Non-layer-wise fracture and deformation mechanism in beta titanium cubic lattice structure manufactured by selective laser melting

Abstract

The layer-wise fracture is a common fracture in porous metal structures, where the whole layer with stress concentration fractures firstly, and then the adjacent layers fracture sequentially. The brittle layer-wise fracture has always been a major obstacle that restricts the development of metallic porous structures, which may induce catastrophic failure under occasional overloading. This work reports that the non-layer-wise fracture was achieved in a beta-type Ti–25Nb–3Zr–3Mo–2Sn (TLM) titanium alloy with 50% porosity cubic lattice structure manufactured by selective laser melting (SLM), mainly due to the stress-induced progressive martensitic transformation (β→α″) supplemented by deformation twinning, resulting in uniform deformation without cracks in the struts even under a large compression strain of ~50%. The in situ synchrotron high-energy X-ray diffraction (HE-XRD) was utilized to study the dynamic deformation behavior of a strut, revealing that the martensitic transformation initiates at the early stage of the compression with a strain of 1.47%, followed by progressive martensitic transformation upon further straining, which probably contributes to the “double-yielding” platform. TEM microstructural observations of the {112}<111>β twins in the strut implies that deformation twinning was activated as a supplementary deformation mechanism for the accommodation of large strain. The findings in this work provide an alternative strategy of introducing non-layer-wise fracture in porous metallic materials through the coupled effect of phase transformation and twinning.