Stiffness and hardness gradients obtained by laser surface treatment of aged β-Ti alloys: The role of ω phase

Abstract

Functional gradient materials are interesting for optimizing the properties of engineering parts. Laser surface treatment makes it possible to obtain a stiffness gradient on titanium parts; such graded material is attractive due to the potential for orthopedic implant performance improvement and other applications as well. This work used laser surface treatment without fusion on two aged titanium alloys: Ti8Mo and Ti30Nb1Fe. This processing route aimed to fully dissolve the α-phase precipitates in a surface layer and, consequently, reduce the elastic modulus, producing a stiffness gradient from the surface to the inner material. Firstly, single tracks were obtained for different laser powers, kept constant the other parameters, aiming to determine the optimum condition to avoid surface fusion and produce the thickest-possible modified layer. Then, multi-track overlapping produced a modified continuous layer. The aged titanium alloys were characterized by optical and scanning electron microscopy and X-ray diffraction. The single tracks were characterized by stereoscopic and optical microscopy. Transmission electron microscopy and instrumented indentation completed the characterization of the laser-modified layer. Laser surface treatment produced a less-rigid surface layer in the Ti8Mo alloy due to the obtainment of β and α″ titanium phases with lower elastic modulus. The surface treatment did not change the material's hardness in this case. On the other hand, the precipitation of the ω phase during surface cooling of the Ti-30Nb-1Fe alloy prevented the elastic modulus from decreasing and increased the modified-layer hardness. Hence, the absence of ω phase precipitation is a condition to obtain a less-rigid surface layer by laser surface treatment on aged titanium alloys.