Ultrahigh Stiffness and Energy Absorption Properties of Isotropic Metallic Closed Cell Microlattices

To validate leading theories on isotropic mechanical metamaterial designs, pSC-pFCC closed cell microlattices are fabricated from SS304L sheets using the LAPIS additive manufacturing technique. By removing excess material at each layer, the fully enclosed voids in the lattice design are faithfully reproduced, confirmed by micro-CT scan, without the need to introduce release holes for precursor materials. Material anisotropy caused by the layer-by-layer fabrication process is removed with a post-print heat treatment. The microlattices exhibited highly similar elastic deformation in the <100> and <110> axes, with stiffnesses at the Hashin-Shtrikman theoretical limit, as predicted previously. However, this isotropy in stress-strain response is unexpectedly extended to the plastic regime as well, even though the microlattices failed via plate buckling in <100> orientation, but by shear banding in <110>. Moreover, the microlattices also displayed remarkable specific energy absorption (15-33 J g^-1) and energy absorption efficiencies up to 44%, at stresses as high as 410 MPa. Material work hardening is key to this breakthrough performance, as it raised the plateau stress of the plate buckling failure to approximately the same level as the stretch-dominated elastic limit, which allowed ultrahigh stiffness to be united with excellent energy absorption characteristics in these mechanical metamaterials.