Microstructure evolution and mechanical properties of a Ti–35Nb–3Zr–2Ta biomedical alloy processed by equal channel angular pressing (ECAP)

• The grains of Ti–Nb–Zr–Ta alloy are significantly refined by the ECAP method.
• Intersectant networks of shear bands are formed due to large plastic strains.
• Severe plastic strain led to stress-induced martensite transformation.
• The unique herringbone morphology of twinned martensitic variants is explained.
• The Ti–Nb–Zr–Ta alloy pressed at 500 °C via the 4th pass shows excellent superelasticity.

In this paper, an equal channel angular pressing method is employed to refine grains and enhance mechanical properties of a new β Ti–35Nb–3Zr–2Ta biomedical alloy. After the 4th pass, the ultrafine equiaxed grains of approximately 300 nm and 600 nm are obtained at pressing temperatures of 500 and 600 °C respectively. The SEM images of billets pressed at 500 °C reveal the evolution of shear bands and finally at the 4th pass intersectant networks of shear bands, involving initial band propagation and new band broadening, are formed with the purpose of accommodating large plastic strain. Furthermore, a unique herringbone microstructure of twinned martensitic variants is observed in TEM images. The results of microhardness measurements and uniaxial tensile tests show a significant improvement in microhardness and tensile strength from 534 MPa to 765 MPa, while keeping a good level of ductility (~ 16%) and low elastic modulus (~ 59 GPa). The maximum superelastic strain of 1.4% and maximum recovered strain of 2.7% are obtained in the billets pressed at 500 °C via the 4th pass, which exhibits an excellent superelastic behavior. Meanwhile, the effects of different accumulative deformations and pressing temperatures on superelasticity of the ECAP-processed alloys are investigated.