Size effects in metallic materials: From yielding and plastic flow to fracture mechanisms and engineering implications

Metallic materials are widely used in modern engineering owing to their excellent mechanical properties and broad processing adaptability. At macroscopic scales, their mechanical behavior is generally regarded as size-independent and can be described by classical continuum plasticity theories. However, with the rapid development of micro- and nano-manufacturing technologies, metallic components are increasingly fabricated at reduced length scales, where pronounced size effects emerge. These effects are manifested by enhanced strength, altered plastic deformation behavior, increased property scatter, and distinct fracture modes. Although extensive experimental and theoretical studies have investigated size-dependent mechanical behavior, existing reviews mainly focus on isolated phenomena or specific material systems. A systematic and mechanism-based understanding of how size effects govern the entire mechanical response, from yielding and plastic flow to fracture, remains insufficient. Therefore, this review provides a comprehensive overview of size effects in metallic materials, with emphasis on their influence on yielding behavior, plastic deformation evolution, and fracture mechanisms. Representative theoretical models are critically assessed, and the implications of size effects for micro-manufacturing and engineering applications are discussed.