Strain hardening in metallic materials: Mechanisms, applications, and multiscale pathways for next-generation alloys

Strain hardening (work hardening), the rise in flow stress with plastic strain, governs the strength–ductility balance of metallic alloys and strongly influences uniform elongation and crash/fatigue tolerance. This review synthesizes 209 studies (2017–2025) identified through Scopus, Web of Science, and ScienceDirect searches (Jan–Oct 2025) plus reference-chaining, and selected using explicit inclusion criteria (metallic systems; quantitative stress–strain or hardening indices; mechanistic interpretation) following PRISMA-style screening. Strain-hardening pathways across FCC, BCC, and HCP systems are examined, emphasizing dislocation storage vs dynamic recovery, grain-boundary/heterogeneous-structure hardening, precipitation–dislocation interactions, and twinning/phase-transformation–assisted hardening (TWIP/TRIP). Classical constitutive laws (Hollomon, Swift, Voce, Kocks–Mecking) are contrasted with emerging physics-informed and machine-learning approaches for predicting θ(ε) and n evolution. Where comparability allowed, descriptive group summaries are reported (median, IQR; bootstrap CIs when n ≥ 8 ; otherwise, findings are integrated via structured narrative synthesis. Finally, processing–microstructure levers (SPD, gradient architectures, AM + post-processing) are mapped and sustainability-relevant constraints (processing energy, life extension, recyclability) are framed for alloys that delay localization while maintaining high strength.