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Ali Behroozfar1 Soheil Daryadel1 S. Reza Morsali1 Rodrigo A. Bernal1 Majid Minary-Jolandan1

1, University of Texas at Dallas, Dallas, Texas, United States

Nanotwinned (nt) metals have a unique microstructure with grains that contain a high density of layered nanoscale twins divided by coherent twin boundaries (TBs). These metals exhibit superior mechanical and electrical properties compared to their coarse-grained and nanocrystalline counterparts. Since TBs can effectively block dislocation motion, the nanotwinned metals often show higher strength and ductility compared to their nanocrystalline counterpart. In addition, these metals show more resistance to electromigration which is a common problem for metals at the nano/microscale. Several processes including pulsed electrodeposition (PED), plastic deformation, recrystallization, phase transformation, and sputter deposition have been used so far to make nt-metals in film and bulk forms. However no additive manufacturing technique has been introduced so far to fabricate nanotwinned metals in microscales. Here we report on a new process for 3D printing of nt-metals, termed localized pulsed electrodeposition (L-PED) to perform microscale 3D printing of nt-Cu with high density of coherent twin boundaries (TBs). We also show that the twin density and grain size of the material can be controlled in this process. This cost-effective process, which is performed at ambient environment can be used for direct 3D printing of layer-by-layer and complex 3D micro-scale nt-cu structures with various applications including electronics, micro/nanoelectromechanical systems (MEMS, and NEMS), metamaterials, plasmonic, and sensors. We also show that L-PED process can be performed on non-conductive substrates to make interconnections between two conductive pads.
This process will enable incorporation of metals with high strength and ductility and low electromigration susceptibility into various applications. The characterization tests performed on the 3D printed samples show that the structure is fully dense, with low to none impurities, and low microstructural defects, and without obvious interface between printed layers, which overall result in good mechanical and electrical properties so there is no need to perform any post-processing steps. We show that the L-PED process can be performed with in situ control over twin lamella thickness (λ) and twin density during printing through control of the pulsed ED parameters. Such spatially varying microstructure control may enable spatial tuning of mechanical and electrical properties of the printed metal.

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