In situ fabrication of a titanium-niobium alloy with tailored microstructures, enhanced mechanical properties and biocompatibility by using selective laser melting

A titanium-niobium (Ti-Nb) alloy with tailored microstructures, enhanced mechanical properties and biocompatibility was in situ fabricated by selective laser melting (SLM) using a blended powder with 25 wt% Nb content. The effect of laser energy density from 70 J/mm(3) to 110 J/mm(3) on the phase transformation, microstructure, and mechanical properties of the SLM-printed Ti-25Nb alloy was investigated. The results indicate that the energy density of 110 J/mm(3) results in the highest relative density and homogeneous element distributions in the alloy. The alpha' and beta phases with an orientation relationship of [023] beta//[-12-16] alpha' were identified through X-ray diffraction and transmission electron microscopy, and their proportions are crucially determined by the laser energy density. With an increase in the energy density, the microstructure of the Ti-25Nb alloy varies from acicular-shaped grains to coarsened lath-shaped grains and to lath-shaped grain + cellular-shaped subgrains, due to the decrease in cooling rate and the rise in temperature gradient. The yield strength and microhardness of the printed Ti-25Nb alloy decrease with the increase in energy density from 70 J/mm(3) to 100 J/mm(3), and then increase to the highest values of 645 MPa and 264 HV at 110 J/mm(3), respectively. This variation of mechanical properties is dependent on both the coarsening of alpha' phase and the formation of beta (Ti, Nb) solid solution. Besides, the SLM-printed Ti-25Nb alloy exhibits both the excellent in vitro apatite-forming capability and better cell spread and proliferation compared to pure Ti.