  The mimicry of crystalline microstructure at meso-scale creates a new class
of architected materials, termed meta-crystals, and offers effective ways to
significantly improve the toughness and eliminate the post-yield collapse of
architected materials. This study investigated the mechanical behaviour of
polygrain-like meta-crystals fabricated from Ti6Al4V by laser powder bed
fusion. The mechanical behaviour of Ti6Al4V meta-crystals is governed by
lattice structures across length-scales: the crystalline microstructure,
architected crystal-like mesostructures and the quality of lattice struts. Due
to the intricate architecture, significant processing defects were seen in the
printed meta-crystals, in particular notch-like defects due to lack of fusion
at the free surface of struts. Such defects raised stress concentration and
reduced the load-bearing area of struts, hence significantly weakening the
lattice struts. In addition, the as-printed condition was brittle due to the
presence of acicular alpha' martensites. The defects and the as-printed
brittleness led to the premature fracture of struts and compromised the
benefits of the crystal-inspired mesostructures. The study subsequently
conducted multiple measures to resolve this issue: increasing the strut
diameter to reduce the influence of the process defects, and annealing to
relieve internal stresses and regain ductility. The combination of the increase
in strut diameter and annealing successfully enabled the crystal-like
architected mesostructure to effectively improve the toughness of the
meta-crystals.
