Microstructural adjustments and mechanical properties of a cold-rolled biomedical near β−Ti alloy sheet

In this study, microstructural adjustments and mechanical properties of a cold-rolled near β-type alloy Ti−25Nb−3Zr−3Mo−2Sn (wt%) sheet were investigated. Microstructures and phase transformation products strongly depended on aging temperatures. Solution treatments within single β-phase field removed the stress-induced α″ martensites and produced a few new lath-shaped ones, but metastable β phase still dominated. This is exactly the reason why current alloy exhibits the lowest modulus (54 GPa) and best elongation to fracture (39 %), but the worst yield strength of only 340 MPa, at solution-treated state. A fairly large number of ellipsoidal ω phase nanoparticles precipitated throughout parent β phase during aging at 380 °C. These ω nanoparticles possess remarkable strengthening effect, but deteriorate ductility seriously. A novel post-aging process was proposed to remove brittle ω phase. By contrast, aging at 450 °C resulted in sufficient precipitation of fine needle-like α phase. This brought about the best combination of high yield strength (770 MPa) and moderate elastic modulus (75 GPa) and good elongation (15 %) for biomedical implants.