Variant selection of α precipitation in a beta titanium alloy during selective laser melting and its influence on mechanical properties

A β titanium alloy was fabricated horizontally and vertically at two different energy densities by selective laser melting. The microstructures of the as-fabricated samples were examined using a range of characterization techniques and the properties evaluated by tensile testing. It was found that the samples that were horizontally built at the lower energy density were dominated by both β and athermal ω phases while the vertically built samples consist of not only β and ω but also nano-sized α laths. Increased energy density led to a significant increase of α in horizontally built samples which tended to constitute a grid-like structure in the β matrix. All the α-bearing samples showed preferential α variant selection and pronounced α micro-texture. The α-free samples all showed high 0.2% yield strengths (922-934 MPa), high ultimate tensile strengths (935-939 MPa) and good ductility (10.2%–13.8% elongation) whereas the α-bearing samples all exhibited poor ductility (elongation < 1.2%). A significant anisotropy in plasticity was observed in the samples made with the lower energy density. Transmission electron microscopy examination revealed that the α-free samples experienced global plastic deformation via both long-range dislocation slipping and twinning whereas the α-bearing samples underwent localised plastic deformation where slipping was either interrupted by the nano-sized α laths or contained within the α-grid demarcated β matrix, which may account for their poor ductility.