Low Temperature Rolling And Duplex Aging Behavior Of A New Bate Type Titanium Alloy For Biomedical Applications

Titanium and titanium alloys with high strength, low elastic modulus, excellent biocompatibility and anticorrosion property, gradually replace the traditional biomedical metal materials as the first choice for medicalimplant materials. Such as artificial hip joints, cardiovascular stents, and dental implants, etc. At present, biomedical titanium alloy materials have developed to the third generation of beta medical titanium alloy. Compared with the medical Ti-6Al-4V alloy which is widely used in clinical application, β titanium alloy with lower elastic modulus, superior biocompatibility and better corrosion resistance has attracted more and more attentions and become a hot topic in present research.In this paper, the duplex aging precipitation and the low temperature rolling deformation behaviors of new biomedical beta titanium alloy Ti-25Nb-10Ta-1Zr-0.2Fe(TNTZF,wt.%) are studied by optical microscope(OM), scanning electron microscope(SEM), X-ray diffraction(XRD), transmission electron microscopy(TEM), combining with the microhardness and mechanical test. The main results are summarized as follows.After duplex aging, the hardness curve of TNTZF alloy mostly appears above the single ageing and shows obvious bimodal characteristic. The aging time and hardness value of the alloy in the peaks under four groups of duplex aging(300°C-20min/1h+550°C,400°C-20min/1h+550°C) are: 4h(295HV),2h(307HV);4h(300HV),4h(332HV), respectively. At the process of single ageing at 550°C, martensite α" is precipitated from β matrix in short time, then α" changes into α phase. The precipitation sequence is as follows: β�'β+α"�'β+α. At the process of duplex aging, ω phase and martensite α" are precipitated in the first stage aging at 300°C/400°C, and they change into α phase in the second stage aging at 550°C. The precipitation sequence is as follows: β�'β+ω�'β+α"�'β+α. After duplex aging, in the alloy’s intracrystalline part from lamella α phase composed by ladder-shaped α phase which has greater contribution to hardness. Within a certain aging time, the alloy elastic modulus and elongation increase at 550°C single aging; and the alloy elastic modulus rises after decreasing at first, while elongation decreases after increasing at first at 400°C/1h+550°C duplex aging. Compared with single aging, duplex aging has more stable tensile strength and plasticity which benefit for controlling on the property of the alloy. The tensile fracture morphology appears to be dimple pattern with feature of plastic fracture. The plastic property of alloy has improved in a certain degree after duplex aging. The optimal process parameter is 400°C/1h+550°C/2h, under this condition, the elastic modulus、tensile strength and elongation are: 65GPa、848MPa、14.3% respectively. This good comprehensive mechanical property meet implant materials’ requirements for biomechanical performance.At the process of low temperature(-20°C)rolling, the stress-induced martensite α " transformed to lath-shaped martensite α" in the alloy TNTZF. In 60% low temperature rolling deformation, the content of martensite α " is in the highest. With higher deformation, the inner alloy has produced inhomogeneous plastic deformation and dislocation cells with high density, and the crystalline grain is measured by nanostructuring. The main deformation mechanism for alloy are stress-induced martensite α" transformation, dislocation glide and nanometer structure. Compared with in the solution treatment state, the microhardness of alloy has better effect after low temperature rolling, and shows some strain hardening effects. With the increase of deformation, the strength increases while the elastic modulus and elongation decreases. Through the tensile fracture morphology, the ways of the alloy fracture has dimple pattern with feature of plastic fracture. Compared with solution state, the surfaces of samples are relatively flat. dimple depth were shallow and small, the plasticity decreased obviously after 40%, 60% and 80% deformation. With the increasing of deformation, the size and depth of dimple are decreasing, which corresponding the decrease trend of the alloy alongation. Under the condition of 60% in the low temperature rolling, the alloy showed good comprehensive mechanical properties, yield strength of 896 MPa, a tensile strength of 1146 MPa, a the lower elastic modulus of 67 GPa, a moderate elongation(5%) and the highest strength to modulus ratio 17.1×10-3.