Atomic-scale strain waves for stronger and more ductile lightweight steels

Reducing weight to save energy and enhancing mechanical properties to increase safety level are both vital for structural materials in modern engineering. The lightweight steels have been promising for addressing these needs. However, the significant loss of ductility and strength in lightweight steels by localized accumulation of dislocations when strengthened by nanoprecipitates is a crucial problem. Here, we show an approach to overcome this dilemma by significant atomic-scale strain waves with extremely small wavelengths (<1 nm) and high amplitudes (up to 3%) in Fe-Mn-Al-C lightweight steels. Such atomic-scale strain waves render the dislocation lines wavy and paired, which tangle into dense hexagonal dislocation networks via cross-slip at medium and late deformation stages, significantly enhancing defect-storage capacity and strain hardening ability. The atomic-scale strain waves also promote the dynamic refinement of slip bands upon deformation. These mechanisms are active in both steel variants with and without nanoscale coherent κ-carbides, enabling unprecedentedly strong and ductile lightweight steels. The strategy of tuning atomic-scale strain waves thus provides an important avenue for designing stronger and more ductile lightweight materials for key structural engineering.