Effects of Doping Elements on the Irradiation Performance of Concentrated Solid-solution Alloys (High Entropy Alloys)

In metallurgy, addition of foreign elements into pure metals to form metallic alloys is a usual way to improve their mechanical properties and irradiation performances. Traditionally, only a small amount of alloying elements is added to a pure metal so that a dilute alloy is produced. The properties of these dilute alloys rely heavily on the matrix and different alloy systems are developed in this way, such as the extensively studied Ni-based alloys, Fe-based alloys and Mg-based alloys. Since the principal element in these alloys is fixed, the performance improvement by incorporating minor components is limited.In contrast to dilute alloys, concentrated solid-solution alloys (CSAs, or high entropy alloys, HEAs) are composed of multiple principal elements all at high concentrations. In CSAs, random arrangement of different elements in a simple underlying crystal lattice results in extreme chemical disorder and local lattice distortions. Because of their excellent mechanical properties, enhanced irradiation resistance, and strong corrosion resistance, CSAs have been proposed as candidate structural materials in advanced nuclear systems. In CSAs, the vast choice of alloy elements provides a unique playground to tune their properties. It has been demonstrated that unusual mechanical strength and irradiation tolerance can be achieved by choosing suitable element combinations. Since the most important feature of CSAs is their intrinsic disordered states arising from random arrangement of different elements, it is anticipated that their properties can be tuned further by manipulating the way in which different atoms are incorporated.Combining the concepts of dilute and concentrated alloys, this proposal will explore the irradiation performance of doped-CSAs by adding a small concentration of doping elements to CSAs. A series of 3d transition-metal CSAs with face centered-cubic structures including NiFe, NiCoCr, and NiCoFeCr, and different doping elements such as Al, Cu, Pd, and Ti will be considered. Since CSAs have demonstrated excellent radiation resistance, it is anticipated that doping will be an effective method to further improve their irradiation performance. Multi-scale simulation techniques will be employed to gain insight into the defect energy landscape of different doped-CSAs with different impurities and CSA matrixes. Specifically, extensive first-principles calculations based on density functional theory (DFT) will be carried out to study defect energies under different local environment. The established local-environment dependent defect database, together with our previous results obtained in equiatomic CSAs, will be utilized to construct a machine learning (ML) model in order to predict defect energies at any given environment. The results will then be used in Atomistic Kinetic Monte Carlo (AKMC) simulations to study defect evolution over long time scales. The obtained results, including effects of different dopants in changing defect energetics and different diffusion coefficients of constitutional elements, will be used to compare with experimental Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) analysis regarding defect size distribution and elemental segregation with the support from our collaborators regarding sample preparation and irradiation setup. The outcome of this multi-scale study will deepen our understanding about the roles that different elements and their concentrations play in doped-CSAs, and finally provide a scientific way to tune the elemental species and their concentrations to enhance their irradiation performance.