Study On The Microstructure And Properties Of Ti-18Nb-xZr(O) Biomedical Alloys

Ti-Ni shape memory alloys are widely used in dentistry, interventional therapy and other medical field due to the excellent superelasticity. The Ni-hypersensitivity leads to research and development of Ni-free titanium shape memory alloys which aims to find substitutes for Ti-Ni shape memory alloys in making some medical devices, such as orthodontic wire and stent. But the recovery strain of Ni-free titanium shape memory alloys is comparatively low. Improving the superelasticity of Ni-free titanium shape memory alloys arouses great interest. In this work, the effect of heat treatment on the phase transformation, microstructure and mechanical behavior of Ti-18Nb-10Zr (at.%), Ti-18Nb-11Zr (at.%) and Ti-18Nb-9Zr-0.5O (at.%) alloys was investigated. The mechanical behavior was studied by tensile tests at room temperature. X-ray diffraction (XRD), electron backscatter diffraction (EBSD), optical microscope (OM) and transmission electron microscope (TEM) were applied to investigate the phase transformation and microstructure. The main results are as follows.Ti-18Nb-10Zr, Ti-18Nb-11Zr and Ti-18Nb-9Zr-0.5O alloys consist of β phase after solution treatment at800℃for0.5h. Solution treated Ti-18Nb-10Zr, Ti-18Nb-11Zr and Ti-18Nb-9Zr-0.5O alloys exhibit partial superelasticity with recovery strain being2.65%,2.36%and2.10%respectively.A large number of isothermal ω phases occur in Ti-18Nb-10Zr alloy solution treated at800℃for0.5h and then aged at350℃for2h, which results in significant increase in the content of β-stabilizing elements in β phase and drastic decrease in martensite start temperature to far below room temperature. The stress-strain curve exhibits typical elastic recovery.Annealing in a certain temperature range for10min is effective for improving the superelasticity of Ti-18Nb-10Zr, Ti-18Nb-11Zr and Ti-18Nb-9Zr-0.5O alloys on account that the critical stress for slip deformation increases due to the fine subgrains and grains. The superelasticity of Ti-18Nb-10Zr alloy annealed at550℃,600℃and650℃is improved markedly with recovery strain being2.87%,2.89%and2.86%respectively. Ti-18Nb-11Zr alloy annealed at55O℃and600℃also exhibits better superelasticity with recovery strain being2.77%and2.78%respectively. Superelasticity improvement is also observed in Ti-18Nb-9Zr-0.5O alloy annealed at600℃and the recovery strain is2.51%.The Ti-18Nb-10Zr alloy annealed at650℃for10min consists of β phase and almost recrystallizes fully with average grain size being10m. Loading and unloading tensile tests with a constant maximum applied strain of3%were carried out until the10th cycle. The critical stress for inducing martensite and recovery strain decrease, and residual strain increases with increasing number of the cycles. After the10th cycle, the recovery strain is2.77%.