Understanding how elemental variations influence defect cluster formation is a longstanding challenge in materials science. By combining defect rates-based long-time dynamics with molecular dynamics and irradiation experiments, we identify a distinct, cluster-mediated mechanism—governed by element-specific interactions—as the dominant driver of vacancy cluster evolution into voids or stacking-fault tetrahedra in irradiated complex concentrated alloys, specifically NiCoCr, ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{30}{\mathrm{Co}}_{10}{\mathrm{Cr}}_{10}$, and Ni at elevated temperatures. Unlike conventional models that focus on point defect behaviors, the proposed mechanism highlights a critical two-step process—vacancy-tetrahedron formation and annihilation—that governs the bifurcation of vacancy clusters. Ni and Co promote void formation by favoring annihilation over formation, leading to $\mathrm{Ni}/\mathrm{Co}$ segregation, whereas larger atoms such as Cr, Fe, and Mn resist annihilation, thus favoring stacking-fault tetrahedra formation. These findings offer new insights into how local chemical environments influence defect evolution and provide strategies for tailoring materials to perform better under extreme conditions.

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• Composition
• Defects
• Diffusion
• Irradiation effects
• Alloys
• Crystal structures
• Ion beam analysis
• Materials modeling
• Molecular dynamics
• Multiscale modeling