Vacancy Engineering in Metals and Alloys

Abstract

Vacancy engineering, the intentional control of atomic-scale vacancies in metals and alloys, is emerging as a powerful yet underexplored strategy for tailoring microstructures and optimizing performance across diverse applications. By enabling excess vacancy populations through quenching, severe deformation, thermomechanical treatments, or additive manufacturing, new microstructures can be obtained that achieve unique combinations of strength, ductility, fatigue life, corrosion resistance, and conductivity. Vacancies are distinct among lattice defects: they are non-conserved entities essential for solute diffusion, yet variably coupled to solutes, dislocations, and phase boundaries. They can accelerate transformations such as nucleation and precipitation or retard kinetics when trapped in clusters, and their transient trapping and release can drive microstructural evolution across time and length scales. This Review synthesizes recent advances in generating, modeling, and characterizing vacancies, highlighting their role in diffusion, precipitation, and phase stability. Case studies in lightweight, high-temperature, fatigue-resistant, electrical, and biomedical materials demonstrate the broad potential of vacancy control. We conclude by emphasizing the opportunity for the metallurgical community to fully exploit excess vacancies as controllable, design-relevant defects that enable new pathways for microstructure and property optimization in next-generation alloys.

Figures (3)

• Figure 1: Processing routes and mechanisms for generating excess vacancies in metals and alloys. The schematic on the left illustrates how different processing routes introduce excess vacancies into metallic lattices through distinct physical mechanisms. The plot on the right compares reported non-equilibrium vacancy concentrations, showing orders-of-magnitude variation among routes. Irradiation, shock loading, and rapid solidification generally yield the highest vacancy fractions boleininger2023microstructurepedchenko1969concerningmajeed2025vacancyhillert2002trappingharaguchi2003determinationyang2010anomalygavsparova2024positron, while severe plastic deformation (SPD) and solid state quenching (e.g. water and liquid nitrogen) produce intermediate levels ma2019dynamiccizek2019developmentsun2019precipitationwu2022freezingchen2023investigation. Vapor deposition-based processes, span a broad range depending on growth kinetics and substrate conditions zhou1997vacancy. Together, these pathways demonstrate how excess vacancies can be engineered to control diffusion, phase stability, precipitation behavior, and mechanical performance.
• Figure 2: Thermodynamic and kinetic principles governing vacancies in metals and alloys.(a) Vacancy formation is thermally activated, requiring a Gibbs free energy barrier $G_f$, and typically occurs via atomic motion. Gibbs free energy $G = H - T S$ decreases with temperature, promoting vacancy stabilization at high $T$. (b) Vacancies drive diffusion-based transformations, forming solute clusters, vacancy clusters, and precipitates through solute redistribution ma2019dynamicpeng2020solutesun2019precipitation. Other products like stacking fault tetrahedra (SFTs), which form from partials and vacancies under irradiation, are mechanically harmful outcomes of vacancy supersaturation uberuaga2007direct. (c) Solute--vacancy binding energies $E_b$ in Mg (blue) and Al (red), plotted against the change in volume produced upon addition of a single impurity into the systemwolverton_solutevacancy_2007shin2010first. A size-dependent trend is shown: larger solutes exhibit more negative $E_b$, indicating stronger elastic interactions. These drive clustering, segregation, and defect stabilization, which are essential to vacancy-guided alloy design.
• Figure 3: Vacancy engineering enables atomic-scale control over solute partitioning, defect redistribution, and phase evolution across structural and functional material systems.