On vacancy clustering behavior in quenched pure aluminum

Excess vacancies introduced by rapid quenching or irradiation strongly influence phase transformations and solute diffusion in aluminum alloys. The supersaturation of vacancies leads to vacancy clustering, which in turn causes the formation of Frank loops. By correlating transmission electron microscopy (TEM) observations of Frank loop distributions with simulations of vacancy generation and annihilation, we investigate the evolution and spatial distribution of excess vacancies during quenching and natural aging. TEM analysis shows that up to ∼ 1 0 − 4 site fraction of vacancies are condensed into Frank loops in the grain interior after quenching. A local equilibrium between these loops and the matrix sustains the residual vacancy supersaturation. The interplay between loop nucleation and growth produces distinct spatial variations in loop characteristics: the loop size does not always vary monotonically with distance from the grain boundary, leading to the formation of an “abnormal-size zone,” where the Frank-loop density continues to increase while their sizes fall below 20 nm. This zone begins at ∼ 1.4 µm from the grain boundary. These microstructure-dependent features arise from the coupled effects of vacancy diffusion, vacancy absorption at grain boundaries, and the kinetics of loop nucleation and growth, and is therefore highly dependent on the quenching rate. The results demonstrate that the supersaturated vacancy concentration in aluminum is governed by a local equilibrium between the density and size of Frank loops and the surrounding matrix. This study highlights the critical role of localized vacancy clustering in controlling vacancy-concentration evolution and provides fundamental insights into vacancy-mediated processes in aluminum alloys.