up to 3.0 V vs. Ag/AgCl could be achieved when x< 0.64. The densification effects were explored by ion energy for (TiZrTaMe)N1–x (Me = Hf, Nb, Mo, or Cr) films and by high-power impulse magnetron sputtering technique for TiNbCrAl films.The local chemical distortions in the TiZrTaNb-based system with different nitrogen content were investigated by X-ray absorption fine structure spectros-copy. The influence of crystallinity on superconducting transition behavior was studied in (TiZrHf)x(TaNb)1-x, (TiZrNbTa)1-xWx, and (TiZrNbTa)1-xVx systems. The highest superconducting transition temperature (Tc) reaches 8.05 K for the TiZrNbTa film (x=0). The superconducting transition temperature Tc of these films deposited at the fixed temperature decreases monotonically as a function of x, and Tc can be increased by elevating the deposition temperature. Furthermore, the structural stability and elemental segregation under Xe-ion irradiation of TiZrNbTaV-based HEA and HEN films, and high-entropy TiZrN-bTa/CrFeCoNi metallic and nitride multilayer coatings were investigated. The microstructure of TiZrNbTaVN film remain stable after irradiation at room temperature and 500 °C. The as-deposited TiZrNbTaV film exhibited an amorphous structure and became a bcc phase structure after irradiation at 500 °C. Thermal-induced and irradiation-induced grain growth resulted in a grain-size distribution. For the multilayer coatings, the microstructure of metallic multilayers was not stable and the interdiffusion or mixing of the constituent elements is prominent under ion irradiation and/or heat treatment. no diffusion and phase trans-formation were observed for the nitride multilayers after irradiation at 500 °C.