Circumventing the strength–ductility trade-off of β-type titanium alloys by defect engineering during laser powder bed fusion

In this study, we propose a novel defect engineering strategy for circumventing the strength–ductility trade-off of a β-type Ti–35Nb–7Zr–5Ta alloy and discuss its underlying mechanism. This strategy is based on the simultaneous introduction of dislocations and twins into the alloy structure through the thermal stress generated during additive manufacturing via laser powder bed fusion (LPBF). As a result, the LPBF-fabricated alloy contains high-density dislocations and two different types of nanosized {112}〈111〉 mechanical twins: unique zigzag-shaped and conventional lamellar ones. Interestingly, the produced alloy exhibits a high tensile yield strength of 816 MPa and large elongation of 16.5%, which significantly exceed the published values for other representative β-type titanium alloys fabricated by various material processing methods. Such a high yield strength is predominantly attributed to the introduced defects, and the relative contributions of dislocation strengthening and twin strengthening are equal to 58.0% and 26.1%, respectively. Meanwhile, the large elongation results from the inhibition effect of the produced twins on the dislocation motion as well as from their dislocation pass-through and growth. The obtained results can provide guidelines for the microstructural design and fabrication of novel β-type titanium alloys with excellent mechanical properties by defect engineering during additive manufacturing.