Design and development of low elastic modulus Ti-Nb-Zr alloys for biomedical applications

The demand for implants has been increasing globally due to the rising population of the older people (aged ≥80 years), bone diseases, e.g., bone cancers, congenital disabilities, birth defects, revision needs, and accidents. It is essential to select both biologically and mechanically compatible implant materials for such applications. The commonly used implant materials today are austenitic stainless-steel alloys, Co–Cr alloys, Ti, Ta, and their alloys. Recently zirconium (Zr) alloys for biomedical applications are receiving increasing attention due to their two unique properties: 1) the formation of an intrinsic bonelike apatite layer on their surfaces in body environments, and 2) better compatibility with magnetic resonance imaging (MRI) diagnostics due to their intrinsically low magnetic susceptibility, as well as their overall excellent biocompatibility, mechanical properties, and bio-corrosion resistance. In particular, since both of the MRI quality and speed depend on magnetic field strength, there is a compelling drive for the use of high magnetic field strength (>3 Tesla) MRI systems. This requires the availability of implant alloys that can offer much lower susceptibility than the current Ti implant alloys. In that regard, Zr-based alloys offer more promise than Ti-based alloys.<br><br>This thesis first presents a comprehensive review of the characteristics of commercially pure (CP) Zr and Zr-based alloys as potential orthopaedic and dental implant materials. These include their 1) phase transformations; 2) unique properties including corrosion resistance, biocompatibility, magnetic susceptibility, shape memory effect, and super-elasticity; 3) mechanical properties; 4) current orthopaedic and dental applications, and; 5) the d-electron theory for Zr alloy design and novel Zr-alloys, and 6) future directions for extending the use of Zr-alloys as orthopaedic and dental implants are discussed. Then following a detailed analysis of the design methods for low elastic modulus Ti alloys, the d-electron theory and the ⁄ ratio approach are used together to design nine strong, ductile, and low elastic modulus Ti-Nb-Zr alloys. Among them, five are Zr-based Ti-Nb-Zr alloys, and four are Tibased Ti-Nb-Zr alloys. To assess Ti-Nb-Zr alloys, it is important to understand the influence of Zr on the β-phase stability of Ti-Nb-Zr alloys. The concept of the Mo equivalence (MoEq), proposed by Molchanova (Phase Diagrams of Titanium Alloys, 1965), has been commonly used as a general guideline to gauge the stability of a β-Ti alloy. A critical literature review has shown that all four existing Mo-Eq expressions deviate substantially from experimental observations and the well-established d-electron theory in predicting the β-phase stability of Ti-Nb-Zr alloys. The reasons are that existing Mo-Eq expressions either completely neglect or significantly overestimate the β-stabilizing effect of Zr. In this thesis, a new Mo-Eq expression, i.e., (Mo-Eq) Ti-Nb-Zr = 0.238Nb (wt.%) + 0.11Zr (wt.%) + 0.97, has been defined for Ti-Nb-Zr alloys in order to properly address the β-stabilizing effect of Zr. This new Mo-Eq expression shows good consistency with both experimental observations and the d-electron theory in predicting the β-phase stability of various Ti-Nb-Zr alloys. With necessary modifications, the approach developed is expected to be also applicable to the assessment of the β-phase stability in other Zr-containing Ti alloys.<br><br>Three different methods: tension, compression, and ultrasonic tests, are used to determine the elastic modulus of the five Zr-Ti-Nb alloys (Zr-45Ti-15Nb, Zr-33Ti-15Nb, Zr-28Ti-15Nb, Zr-35Ti-10Nb, and Zr-30Ti-20Nb, in at.%) alloys. The as-cast tensile, compressive and ultrasonic elastic moduli of these alloys range from 58-79GPa, 45-57GPa and 60-95GPa respectively. The two Zr-Ti-Nb alloys (Zr-based Ti-6Nb-53Zr and Ti-18Nb-51Zr) from the literature, which reportedly have the lowest elastic moduli, are prepared and tested for comparison as a point of reference. The dependence of elastic moduli on the test methods, phase constitutes as well as and ⁄ ratio is systematically investigated. The reassessed Mo-Eq. values change linearly with the ⁄ ratio for the above seven alloys. The current work also indicates that a small amount of the ω-phase along with β and α″-phases, and the condition of ⁄ ≈ 4.15, can lead to low elastic modulus for Zr-Ti-Nb alloys. Therefore, a modified relationship between the phases and the elastic modulus has been suggested, which is: Eα″ < 40 GPa < Eβ ≈ 60-90 GPa < Eα ≈ 100 GPa < Eω ≈ 130-220 GPa. This study identifies that as-cast Zr-28Ti-15Nb and Zr-33Ti-15Nb alloys can offer low elastic modulus (~60 GPa, tensile and ultrasonic), excellent tensile ductility (~16%), uniform plastic strain (greater than 10%) and sufficiently high tensile yield strength (~650 MPa) for implant applications.<br><br>While designing the Zr-Ti-Nb alloys, this thesis author realized that Ti-Nb-Zr alloys could also offer low elastic modulus. As a result, four new Ti-Nb-Zr alloys (Ti-26Zr-10Nb, Ti25Zr-15Nb, Ti-22Zr-15Nb, and Ti-21Zr-20Nb) are designed by the d-electron theory and ⁄ ratio. The as-cast tensile, compressive and ultrasonic elastic moduli of these alloys are in the range of 58-71GPa, 34-60GPa and 52-83GPa respectively. The effects of alloying elements on microstructures, mechanical properties i.e. tensile strength, yield strength, compressive yield strength, elastic modulus, elastic energy, and microhardness of these newly designed alloys have been investigated. Ti-Nb-Zr alloys also show the linear relationship between MoEq values and ⁄ ratio. The results also confirm that a small amount of ω-phase is not clearly detrimental in reducing the elastic modulus along with β and α″-phase. Therefore, the results from Ti-Nb-Zr alloys strongly agree with the above proposed relationship sequence between the phases and the elastic modulus for Zr-Ti-Nb alloys.