Morphological Evolution and Instability of Microvoids Governed by Competing Surface and Bulk Diffusion

Understanding diffusion-controlled void growth at the nanoscale under extreme environments (e.g., high temperatures, irradiation) is crucial for predicting failure in metallic materials. We develop a micromechanical model that integrates surface diffusion, bulk diffusion, and heterogeneous stress fields to capture the growth, morphological evolution and coalescence of microvoids. The model is validated against well-established analytical solutions, demonstrating high reliability and accuracy. Using this model, we explore the competitive interplay between surface diffusion and bulk diffusion in governing void shape stability and transitions. Our findings reveal that surface diffusion promotes stable, circular or elliptical morphologies, whereas bulk diffusion, especially under heterogeneous stress, induces anisotropic growth and morphological instabilities. As voids grow, the influence of surface diffusion diminishes, facilitating the formation of increasingly complex void shapes. Void coalescence behavior further reflects this interplay: in the absence of inter-void vacancy sources, bulk diffusion alone is insufficient to drive coalescence due to shielding effect on vacancy concentration gradient at inter-void region. In contrast, surface diffusion facilitates void coalescence, with rates increasing alongside surface diffusivity. When surface diffusion is substantially weaker than bulk diffusion, void growth becomes inherently unstable, and minor surface perturbations can trigger the nucleation of micro-cracks. These findings show excellent agreement with existing experimental results. Overall, this study provides a mechanistic understanding of diffusion-controlled void evolution and offers valuable insights into damage precursors in metals subjected to extreme environments.