Xue, Lufeng (2022-11). Mechanical & Electrochemical Characterization of Newly Designed High-entropy Alloy System Al2Cr5Cu5Fe53Ni35 in Marine Environment. Doctoral Dissertation.
Thesis
A new high entropy alloy (HEA) system, Al2Cr5Cu5Fe53Ni35 is designed, synthesized and characterized. In order to improve its properties, several grain refinement treatments including cold rolling and annealing are conducted. To determine the microstructure of this new HEA, and also the HEA after grain refinement process, comprehensive microstructural characterizations such as XRD, SEM, EDS and EBSD are employed. The microstructural characterization experiments reveal that the studied HEA is a single phase FCC alloy, and that the grain refinement process reduces the grain size of the HEA significantly without changing its crystal structure. The influence of temperature and chloride concentration on the corrosion properties of the new HEA system are measured by means of potentiodynamic, potentiostatic and EIS experiments. The XPS and SIMS experiments are employed to analyze the composition of the passive film formed on the surface of the HEA in seawater. The data gathered from the electrochemical experiments are used to build the Point Defect Model (PDM), which is used to analyze the influence of the temperature and chloride concentration on the properties of the passive film. The electrochemical characterization of the Al2Cr5Cu5Fe53Ni35 alloy reveals that the smaller the grain size, the lower the solution temperature, and the lower the chloride concentration of the solution, the better the corrosion resistance is. The mechanical properties of the studied HEA is characterized by conducting comprehensive tensile tests of selected geometry specimens. It can be demonstrated from the experiments that the newly designed HEA possesses good ductility and acceptable strength, and that the grain refinement can improve its mechanical properties. Moreover, the mechanical behavior of the studied HEA is simulated with an uncoupled plasticity / fracture model. The plasticity model incorporates a pressure-sensitive function into the yield condition along with an associated flow rule and Swift-Voce isotropic hardening rule. The phenomenological fracture model used to predict the initiation of fracture is based on the Modified Mohr-Coulomb model. The phenomenological numerical model can serve as a basis for more accurate assessments of deformation processes and fracture response of the studied HEA. In addition, a crystal plasticity-based approach is employed to reveal gliding between crystals, responsible for plastic deformation, during plastic deformation. The outcome indicates that the grain size and the grain orientation affect the dislocation slip during the plastic deformation, influencing the mechanical properties of the HEA.
