Using Spinodal Decomposition to Investigate Diffusion Enhancement and Vacancy Population

Material sustainability requires energy-efficient and rapid strengthening processes. In alloys, strengthening through diffusion-driven precipitation is limited by the low vacancy concentration, with fewer than one vacancy per 100 billion lattice sites at room temperature in metals such as aluminum and iron under thermodynamic equilibrium. Artificially increasing vacancy concentrations by 1 to 7 orders of magnitude above equilibrium levels through quenching, irradiation, or deformation can significantly accelerate material strengthening. However, measuring vacancy concentrations below 10^-7 in alloys and achieving spatial mapping remain challenging. Here, a vacancy-mediated gradient microstructure near grain boundaries is reported and analyzed to investigate diffusion enhancement and the local vacancy population in an Al-Zn system. This method uses cryogenic processes to preserve excess vacancies and halt microstructure evolution, enabling intermittent measurement of compositional fluctuations during ultrafast spinodal decomposition. It allows for the assessment of diffusion enhancement and determination of vacancy supersaturation in sub-micrometer regions. Liquid nitrogen-quenched Al-12.5 at.% Zn alloy shows a vacancy concentration of ≈10^-7 at room temperature, dropping to 10^-9 after 3 h, with significant spatial variation near grain boundaries. This work addresses gaps in understanding the evolution and distribution of vacancies across various measurement scales, advancing the control of vacancies to enhance the strengthening of engineering alloys.