Kinetically-Driven Microstructure and Mechanical Properties of 3D Micro-Architected Metal Alloys Formed via Hydrogel Infusion Additive Manufacturing (HIAM)

Hydrogel infusion-based additive manufacturing (HIAM) to create 3D micro-architected metals is unique in its absence of melt pool solidification. By nucleating and growing parent oxide phases and subsequently reducing them, HIAM provides a kinetically-driven pathway for forming boundaries in polycrystalline metal microarchitectures. Using EBSD and TEM analysis, we characterize these boundaries, defects, and other microstructural features within Cu, Ni, and Cu-Ni alloys produced by this technique over a span of thermal treatment conditions. These metals and alloys exhibit anomalous nanoindentation hardnesses, exceeding predictions of the classical Hall-Petch relation by at least 30%, which we postulate to be due to the present networks of high-angle grain boundaries, annealing twin boundaries, and higher-order coincident site lattice (CSL) boundaries. We explore the role of formed boundaries by conducting site-specific compression experiments on nano- and micropillars carved from individual grains -- as well as spanning specific boundaries --, isolating the contribution of each type of boundary to plasticity in HIAM-produced metals. We present a phenomenological framework that accounts for the contribution of special boundaries to global mechanical properties, which helps inform HIAM as a novel means to create 3D-architected metals and alloys with non-equilibrium microstructures and compositions.