**2. Titanium base alloys developments**

### **2.1 Commercially pure titanium**

Commercially pure titanium (Ti CP) and extra low interstitial Ti-6Al-4V (ELI) are the two most common titanium base implant biomaterials. These materials are classified as biologically inert biomaterials. As such, they remain essentially unchanged when implanted into human bodies. The human body is able to recognize these materials as foreign and tries to isolate them by encasing it in fibrous tissues. However, they do not promote any adverse reactions and are tolerated well by the human tissues.

These metals do not induce allergic reactions such as has been observed with some stainless steels, which have induced nickel hypersensitivity in surrounding tissues.

Titanium is very light with a density of 4.5 g/cm3. The Ti CP is 98.9 - 99.6 % Ti, being the oxygen content (and other interstitial elements such as C and N) the main element influencing significantly its yield, tensile and fatigue strengths (Table 2). Interstitial elements strengthen the metal through interstitial solid solution strengthening mechanism, with nitrogen having approximately twice the hardening effect (per atom) of either carbon or oxygen.

Pure Ti is an allotropic metal having hexagonal phase (HCP) below 882 ºC and transforming to a cubic phase (BCC) over that temperature. As its typical microstructure is a single alpha phase, cold work is also an applied strengthening mechanism.

Its very good biocompatibility is due the formation of an oxide film (TiO2) over its surface. This oxide is a strong and stable layer that grows spontaneously in contact with air and prevents the diffusion of the oxygen from the environment providing corrosion resistance.

Attempts to use titanium for implant fabrication dates to the late 1930s. It was found that titanium was tolerated as was stainless steels and cobalt alloys. Titanium's lightness and

Titanium was found the only metal biomaterial to osseointegrate (Van Noort, 1987). Also, there were even assumptions on a possible bioactive behaviour (Li et al., 1994) due to the slow growth of hydrated titanium oxide on the surface of the titanium implant that leads to

**Material designation Common name ASTM Standard** 

Cast CoCrMo Wrought CoNiCrMo

Commercially pure Ti Ti6Al4V

Unalloyed Tantalum Nitinol

Commercially pure titanium (Ti CP) and extra low interstitial Ti-6Al-4V (ELI) are the two most common titanium base implant biomaterials. These materials are classified as biologically inert biomaterials. As such, they remain essentially unchanged when implanted into human bodies. The human body is able to recognize these materials as foreign and tries to isolate them by encasing it in fibrous tissues. However, they do not promote any adverse

These metals do not induce allergic reactions such as has been observed with some stainless

Titanium is very light with a density of 4.5 g/cm3. The Ti CP is 98.9 - 99.6 % Ti, being the oxygen content (and other interstitial elements such as C and N) the main element influencing significantly its yield, tensile and fatigue strengths (Table 2). Interstitial elements strengthen the metal through interstitial solid solution strengthening mechanism, with nitrogen having approximately twice the hardening effect (per atom) of

Pure Ti is an allotropic metal having hexagonal phase (HCP) below 882 ºC and transforming to a cubic phase (BCC) over that temperature. As its typical microstructure

Its very good biocompatibility is due the formation of an oxide film (TiO2) over its surface. This oxide is a strong and stable layer that grows spontaneously in contact with air and prevents the diffusion of the oxygen from the environment providing corrosion resistance.

steels, which have induced nickel hypersensitivity in surrounding tissues.

is a single alpha phase, cold work is also an applied strengthening mechanism.

Table 1. Examples of metallic biomaterials (An introduction to biomaterials , 2006)

ASTM F 75 ASTM F 572

ASTM F 67 ASTM F 136

ASTM F 560 ASTM F 2063

Fe-18Cr-14Ni-2.5Mo 316L Stainless Steel ASTM F 138

good mechano-chemical properties are salient features for implant application.

the incorporation of calcium and phosphorous.

**2. Titanium base alloys developments** 

reactions and are tolerated well by the human tissues.

**2.1 Commercially pure titanium** 

either carbon or oxygen.

*Stainless Steel* 

*Cobalt base alloys*  Co-28Cr-6Mo Co-35Ni-20Cr-10Mo

*Titanium base alloys*  Ti CP (grade 1 to 4) Ti-6Al-4V ELI

*Specialty metallic alloys* 

Ta Ni-45Ti


It is a biomaterial with a high superficial energy and after implantation it provides a favourable body reaction that leads to direct apposition of minerals on the bone-titanium interface and titanium osseointegration (Acero et al., 1999).

a Aluminium 6%, Vanadium 4%

Table 2. Chemical composition of Ti CP (ASTM F 67) and Ti6Al4V alloy (ASTM F 136)

### **2.2 Ti6Al4V alloy**

Ti6Al4V alloy is widely used to manufacture implants and its chemical composition is given in Table 2. The addition of alloying elements to titanium enables it to have a wide range of properties because aluminium tends to stabilize the -phase and vanadium tends to stabilize the -phase, lowering the temperature of the transformation from to . The alpha phase promotes good weldability, excellent strength characteristics and oxidation resistance. The addition of controlled amounts of vanadium as a -stabilizer causes the higher strength of beta-phase to persist below the transformation temperature which results in a two-phase system. The -phase can precipitate by an ageing heat treatment. This microstructure produce local strain fields capable of absorbing deformation energy. Cracks are arrested or deterred at the particles. The mechanical properties of the Ti CP and the Ti6Al4V are given in Table 3.


Table 3. Mechanical properties of Ti CP (ASTM F 67) and Ti6Al4V alloy (ASTM F 136)

The modulus of elasticity of these materials is about 110 GPa. This is much lower than stainless steels and Co-base alloys modulus (210 and 240 GPa, respectively (Dadvinson & Gergette, 1986). When compared by specific strength (strength/density) the titanium alloys exceed any other implant materials.

Titanium and titanium alloys, nevertheless, have poor shear strength, making them less desirable for bone screws, plates and similar applications. They also tend to gall or seize in sliding contact with itself or another metal.

### **2.3 Low modulus titanium alloys**

The Ti6Al4V alloy has some disadvantages: its elastic modulus, although low, is 4 to 6 times that of cortical bone and has low wear resistance that is a problem in articulations surfaces.

Titanium as a Biomaterial for Implants 153



A great problem of these new alloys is its fabrication processes because most beta titanium alloys contain considerable amounts of refractory elements with high melting temperatures. This results in heavily weight, difficult melting and solidification processing, low plastic

The various refractory materials employed in casting are attacked by titanium with such severity that sounds castings, possessing good mechanical properties are difficult to obtain.

The molten metal and the hot casting are susceptible to atmospheric contamination. Because Ti is very reactive with oxygen and other atmospheric gases, the melting and casting processes implies high temperature fusion and casting under vacuum or protective neutral atmospheres. Another casting problem is the maintenance of good flow over severe

Powder metallurgy (P/M) is an alternative method of fabrication in which metal powders are utilized by compacting and sintering to form useful products. This method is employed primarily to produce simple shapes with good dimensional stability, to form shapes with material of extremely high melting temperatures and to produce parts not

Production of cast titanium today takes 16 times more energy per tonne than the production of steel. Instead of conventional melting, milling and machining, P/M techniques implies powders that remain in solid form during the entire procedure. This saves a tremendous

As was said, the elastic moduli and strength of titanium and its alloys are much higher than those of human bones, which may result in stress shielding and the failure of implants. People have tried to develop new types of titanium alloys, such as -Ti alloys, to reduce the modulus of the implants to the level approaching human bones. On the other hand, the mechanical properties of porous titanium can be adjusted by pore fraction and morphology, and the stress shielding effect will be reduced. Porous titanium with porosity in a wide range can be prepared with powder metallurgy methods (Zhiguang et al., 2009), from which other kinds of powders as second phase in green bodies would be removed during

Despite the great progress achieved in orthopaedic biomaterials, fixation of implants to the bone host remains a problem. As titanium has an inert behaviour, the body tries to encapsulate the Ti-based implant. However, titanium does not bond directly to bone resulting in micro-movements and, eventually loosening of the implant. Undesirable

amount of processing energy with a reduction of over 50% (Mehta, 2008).

Bulk titanium alloys used in implants present three main problems:

**3. Problems and solutions** 

**3.1 The processing** 

feasible by other means.

**3.2 The elastic modulus** 

subsequent heat treatment.

**3.3 The inert behaviour** 


deformability and high materials costs.

because it do not react with the human tissues

So, conventional methods are not practical with titanium.

changes of dimensions or direction within the mold.

Also, V can cause potential citotoxicity and adverse tissue reactions (Steinemann, 1980), and Al ions from the alloy might cause long-term Alzheimer diseases (Rao et al., 1996).

Briefly, a biocompatible titanium base alloy suitable for bone implant should meet at least the following requirements (Mehta, 2008):


Consequently, the recent trend in research and development of titanium for biomedical applications is to develop alloys composed of non-toxic and non-allergenic elements with excellent mechanical properties (low modulus-high strength) and workability (Niinomi, 1999).

The first generation of design orthopaedic alloys try to replace the V and Al alloys with other non toxic components such as Nb, Fe and Mo (for the V) and Ta, Hf and Zr (for the Al).

Subsequent developments in orthopaedic Ti-base alloys have been motivated by the requirement of low elastic modulus. The stiffness of titanium and its alloys is still largely greater than that of cortical bone, although it is less than that of Co-Cr type alloys and stainless steels used for biomedical applications. This difference of rigidity produced the stress-shielding phenomenon. Stress shielding occurs because of the mismatch between the stiffness of the bone, which has a Young modulus of 7–25 GPa (Currey, 1998), and that of the metal implant stem.

Various methods of solving this problem have been considered, including changing the size and shape of the stem to reduce the differences in the structural stiffness of the implant and the surrounding bone and changing the implant material from steel to commercially pure titanium or Ti alloys with low modulus (Sarmiento et al., 1979).

Metastable -Ti alloys were developed for this purpose, with low elastic modulus. In Table 4 it could be observed some old and new Ti-base alloys develop specifically for biomedical purpose (Guilemot et al., 2004).

Low modulus alloys are nowadays desired because the moduli of alloys are required to be much more similar to that of bone. These new alloys have an elastic modulus ranging 55-85 GPa, so it could be minimized the stress shielding phenomena because it is more proximally to the bone modulus. However, they are still greater than that of cortical bone.


Table 4. Ti alloys developed for biomedical applications (in bold) and for other uses
