  The mechanical performance of metallic metamaterials with 3-dimensional solid
frames is typically a combination of the geometrical effect ("architecture")
and the characteristic size effects of the base material ("microstructure"). In
this study, for the first time, the temperature- and rate-dependent mechanical
response of copper microlattices has been investigated. The microlattices were
fabricated via a localized electrodeposition in liquid (LEL) process which
enables high-precision additive manufacturing of metal at the micro-scale. The
metal microlattices possess a unique microstructure with micron sized grains
that are rich with randomly oriented growth twins and near-ideal nodal
connectivity. Importantly, copper microlattices exhibited unique temperature
(-150 and 25 degree C) and strain rate (0.001~100 s-1) dependent deformation
behavior during in situ micromechanical testing. Systematic compression tests
of fully dense copper micropillars, equivalent in diameter and length to the
struts of the microlattice at comparable extreme loading conditions, allow us
to investigate the intrinsic deformation mechanism of copper. Combined with the
post-mortem microstructural analysis, substantial shifts in deformation
mechanisms depending on the temperature and strain rate were revealed. On the
one hand, at room temperature (25 degree C), dislocation slip based plastic
deformation occurs and leads to a localized deformation of the micropillars. On
the other hand, at cryogenic temperature (-150 degree C), mechanical twinning
occurs and leads to relatively homogeneous deformation of the micropillars.
Based on the intrinsic deformation mechanisms of copper, the temperature and
strain rate dependent deformation behavior of microlattices could be explained.
