The origin of dense dislocations in many additively manufactured metals remains a mystery. We here employed pure Cu as a prototype and fabricated the very challenging high-purity (>99.9%) bulk Cu by laser powder-bed-fusion (L-PBF) technique. We found that high-density dislocations were present in the as-built samples and these high-density dislocations were introduced on the fly during the L-PBF process. A newly developed multi-physics modeling was further employed to interpret the origin of these pre-existing dislocations, demonstrating that the compression-tension cycles rendered by the localized heating/cooling heterogeneity upon laser scanning are responsible for the residual high-density dislocations. GRAPHICAL ABSTRACT Impact statement The origin of dense dislocation structures in many as-built additively manufactured metals, which incurs a long-standing dispute, was found to be inherent to the additive manufacturing process.

A grain structure tailoring strategy was implemented by powder assembly & alloying in carbon nanotube reinforced Al-Cu-Mg composites. Tensile tests revealed that composites with trimodal grain structure had an outstanding combination of strength and ductility, surpassing that of composites with simpler grain structures (e.g. bimodal and unimodal structure). An in-situ tracking of the strain distributions and computational simulations were jointly employed to provide insight into the underlying mechanisms. The results unravel that the trimodal grain structure is more prone to facilitate strain hardening and alleviate strain/stress concentrations, thereby rendering the composites high yield strength and large tensile ductility. IMPACT STATEMENT A novel trimodal grain structure was designed for the first time to achieve an excellent strength-ductility combination in CNT/Al-Cu-Mg composites. GRAPHICAL ABSTRACT

Nanoparticles or precipitates are long used to block dislocations to strengthen metals. However, this strengthening mechanism unavoidably adds stress concentrations at the obstacles, instigating crack initiation that hampers ductility. Here, we demonstrate a strategy that replaces the traditional crystalline dispersions with dense amorphous nanoparticles, which is made possible via laser powder bed fusion. Porosity-free copper-based nanocomposites are demonstrated as a prototype, consisting of densely and uniformly distributed amorphous boron–carbide nanoparticles (~47 nm in average diameter, up to 12% volume fraction) via an in situ nanofragmentation and melt-quench process. The amorphous nanoparticles act as dislocation sinks, thereby alleviating local stress concentration. They also self-harden along with tensile deformation, promoting strain hardening and therefore homogeneous plastic flow. The as-built composite achieves a tensile strength of more than one gigapascal and a total elongation of approximately 10%, more than twice that of its crystalline dispersion counterpart. Defect accumulation is also suppressed upon cyclic deformation of the as-built bulk nanocomposites, delivering a fatigue strength limit (at > 10⁷ cycles) of more than 70% of the tensile strength. Our results demonstrate an effective strategy for additive manufacturing of metallic materials with superior properties.

Nanocrystalline (nc) metals typically possess high strength but low ductility. Here, we report an interface nanostructuring design via plasma assisted ball milling (PABM) to fabricate disordered multilayer graphene (DMGr)/Cu composites that are ultra-strong yet plastic, achieving a compressive strength of 1.56 GPa and plastic strain of exceeding 0.6. Our strategy relies on the uniformly and densely dispersed DMGr with sp²-sp³ hybridization. Interlayer sliding of DMGr but much stronger than van der Waals forces which can be anticipated to mediate plastic deformation and improve the plasticity. The high intrinsic strength of DMGr and associated Cu lattice strain near the interface due to strong interactions between DMGr and matrix can significantly impede dislocation motion and promote dislocation accumulation within nanograin interior. Ex-situ and in-situ TEM characterizations revealed that substantial dislocation interactions and accumulations induced by DMGr and the associated lattice strain along with interlayer sliding of DMGr, led to high strength, enhanced strain hardening capacity and superior plasticity. Such a design strategy provides a pathway for mitigating the trade-off between strength and plasticity in nanograined metals.