Characterization of nickel-based microlattice materials with structural hierarchy from the nanometer to the millimeter scale

Novel nickel-based microlattice materials with structural hierarchy spanning three different length scales (nm, mu m, mm) are characterized microstructurally and mechanically. These materials are produced by plating a sacrificial template obtained by self-propagating photopolymer waveguide prototyping. Ni-P films with a thickness of 120 nm to 3 mu m are deposited by electroless plating, whereas thicker films (5-26 mu m) are obtained by subsequent electrodeposition of a pure Ni layer. This results in cellular materials spanning three orders of magnitude in relative density, from 0.01% to 8.5%. The thin electroless Ni-P films have ultra-fine grain size (7 nm) and a yield strength of similar to 2.5 GPa, whereas the thicker electrodeposited Ni films exhibit a much broader distribution with average grain size of 116 nm and strong (100) texture in the plating direction, resulting in a yield strength of similar to 1 GPa. Uniaxial compression experiments reveal two distinct mechanical responses. At ultra-low densities (<0.1%), these lattices exhibit nearly full recovery after strains up to more than 50%, and damping coefficients an order of magnitude larger than for conventional Ni foams. At higher densities (0.1-10%), the compression behavior is fully plastic, similar to traditional cellular metals. A simple mechanical analysis reveals that the transition occurs when the thickness-to-diameter ratio of the truss elements is of the order of the yield strain of the material, in agreement with experimental observations. Optical and electron imaging of deformed lattices show that the deformation largely localizes around the nodes. In the ultra-light regime, the microlattice materials are stiffer and stronger than any existing alternative. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.