Development and Application of Biomedical Titanium Alloys

Titanium alloys have been widely used in medical or dental applications due to their superior biocompatibility, high strength and corrosion-resistant as well as low modulus relative to other implantable metals. In order to meet the stringent medical regulations and the advancement of bioengineering, material scientists have therefore designed a series of advanced titanium alloys. This chapter aims at reviewing the development process of new medical grade titanium alloys in terms of composition design, biocompatibility and shape memory effect etc.

Currently, Ti-Mo, Ti-Nb, Ti-Ta and Ti-Zr-based β-titanium alloys have been widely studied. Compared with other titanium alloys, these alloys can achieve lower elastic modulus and higher strength. Since Ti-Nb alloy has low elastic modulus and good shape memory effect, it is the most promising medical titanium alloy to develop and utilize. Beta titanium alloys with the elements of Nb,Ta,Zr are being studied as the most important biomedical materials. This chapter presents the microstructure and mechanical properties of TiNbTaZr titanium alloy. The results show that the TiNbTaZr β titanium alloy with lower elastic modulus and non-toxic alloying elements has much more important application in biomedical field. The Ti35Nb2Ta3Zr β titanium alloy is studied and the alloy has the complex properties of lower elastic modulus, high strength, high elongation and excellent shape memory effect. Compared with direct rolling, cross rolling is beneficial to the isotropic of the microstructure and mechanical properties.

Novel non-toxic β-Ti alloys have been developed and used in the next generation of metallic implants to replace the present utilized near α-type CP-Ti and (α+β)-type Ti-6Al-4V alloy in orthopaedic applications. Nevertheless, the vast majority of these newly β-type Ti alloys are containing a substantial concentration of costly elements like Ta, Nb and Zr with high density and high melting points. Therefore, it is highly desirable to design new β-type biomedical Ti alloys composed of non-toxic, low-cost, abundant metals to lower fraction of high-cost elements. Very recently, some serials of Ti-xFe-yTa, Ti-Fe-xNb and Ti-Nb-xFe alloys have been developed by using the DV-Xα molecular orbital method. The mechanical properties of the alloys can be evaluated by studying the effects of Ta, Fe and Nb contents on phase transition, β phase stability and microstructure evolution, and compared with the currently applied biological materials to determine its suitability. In the currently designed alloys, Ti- 10Fe-10Ta, Ti-7Fe-11Nb and Ti-11Nb-9Fe display the excellent combination of mechanical properties, which make them more attractive compared with the conventionally used CP-Ti and Ti-6Al- 4V materials for biomedical applications. Compared to CP-Ti and Ti-6Al-4V alloys, a new type of Ti alloy with better performance for orthopaedic applications can be designed by appropriate alloy design.

Although titanium alloys have exhibited a combination range of excellent properties. However, various potential applications of titanium alloys are hampered by hard machinery and/or high cost due to material removal in conventional manufacturing processes. Emerging additive manufacturing techniques are providing a perfect opportunity for creating titanium and its composites, especially with complex dimensions, such as selective laser melting (SLM). So far, many types of titanium alloys components have been successfully manufactured by SLM, and they show comparable properties compared with their traditional counterparts. This chapter first briefly introduces the characteristics of the SLM process and parameters involved, then reviews some of the latest developments in the processing, microstructure and mechanical properties of Ti-based alloys and porous structures produced by SLM.

Electron beam melting (EBM) is a relatively new rapid, additive manufacturing technology which is capable of fabricating complex, multi-functional metal or alloy components directly from CAD models, selective melting of precursor powder beds. Compared with Ti-6Al-4V samples with same porosity level, the EBMproduced β-type Ti-24Nb-4Zr-8Sn (Ti2448) porous components exhibit a higher normalized fatigue strength owing to super-elastic property, greater plastic zone ahead of the fatigue crack tip and the crack deflection behavior. The super-elastic property can be improved by increasing porosity of porous samples as a result of increasing the tensile/compressive stress ratio of the porous structure. EBM-produced components exhibit more than twice the strength-to-modulus ratio of porous Ti-6Al-4V counterparts. The position of fatigue crack initiation is defined in strain curves based on the variation of the fatigue cyclic loops. The unique manufacturing process of EBM results in the generation of different sizes of grains, and the apparent fatigue crack deflection occurs at the grain boundaries in the columnar grain zone due to substantial misorientation between adjacent grains.

The excellent mechanical properties of NiTi alloys, such as unique shape memory effect, superelasticity, as well as good biocompatibility, excellent corrosion resistance, low elastic modulus and good ductility, make it an ideal choice for the biomedical, aerospace and intelligent materials. However, NiTi alloy with itself and dissimilar material connection problems NiTi alloy can only have a simple geometry, thus limiting the NiTi alloy more widely used. For this reason, resistance welding and laser welding are proposed by scholars, but these methods are not widely used due to the fact that welding makes the joint parts easy to produce brittle second phase and large area heat-affected zone, which also needs a large amount of solder. In this chapter, NiTi / NiTi-Nb alloy was fabricated by heating niobium and NiTi wires in a high-temperature argon atmosphere furnace to 1185 °C for 6min. The results show that there are 4 diffusion layers, including Nb foil, NiTi-Nb eutectic region, pre-eutectic NiTi region and NiTi matrix at the junction of NiTi wire and Nb foil. In addition, martensite phase was found inside the NiTi matrix. In the NiTi-Nb eutectic diffusion layer, niobium is inhomogeneously distributed, forming a rod-like Nb-rich phase, facetted Ti-rich phase and stripe-like or equiaxed NiTi-Nb eutectic structure. The experimental study on the microstructure and mechanical properties of interface interface between NiTi and Nb will help to understand the advantages of using Nb as the NiTi alloy connection material, provide theoretical support for preparation of NiTi lattice materials, and promote NiTi alloy more extensive application.

Possessing excellent bio-compatibility and mechanical performance, Ti-6Al- 4V (TC4) alloy is widely used as implant. Aiming at solving the problem of poor surface wear properties and improving bio-compatibility of TC4 alloy, friction stir processing(FSP) is applied to it filled with TiO2 powder in the groove to realize surface modification and build nano-sized composite biomedical material. Change in microstructure and its relationship with mechanical performance such as hardness will be discussed. A series of experiments in biology, including cytotoxicity test, cell culture, adhesion, proliferation and alkaline phosphatase activity is carried out to verify bio-compatibility of the material, compared with original TC4. The improved material is expected to provide a better environment for cells to grow.

It is well known that different manufacturing technologies of titanium alloys have a substantial impact on its performance; for example the Ti-6Al-4V manufactured by selective laser melting (SLM) exhibit comparable even better mechanical properties than the counterpart produced by traditional manufacturing methods. Yet, the understanding of the corrosion behavior of SLM-produced materials is unknown. This chapter reviews the recent progress of the corrosion behavior of SLM-produced Tibased alloys, such as Ti-6Al-4V and Ti-TiB composite. The corrosion behavior and corrosion mechanism are compared and discussed between the SLM-produced titanium alloys and their counterparts processed by traditional methods.