Heterogeneous microstructures by combining laser additive manufacturing with auxiliary ultrasound field for strength-ductility betterment

Heterogenization is broadly employed to metallic alloys to improve their strength and ductility. In the current work, taking advantage of layer-wise melting by laser additive manufacturing (LAM) and grain refinement by auxiliary ultrasound field (USF), we combine LAM with USF to realize heterogeneous structures in 316 L stainless steel (SS). The as-built heterogeneous 316 L SS exhibits hierarchical microstructures spanning multiple length scales, ranging from millimeter-sized grains and micrometer-scale intragranular cellular domains down to nanoparticles of Fe-Cr σ-phase and L1₂ precipitates heterogeneously dispersed within the cellular matrix. This unique spatial distribution of microstructures with a broad size-span and chemical fluctuations give rise to an ultra-high tensile strength in heterogeneous 316 L SS with a ultimate strength up to 1 GPa and an elongation of ∼12.5 %. We further reveal that the synergy effect from hierarchical twins and metastable cells on dislocation gliding gives rise to the superior strength-ductility performance. The demonstration of using LAM with auxiliary USF to achieve controllable hierarchical heterogeneous microstructure paves a practical way for microstructure design and manufacturing.