Vacancy annihilation, clustering, and trapping in aluminum alloys studied by simulations and experiments
Chen, Xinren; Raabe, Dierk (Thesis advisor); Svendsen, Bob (Thesis advisor)
Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2023
The phase transformation kinetics of solid-state materials largely depends on the diffusivity of atoms. Diffusion of substitutional atoms proceeds via lattice vacancies. Processes like quenching, irradiation, and plastic deformation increase the vacancy concentration and thus accelerate the diffusion rate. The excess vacancies can be absorbed by defects such as dislocations and grain boundaries. However, the vacancy distribution near defects such as grain boundaries remains less well understood. The investigations on vacancies in this thesis are separated into two parts: 1. simulation methods and results of the vacancy concentration field in aluminum and aluminum alloys; 2. experimental measurements of the vacancy concentration in aluminum alloys. The simulations of vacancy fields combine current physical models, including the diffusion of vacancies, the vacancies' annihilation at lattice defects, solute-vacancy binding, and the condensation of excess vacancies within grains. For the specific quenching process and grain size in this work, the results demonstrate that the fast nucleation of Frank loops significantly reduces the supersaturation of vacancies. The results also show an abnormal size distribution of Frank loops near grain boundaries, as further demonstrated by the experimental results in this thesis. Based on the simulation results, experimental methods were designed and conducted to reveal the distribution of vacancies. The first method is using electron transmission microscopy to observe the local density and size of vacancy Frank loops, which translates to the vacancy concentration that is annihilated in the vacancy Frank loops. The second method is to analyze the local solute diffusivity by spinodal decomposition, which is based on the proportional relationship between local free vacancy concentration and atomic diffusivity. This is enabled by atom probe tomography with three-dimensional spatial nanometer resolution and a cryogenic transfer technique that is used to freeze vacancies and atoms.
- Division of Materials Science and Engineering 
- Chair of Materials Physics and Institute for Physical Metallurgy and Materials Physics