Novel beta titanium alloys with trace addition of the rare earth elements scandium and yttrium for biomedical applications

<p>The rapid growth in recent decades of aged populations around the world has led to significantly increased demands for metallic biomaterials for biomedical applications. Although conventional metallic biomaterials including stainless steels, cobalt-chromium-based alloys and titanium (Ti)-based alloys can meet most immediate needs, there are a few concerns over their long-term success due to the extended lifespan and more diverse pursuit of leisure and quality-of-life activities of the aged populations. Thus, there are higher requirements and expectations on the service quality of the metallic biomaterials.</p> <p>Currently, the two major concerns for conventional metallic biomaterials are the insufficient strength of the implant, and the stress shielding caused by a mismatch in the Young's modulus (E) between the implant and natural bone. To resolve the latter problem, beta (ß)-type Ti alloys are considered among the most promising metallic biomaterials to replace conventional metallic biomaterials, owing to their superior biocompatibility and relatively low E. Unfortunately, the insufficient mechanical properties of biocompatible ß Ti alloys, including low strength and wear resistance, are still significant enough factors that can reduce the service life and quality of ß Ti alloys.</p> <p>To overcome this issue, this thesis aims to develop the advanced ß Ti alloys with new compositions that can inherently provide higher mechanical performance for biomedical applications. Trace addition of rare earth elements (REEs) were additionally added into the designed ß Ti alloys in an attempt to further improve the mechanical properties without affecting the biocompatibility. The thesis first presents a brief introduction to the development of biomedical Ti alloys, and a summary of the effects of REEs on the microstructure and mechanical properties of Ti alloys, along with the biocompatibility of individual REEs. It then delves into the effects of some commonly used alloying elements on tensile strength, E and microstructure of Ti alloys for biomedical applications. It was found that niobium (Nb), tantalum (Ta), molybdenum (Mo), tin (Sn) and zirconium (Zr) are the optimum additions for biomedical Ti alloys. Moreover, interstitial elements oxygen (O) and nitrogen (N) show significant effects on the microstructure and phase transformations in ß Ti alloys, and so can be considered for the development of high-strength Ti alloys; conversely, neither carbon (C) nor hydrogen (H) brings obvious benefit to Ti alloys, and so their additions need to be limited and/or avoided. Based on these, a new series of ß Ti-Nb-Zr-Mo alloys were then designed using d-electron alloy design method and fabricated by cold-crucible levitation melting. Among all designed ß Ti-Nb-Zr-Mo alloys, the Ti-24Nb-38Zr-2Mo alloy exhibited a high tensile strength (682 MPa), tensile yield strength (675 MPa), elongation (13 %), compressive yield strength (691 MPa), microhardness (258 HV), and the lowest E (68 GPa), significantly lower than that of Grade-4 biomedical CP-Ti (~104 GPa) and Ti-6Al-4V alloy (~110 GPa). Thus, Ti-24Nb-38Zr-2Mo alloy was selected as the base model, and trace addition (0.10 wt.%) of REEs scandium (Sc) and yttrium (Y) were separately added into the base model in an attempt to investigate their effects on the microstructure, mechanical properties and biocompatibility. It was found that Sc showed solid solution strengthening that improved microhardness, tensile strength, tensile yield strength of Ti-24Nb-38Zr-2Mo alloy without any change to the high ductility, low E and high biocompatibility. Y precipitated as heterogeneously distributed, large, Y-rich oxides in Ti-24Nb-38Zr-2Mo alloy, showing minor effects on the mechanical performance and the high biocompatibility. Furthermore, the impact of trace addition (0.10 wt.%) of Sc and Y on nanohardness (H), reduced Young's modulus (Er) and nanowear properties of the Ti-24Nb-38Zr-2Mo alloy for biomedical applications were investigated. It was found that both Sc and Y in Ti-24Nb-38Zr-2Mo alloy increased H and reduced Er; and Sc also improved wear resistance of Ti-24Nb-38Zr-2Mo alloy by inhibiting wear formation and increasing plastic shear resistance. Sc was thus highlighted as the suitable alloying element for Ti-24Nb-38Zr-2Mo alloy; Y was less recommended.</p> <p>Despite trace addition of Sc and Y in Ti-24Nb-38Zr-2Mo alloy showing promising value for biomedical applications, the investigated compositions cannot stand alone as representative for all REE alloying behavior; different results may well be achieved if the base model or selection of REEs are changed. REE concentration and REE particle behavior were considered the important factors that influence the effects of REEs on refining microstructure and improving mechanical properties of Ti alloys.</p>