Microstructure and enhanced strength-ductility of TiNbCu alloys produced by laser powder bed fusion

The beta-type Ti35NbxCu alloys (x = 0, 1, 3, 5, and 7 wt%) were produced by laser powder bed fusion (LPBF) and the influence of Cu content on the microstructure and mechanical properties was studied. The results indicate that when the content of Cu is less than 3 wt%, Cu can diffuse completely into the Ti matrix to form solid solution without element segregation. Moreover, the elastic modulus of Ti35Nb1Cu alloy (49.1 +/- 1.6 GPa) is reduced by 33 % compared to that of Ti35Nb alloy (73.2 +/- 1.4 GPa). When the content of Cu exceeds 3 wt%, the eutectoid reaction is activated during the cyclic heating and rapid cooling of LPBF, and the partially supersaturated beta phase is transformed into nano-scale alpha and Ti2Cu phases (beta ->alpha+Ti2Cu). Nano-scale Ti2Cu particles precipitated along beta grain boundary results in a pinning effect, which inhibits the growth of beta grain, hinders dislocation movement, and improves the strength of the alloy. However, when the content of Cu is 7 wt%, the dislocation density (4.179 x 1015 m  2) and the ratio of brittle phase Ti2Cu (8.5 %) are too high, and the stress concentration caused by the initiation of inherent micro-cracks are the main reason for the tensile brittle fracture of Ti35Nb7Cu alloy. As such, the Ti35Nb5Cu alloy has the highest tensile strength of 867.3 +/- 24 MPa and the elongation reaches 11.7 +/- 0.5 %. Therefore, solution strengthening of Cu, fine grain strengthening of beta, dislocation strengthening, and precipitation strengthening of Ti2Cu are the main reasons for the excellent comprehensive mechanical properties of Ti35Nb5Cu alloy.