Well-Ordered Nanonetwork Invar from Templated Electrochemical Deposition as Mechanical Metamaterials

This work presents a novel bottom-up approach using a template from a self-assembled block copolymer followed by selective etching for templated electrochemical deposition to fabricate well-ordered nanonetwork Invar (Fe₆₄Ni₃₆) thin film, giving exceptional mechanical properties as mechanical metamaterials. By utilizing pulse-current electrochemical deposition, it is feasible to achieve complete pore-filling for the template, giving diamond-structured Invar with a strut size of ≈20 nm that is one-order magnitude less than the smallest diameter from a top-down approach. The nanoindentation test reveals that the reduced modulus of the diamond-structured Invar exceeds the upper bound of the Gibson-Ashby scaling law due to its deliberate structuring. Following the same reasoning, as evidenced by the micro-compression test, the diamond-structured Invar exhibits an extensive plastic plateau region under loading, different from the catastrophic failure for intrinsic Invar. These findings also demonstrate that the specific strength and energy absorption per volume of the diamond-structured Invar surpasses that of nickel microlattices, in agreement with the "smaller is stronger and tougher" phenomenon. This work highlights the feasibility of fabricating lightweight Invar without sacrificing the inherent properties of Invar, offering a significant improvement over metallic micro-/nanolattices derived from a top-down approach.