Development of Ti-Mo-Fe alloys combining different plastic deformation mechanisms for improved strength-ductility trade-off and high work hardening rate

Titanium-based biomaterials are the gold standard for orthopedic implants; however, they are not generally suitable for the manufacture of intravascular stents. Their low strength-ductility trade-off and low work hardening rate are their main limitations. However, Ni-free alloys are desirable for such application in order to avoid allergic reactions caused by the high Ni-content materials currently applied. Therefore, in this study, three alloys of the Ti-Mo-Fe system (Ti-8Mo-2Fe, Ti-9Mo-1Fe and Ti-10.5Mo-1Fe) were designed to present high strength-ductility compromise and high work hardening rate. Their microstructures, me-chanical properties and plastic deformation mechanism were investigated. Athermal omega precipitates were observed in the beta matrix of all solution-treated alloys. In the solution-treated beta matrix of the Ti-9Mo-1Fe alloy, additional nanometer-sized alpha" particles were detected by transmission electron microscopy (TEM). Although the combined TWIP/TRIP effects were expected by the design method on the Ti-8Mo-2Fe and Ti-9Mo-1Fe alloys, no TRIP effect was actually observed. In fact, stress-induced martensitic (SIM) transfor-mation occurred mainly at the {332} < 113 > twins/matrix interfaces for all the strained microstructures and acted as a localized stress-relaxation mechanism, delaying the fracture. Based on the electron backscatter diffraction (EBSD) analyses, in the Ti-8Mo-2Fe and Ti-10.5Mo-1Fe alloys, the formation of a dense network of {332} < 113 > twins was responsible for their high and steady work hardening rates (1370 and 1120 MPa) and large uniform elongations (22% and 34%). The absence of SIM alpha" as the primary mechanism of plastic deformation and solid solution hardening of Fe resulted in their high strengths (yield strength of 772 and 523 MPa). In Ti-9Mo-1Fe, the formation of mechanical twinning was hindered, resulting in limited strain -hardening capability and low uniform elongation (6%). The nanometer-sized alpha" particles in its beta matrix along with the athermal omega precipitates are thought to impair the mechanical twinning and the ductility of this alloy. (c) 2022 Elsevier B.V. All rights reserved.