Achieving excellent strength-ductility synergy in an additively manufactured titanium alloy by forming a bi-heterogeneous structure

Additively manufactured (AM) titanium alloys often suffer strength-ductility tradeoff and need new strengthening mechanisms to overcome this issue. In recent years, heterostructures have been synthesized in a number of metallic materials many leading to enhanced strength-ductility synergy. Heterostructures were also reported in several AM-processed titanium alloys lately but the role of the heterostructures in mechanical property development and deformation mechanisms in these materials remains unclear. In this study, by taking advantage of microscale chemical inhomogeneity in a novel AM-processed titanium alloy, we achieve a unique heterostructure characterized by alternated layers of α-enriched fine grains and α-free coarse grains through controlled heat treatment, which leads to a tensile 0.2 % yield strength of 1033 MPa and a large elongation of 19 %. Loading-unloading-reloading test demonstrates that back stress strengthening contributes significantly to the strength of the material. In-situ tensile test and microstructural characterization reveal that the fine-grained zones and the interfaces between fine- and coarse-grained zones experience more pronounced strain accumulation than the interior of large grains during tensile deformation. Strain gradients form from the zone boundaries into the interior of the large grains. These indicate that back-stress strengthening has been indeed operative during deformation. Moreover, increased slip bands and shear bands form within grains with increased strain and dislocations were found to cut through both the β matrix and α precipitates, which accounts for the excellent ductility. The present work paves the way for enhancing strength-ductility synergy in AM-processed titanium alloys by creating heterostructures.