Mechanical properties of biomedical titanium alloys
Introduction
Pure titanium and titanium alloys are now the most attractive metallic materials for biomedical applications. Ti–6Al–4V has been a main biomedical titanium alloy for a long period. New types of alloys like Ti–6Al–7Nb [1]and Ti–5Al–2.5Fe [2]have been, however, recently developed because of the problem of toxicity of elements in the Ti–6Al–4V alloy and the development of the required performance of the alloy. Biomedical titanium alloys with much greater biocompatibility have been proposed and are currently under development [3]. They are mainly β type alloys composed of non-toxic elements. The β type alloys have greater biocompatibility because their moduli are much less than those of α+β type alloys like Ti–6Al–4V and so on. They are also able to gain greater strength and toughness balance compared with α+β type alloys.
These titanium alloys are mainly used for substituting materials for hard tissues. Fracture of the alloys is, therefore, one of the big problems for their reliable use in the body. The fracture characteristics of the alloys are affected by changes in microstructure. Therefore, their fracture characteristics, including tensile and fatigue characteristics should be clearly understood with respect to microstructures. The fracture characteristics in the simulated body environment should also be identified because the alloys are used as biomedical materials. The effect of living body environment on the mechanical properties is also very important to understand.
The mechanical properties such as tensile characteristics, fracture toughness, fatigue characteristics and so on of various biomedical titanium alloys developed to date will be described in this paper in as much detail as possible. The effects of simulated body environment and living body environment on the mechanical properties will be also described in the paper.
Section snippets
Titanium alloys for implant and dental materials
Titanium alloys developed for implant materials to date are listed in Table 1 1, 2, 3, 5, 6, 7, 8, 9, 10, 11, 12, 13. Commercial pure titanium, Ti–6Al–4V and Ti–6Al–4V ELI have basically been developed for structural materials although they are still widely used as representative titanium alloys for implant materials. More recently, V free α+β type alloys such as Ti–6Al–7Nb [1]and Ti–5Al–2.5Fe [2]have appeared as implant materials. In addition, V and Al free α+β type alloys composed of
Tensile properties
The tensile properties of titanium implant materials developed to date are listed in Table 3 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. The data of tensile yield stress and elongation in Table 3 are plotted in Fig. 1 with those of structural α, α+β and β type titanium alloys. The data of the structural titanium alloys are shown as a scatter band in Fig. 1. The yield strength of biomedical titanium alloys are distributed nearly between 500 and 1000 MPa. The yield strength of pure titanium
Modulus
The moduli of elasticity of biomedical titanium alloys are shown in Fig. 2 although their values have been already shown in Table 3 3, 5, 6, 10, 11, 13. The moduli of other metallic biomaterials like stainless steel and Co type alloy are round 206 and 240 GPa, respectively [6]. They are much greater than that of bone whose modulus of elasticity is generally between 17 and 28 GPa [6]. The moduli of elasticity of biomedical titanium alloys are much smaller than those of other metallic
Fatigue strength
Fatigue strength of biomedical titanium alloy at 107 cycles are shown in Fig. 3 8, 10, 13with those of other metallic biomedical materials such as stainless steels; AISI 316 LVM and SUS 316L and Co type alloys; Co–Cr–Mo and Co–Ni–Cr–Mo. Fatigue strength of biomedical titanium alloys listed in Table 3 is from 265 to 816 MPa.
Fracture toughness
Fracture toughness of biomedical titanium alloys are shown in Fig. 4 4, 6, 13. The fracture toughness of β type medical titanium alloys are similar to those of α+β type ones.
Fatigue strength
S–N curves of Ti–6Al–4V ELI with equiaxed and Widmanstätten α structure and annealed SUS 316L in air and Ringer's solution obtained from rotating bending fatigue tests have been reported [15]. The rotating bending fatigue strength of Ti–6Al–4V ELI in air and Ringer's solution are equivalent. The fatigue strength of SUS 316L is degraded in Ringer's solution at relatively greater number of cycles to failure comparing with that in air. The concentration of oxygen in body liquid or muscle tissue
Mechanical properties after implanted in living body
The Vickers hardness changes on the specimen surfaces of implant materials Ti–5Al–2.5Fe and Ti–6Al–4V ELI with equiaxed α structure and SUS 316L before and after implanting into the paravertebral muscle of living rabbit for about 11 months are shown in Fig. 6 [15]. The Vickers hardness of both titanium alloys are not changed before and after implanting. The Vickers hardness of SUS 316L after implanting is however increased compared with that before implanting. The surface hardened area
Summary
The tensile strength of biomedical titanium alloys developed to date lies between 500 and 1000 MPa. The elongation lies between 10 and 20%. The moduli of elasticity of the low modulus β type biomedical titanium alloys developed to date is between 55 and 85 MPa.The rotating bending fatigue strength of the biomedical titanium alloy is degraded in simulated body environment with rather lower oxygen concentration. The crack propagation rate of biomedical titanium alloys is grater in simulated body
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