Low-cost and biocompatible nickel-free superelastic titanium alloys

<p>The superior mechanical properties, biocompatibility and corrosion resistance of titanium and its alloys have led to their successful integration with the biomedical and dental industries. However, titanium alloys that are currently used for bone and dental implants or stents contain toxic elements which can lead to serious biological complications. For example, the vanadium in Ti-6Al-4V (for bone and dental implants) and the excessive nickel in superelastic NiTi (used as self-expandable stents) are carcinogenic, genotoxic, mutagenic, cytotoxic and allergenic. These concerns have re-evoked a recent global attempt for the development of novel biocompatible titanium alloys that can offer both low elastic modulus and superelasticity. Superelasticity is necessary for a broad range of applications in the field of biomedical and dental implants and devices. For example, the excellent superelasticity of the NiTi alloy has resulted in its extensive use in a variety of biomedical and dental applications. Following a thorough review of the latest developments in biomedical Ni-free superelastic titanium alloys, this thesis first discusses superelastic titanium alloy design considerations and then introduces two novel classes of low-cost Ni-free superelastic titanium alloys. In addition to superior superelasticity, biocompatibility and corrosion resistance, these novel alloys were designed to overcome some important drawbacks of the current Ni-free superelastic titanium alloys such as the high material cost, demanding processing routes and homogenisation-related issues.</p> <p>The two classes of novel alloys introduced in this thesis were fabricated via vacuum arc melting followed by a homogenization heat treatment and quenching. Their as-quenched and deformed microstructures were studied using scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). Their mechanical properties and superelasticity were quantified using instrumented nano-indentation and cyclic compression tests, respectively. Their biocompatibility was evaluated via in-vitro mammalian cell-density, cell attachment and cell viability assessments. In addition, their corrosion response in a simulated body fluid at 37 °C and a pH of 7.4 (human body's environment) was investigated via open circuit potential and potentiodynamic polarization analyses. Furthermore, the relationship between the crystallographic orientation of the Beta crystals of the novel alloys in as-quenched state and their recoverable strain was investigated. The major achievements are summarised below:</p> <p>The Bo-Md map derived from the d-electron theory is well developed for designing Beta-Ti alloys with low elastic modulus but much less established for designing superelastic titanium alloys. Important modifications were made to the existing Bo-Md map on the basis of a detailed analysis of the literature data, leading to its greater accuracy. In addition, the consistency of the existing molybdenum equivalence (Mo-Eq) equations and the electron to atom (e/a) ratio were both assessed for the design of Ni-free superelastic titanium alloys. The effort resulted in an alloy design approach specifically for Ni-free superelastic Beta-Ti alloys, comprising of the coherent use of the modified Bo-Md map, modified Mo-Eq equations and the e/a ratio.</p> <p>The Ti-19Zr-10Nb-1Fe (at.%) alloy is an existing superelastic alloy which exhibits a recoverable strain of 4.7%, ultimate tensile strength of 723 MPa and low elastic modulus of 59 GPa after undergoing homogenisation, severe rolling and recrystallization. Using the proposed alloy design approach, it is demonstrated that an excellent combination of a total recoverable strain of 3.5%, yield strength of 1150 MPa and an elastic modulus as low as 60 GPa can be obtained by reducing the Ti-19Zr-10Nb-1Fe alloy to Ti-10Zr-5Nb-(0-2.5)Fe, and simplifying the thermomechanical process to only homogenisation. In addition to the significant reduction in the material cost, density and manufacturing difficulties (due to higher cost, density and melting points of Nb and Zr compared with Fe), the overall reduction in alloying elements increases the transformation strain leading to a potentially greater superelasticity. In addition, the atomic-scale deformation mechanisms of the Ti-10Zr-5Nb-(1-2.5)Fe alloys were thoroughly investigated via TEM. For the first time, a preferential formation of dislocations within the stress-induced a' was discovered which is believed to play a major role in deterioration of superelasticity in Beta-Ti alloys. The experimental findings were used to improve the accuracy and reliability of the Bo-Md map for designing superelastic and shape memory alloys.</p> <p>Recent studies have revealed that Sn can enhance the superelasticity of Beta-Ti alloys due to its effect on the a' lattice parameters and decrease their elastic modulus. Additionally, Sn has greater potential for suppressing the a' martensite and athermal Omega phases than Zr. This suggests that replacing the Zr content of the newly developed Ti-10Zr-5Nb-2.5Fe alloy with Sn has the potential to allow for further reduction of the Nb content, which consequently is expected to increase the transformation strain and ultimately the superelastic recoverable strain. Hence, novel Ti-2.5Nb-2.5Fe-(0-6)Sn alloys were introduced which, as expected, exhibited superior superelastic recoverable strain up to 5%. Additionally, experimental results indicated their greater elastic stored energy, biocompatibility and corrosion resistance compared with those of NiTi. Furthermore, the deformation mechanisms, in particular the stress-induced martensitic transformation and the associated twinning of the Ti-2.5Nb-2.5Fe-(3-6)Sn alloys were investigated closely via electron backscatter diffraction (EBSD). Moreover, the effect of Sn content on the formation of the recently discovered O' metastable phase was also investigated.</p> <p>The strain accommodated by the Beta to a' reversible transformation is dependent on the orientation relationship (OR) as well as the difference in the unit-cell volume of the two phases. The OR between these phases suggests a certain degree of anisotropy in the transformation strain. Hence, the relationship between the orientation of single Beta crystals and the recoverable strain of the most promising Ti-Zr-Nb-Fe and Ti-Nb-Fe-Sn alloys was also investigated via novel experimental techniques. The Ti-Nb-Fe-Sn alloys exhibited a greater degree of crystallographic anisotropy in their superelasticity compared with their Ti-Zr-Nb-Fe counterparts. Moreover, increasing the Sn content of the Ti-Nb-Fe-Sn alloys weakens this anisotropy. Consequently, the Ti-6Sn-2.5Nb-2.5Fe alloy exhibited an exceptional recoverable strain of 9.7% along the <001>Beta direction, indicating its potential for exhibiting comparable superelasticity to that of the NiTi upon the introduction of an appropriate crystallographic texture. To the best of the author's knowledge, the recoverable strain of 9.7% exhibited by the novel alloys is the largest ever reported amongst all Ni-free superelastic titanium alloys, to date.</p>