Twinning-inspired hexagonal close-packed metamaterials for enhanced energy absorption

Additive Manufacturing provides unprecedented opportunities for designing and producing intricate, architected materials with exceptional properties while maintaining reduced weight. One of the most straightforward ways to create such structures is to mimic the lattice geometries of crystals. New approaches are now exploring how to influence the behaviour of these structures by mimicking not only the crystal arrangement but also peculiar mechanisms occurring at the crystallographic scale. In such a context, this study first presents a hexagonal close-packed (HCP) lattice derived from the homonymous crystal structures of Ti or Zr metals. Subsequently, the twinning phenomenon responsible for the plastic deformation of these metals is reproduced geometrically into this structure to improve its energy absorption capability. Compressive tests conducted on 3D-printed samples confirm such a hypothesis: the specific energy absorption of twinned structures is significantly higher than that of the equivalent HCP ones, with an increase of up to 24.3%. Introducing twinned regions stabilises the plastic deformation despite a limited reduction of the peak stress. Therefore, replicating the twinning metallurgical mechanism within an HCP-inspired metamaterial proves successful, further validating the approach of mimicking and transferring at a different scale atomic phenomena to tailor the properties of architected materials.