β-type Ti Alloys for Biomedical Applications

Microstructural Characteristics of β-type Ti Alloys

The properties of titanium-based materials are strongly influenced by their allotropic transformation. Ti has two major phase structures (α and β) depending on the temperature under constant pressure. Pure Ti exists in α phase with a hexagonal close-packed (hcp) crystal lattice structure at low temperatures (T ˂ 883°C) while exists in β phase with a body-centred cubic (bcc) structure above 883°C [1]. The elements added to Ti change the phase transition temperature between α+β. For example, elements such as Al, O, N, C are called α-stabilizers because they increase the allotropic transformation temperature and keep the α phase stable in the microstructure, while the elements such as Nb, Mo, Ta, Fe, Cr are called β-stabilizers since they decrease the transformation temperature and keep the structure in the β phase [2, 3]. Two types of β-alloys, namely semi-stable and stable, can be mentioned depending on the amount of β-stabilizer content in the structure and the cooling rate of the structure from the transformation temperature. The stability of the β phase in the microstructure is determined using the Molybdenum Equivalence (Mo-Eq). [Mo- Eq. = 1.0Mo + 0.67V + 0.44W +2.9Fe + 1.6Cr + 0.3Nb + 0.22Ta - 1.0Al] [4]. If the Mo equivalent is 10 or more, it indicates that the structure will completely have the β phase at room temperature. Morphology and distribution of the phases in the microstructure are considered to affect the properties of the titanium alloys, therefore it can be predicted that β-type Ti alloys with a body-centred cubic crystal structure may have different mechanical, chemical, and physical properties in many ways compared to Ti alloys in α, near-α, and α+β phase structure.

Elastic Modulus of β-type Ti Alloys

One of the most common problems with the metallic implant materials used in the body is biomechanical incompatibility due to the elastic modulus mismatch [5]. For this reason, the mechanical relationship between the body and the implant material should be implemented considering the modulus of elasticity. The stresses on the femoral head without any implantation are distributed homogeneously in the bone and transmitted downward. When an implant with a high modulus of elasticity is placed in the same area, the stress on the bone is transmitted to the lower areas through the implant due to the high modulus of elasticity. Therefore, the bone structure around the implant bears less stress than the implant, resulting in bone resorption. As a result of bone resorption, the implant loosens in the area where it is placed, and this phenomenon is called the stress shielding effect [6, 7]. To prolong the implantation life, it is very important to prevent the above-mentioned biomechanical incompatibility. Although the modulus of elasticity of materials such as pure Ti and Ti-6Al-4V, which are currently used in the body, are 50% lower than the 316L and Co-Cr alloys, the modulus of elasticity remains higher compared to the bone structure [8]. When the studies in the literature are examined in detail, the order of modulus of elasticity of the phases in Ti-based alloys is as follows: ω˃α˃α’˃α’’˃β [9]. Mechanical properties and elastic modulus values of different Ti and its alloys with different phase structures are given in Table 1. As can be seen in Table 1, keeping the β phase in Ti-based biomaterials reduces the modulus of elasticity of titanium alloys and it allows long-term use of the implant by eliminating the stress shielding effect.

Biocompatibility Properties of β-type Ti Alloys

It is an indication of biocompatibility that the implanted biomaterial does not cause any toxic, irritant, inflammatory, allergic, mutagenic, or carcinogenic effects in the host organism. Although stainless steel (316L) and Co-Cr alloys are preferred particularly for hard tissue applications such as hip implants, it has been determined that these materials have properties causing negative effects on the body in terms of biocompatibility due to ion release that have toxic effects on the body [17]. In Figure 1 [18], biocompatibility grades of some elements and alloys are given in detail. At this point, the excellent corrosion resistance of Ti and Ti alloys due to the stable surface oxide film and its superior biocompatibility properties in soft and hard tissue applications makes these β alloys suitable for use as biomaterials. Although Ti-6Al-4V alloy with α+β phase structure is a widely preferred implant composition, the studies in the literature have indicated that vanadium (V) ions cause the toxic effect on the body by altering the kinetics of the enzyme activity associated with inflammatory response cells [19, 20]. Similarly, Al ions have been found to trigger Alzheimer’s disease in the host organism [21].

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The negative issues caused by the mostly preferred Ti alloys in the body have paved the way for the development of β-type Ti alloys. β-stabilizing elements (Ta, Nb, Zr, Mo) added to the alloy composition to maintain the β phase in the structure has a low cytotoxic feature, and they exhibit excellent biocompatibility [13]. Besides, studies conducted on the alloying of Nb, Zr, and Ta elements with Ti reveal that the oxide layer formed on the Ti surface increases the thermodynamic stability. Nb2O5, Ta2O5, and MoO3 protective oxide layers in addition to TiO2, which is formed on the Ti surface, improve the structural integrity of the oxide layer by strengthening the TiO2 layer [22]. The alloying elements of β-type Ti alloys produced to be used in corrosive and aggressive body fluid do not pose any toxic hazard to the host organism compared to the materials preferred in implant production. This feature indicates that these alloys improve the biocompatibility properties of Ti-based materials.

Conclusion

Thanks to its body-centred cubic structure, β-type titanium alloys have a low modulus of elasticity and contain highly biocompatible alloy elements that without toxic effects. These features are the main characteristics that distinguish these materials from conventional materials used in implant production. Since the alloying elements are safe for the body and the material is developed taking into account biomechanical compatibility, it seems like a creative and innovative solution to two frequently encountered problems. In addition to these advantages, excellent forgeability in sheet form, hot-cold formability, and high fracture toughness characteristics of β-type Ti alloys constitute the reason for preferring β-type titanium alloys, particularly in the biomedical field, compared to Ti alloys with near-α and α+β phase structure.