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Materials Classification
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Materials Classification 2.1. 2.2.
General Medical/Dental Titanium and its Alloys 2.2.1 Commercially Pure Titanium (CpTi) 2.2.2 Ti-6Al-4V 2.2.3 Ti-6Al-7Nb 2.2.4 Ti-3Al-2.5V 2.2.5 Ti-5Al-3Mo-4Zr 2.2.6 Ti-5Al-2.5Fe 2.2.7 Ti-Ni 2.2.8 Ti-Cu 2.2.9 Ti-Mo 2.2.10 Ti-5Al-2Mo-2Fe (SP700) 2.2.11 Other Ti-Based Alloys 2.2.12 Intermetallic Alloys References
11 13 13 16 16 16 16 17 17 18 18 18 19 19 20
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Materials Classification Materials research, development, and application should include needs-oriented and seeds-oriented approaches. A good example of needs-oriented materials R&D is the search for a light-weight structural material (particularly demanded by the aerospace industries). This has accelerated the development of titanium alloys. On the other hand, NiTi provides a typical example of seeds-oriented material. Opposing its initial specific aim for development (which was a strategic search for submarine structural materials with a relatively low damping capacity), its unique shape-memory effect (SME), as well as superelasticity (SE) properties, needed a successful application. This was realized when used for medical applications (orthopedic implants, stunt, Harrington bar, and others) and dental applications (blade-type implants, orthodontic wires, splints, as well as endodontic files). Recently owing to advanced quantitative metallography, as well as molecular/ atomic dynamics and designing concepts, these two needs-oriented and seedsoriented concepts have been integrated systematically to develop nano-scale materials designs and manufacturing technologies. 2.1. GENERAL
Basically, titanium and titanium-based alloys can be classified into type (HCP: hexagonal closed-packed crystalline structure), near type, (⫹) type, and type (BCC: body centered-cubic crystalline structure) alloy groups. Alloying elements added to titanium are divided into two groups: alpha () stabilizers and beta () stabilizers. Elements, such as Al, Sn, Ga, Zr, and interstitial elements (either singly of C, O, and N or in combination), dissolve into the titanium matrix, and are strong solid solution strengtheners which produce little change at the transformation temperature (-transus: 885oC for pure Ti) from the HCP () to the BCC () structure of pure titanium when heating, and from BCC to HCP when cooling. Hence they are known as -stabilizers and exhibit good hightemperature performance. Alloying elements which decrease this phase transformation temperature are referred to as -stabilizers. Generally, -stabilizing elements are the transition metals, such as V, Mo, Nb, Ta, and Cr, providing much friability [2-1]. Besides these alloying elements, Fe, Cu, Ni, Si, and B are frequently added to Ti-based alloys for improving mechanical strength, chemical 11
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stability, castability, and/or grain refining. By increasing the -phase portion, it is generally recognized that (1) -transus temperature increases, (2) creep strength as well as high temperature strengths enhance, (3) flow stress increases, and (4) weldability improves. By increasing the -phase portion, it is known that (1) room temperature strength increases, (2) heat treatment and forming capabilities enhance, and (3) strain-rate sensitivity increases, so that superplastic forming is more favorably applicable. When any titanium material possesses either one of the aforementioned types as a major constitutional phase, such titanium material is named after the type of phase. Alpha alloys generally have creep resistance superior to that of beta alloys, and are preferred for high-temperature applications. The absence of a ductile–brittle transition (a feature of beta alloys) makes alpha alloys suitable for cryogenic applications, too. Unlike beta alloys, alpha alloys cannot be strengthened by heat treatment because the alpha structure is a stable phase. Alpha⫹beta alloys have compositions of a mixture of and phases and may contain between 10 and 50% phase at room temperature. The most common type within this group alloy is Ti-6Al-4V1. Within the ⫹ type class, an alloy containing much more alpha than beta is often called a near-alpha alloy. Generally, when strengthening is needed, the alloys are rapidly cooled (i.e., quenched) from a temperature high in the alpha-beta range above the -transus. This solution treatment is followed by an intermediate-temperature treatment (aging) to produce an appropriate mixture of alpha and transformed beta products. Beta alloys. Beta titanium has a wider solubility for alloying elements without precipitating any intermetallic compounds. Beta alloys contain transition elements, such as V, Mo, Nb, Ta, and Cr, which tend to reduce the temperature of the ↔ phase transformation (or simply -transus). They have excellent forgeability over a wider range of forging temperature than alloys, and -alloy sheet is cold formable in the solution-treated condition. Beta alloys have excellent workhardening and heat-treatment capabilities. A common thermal treatment involves solution treatment followed by aging at temperatures ranging from 450 to 650°C [2-2]. Beta alloys are one of the most promising groups of titanium alloys in terms 1
Ti-6Al-4V indicates the Ti-based alloy added with 6 wt.% (or w/o) of Al and 4 wt.% of V. The text uses this alloy description, otherwise if atomic % (a/o) is used, both a/o and w/o will be indicated. For conversion between w/o and a/o, let Wx and Ax be wt.% and atomic % of element X in the X–Y binary alloy, where X and Y are atomic weights of respective elements. For each of the respective elements, conversion from w/o to a/o and from a/o to w/o is calculated as follows: Ax ⫽ 100/{1⫹(X/Y)[(100/Wx) ⫺ 1]} and Ay ⫽ 100 ⫺ Ax Wx ⫽ 100/{1⫹(Y/X)[(100/Ax) ⫺ 1]} and Wy ⫽ 100 ⫺ Wx
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of processing, properties, and potential applications. This group of alloys (including beta, metastable beta, and beta-rich alpha/beta compositions) represents the highest range of strength, fatigue resistance, and environmental resistance among all titanium materials. Of course, in these alloys, the single -phase and single phase regions are separated by a two-phase (⫹) region. As a result, the alloys utilize multi-component elements and are composed of mixtures of and stabilizers. Depending on the ratio of and phases, they can be furthermore subgrouped into near () and near () alloys. Typical beta group Ti alloys include (1) beta type (Ti-35V-15Cr, Ti-40Mo, Ti-13V-11Cr-3Al, Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-30Mo), metastable beta type (Ti-6V-5.7Fe-2.7Al, Ti-12V-11Cr-3Al, Ti-1Al-8V5Fe, Ti-12Mo-6Zr-2Fe,Ti-4.5Fe-6.8Mo-1.5Al,Ti-15V-1Mo-0.5Nb-3Al-3Sn-0.5Zr, Ti-3Al-8V-4Mo-4Zr, Ti-15Mo, Ti-8V-8Mo-2Fe-3Al, Ti-15Mo-2.6Nb-3Al-0.2Si, Ti-15V-3Cr-3Sn-3Al, Ti-11.5Mo-6Zr-4.5Sn, Ti-10V-2Fe-3Al, Ti-5V-5Mo-1Cr1Fe-5Al, Ti-5Al-2Sn-4Mo-4Cr, Ti-4.5Al-3V-2Mo-2Fe), and beta-rich group (Ti-5Al-2Sn-2Cr-4Mo-4Zr-1Fe, Ti-13Nb-13Zr, Ti-4.5Al-3V-2Mo-2Fe) [2-3–2-6]. Table 2.1 summarizes the above discussion. 2.2. MEDICAL/DENTAL TITANIUM AND ITS ALLOYS
Currently, pure titanium and ⫹ type Ti-6Al-4V ELI (Extra Low level of Interstitial content) alloys are widely used as structural and/or functional biomaterials for the replacement of hard tissues in devices such as artificial total hip or knee replacements and dental implants, since they exhibit excellent specific strengths and corrosion resistance, and the best biocompatibility characteristics among metallic biomaterials. They are used more than any other titanium biomaterials; however, other new titanium alloys for biomedical applications have now been included in ASTM standardizations [2-7, 2-8]. For example, -type Ti-15Mo [2-9] has been registered in ASTM standardizations, and - type Ti35Nb-7Zr-5Ta [2-10] and ⫹ type Ti-3Al-2.5V [2-11] are in the process of being registered. Among several dozen commercially available alloys, the following are typical titanium materials which are utilized or experimentally and clinically tried in both the medical and dental fields. 2.2.1 Commercially pure titanium (CpTi) Under the category of “unalloyed grades” of ASTM specification, there are five materials classified in this group; they include ASTM grade 1 (99.5%Ti), grade 2 (99.3%Ti), grade 3 (99.2%Ti), grade 4 (99.0%Ti), and grade 7 (99.4%Ti). Although each material contains slightly different levels of N, Fe, and O, C is specified <0.10 wt.% (wt.% or w/o) and H is also specified <0.015 wt.%. ASTM CpTi
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Table 2.1. Three major types of titanium materials and influencing effects of major alloying elements. Type/material property
and near
⫹
and near
-Stabilizing elements Al, Sn, Ga, Zr, C, O, N -Stabilizing elements Typical materials Commercially pure Ti Ti-5Al-2.5Sn Ti-5Al-6Sn-2Zr-1Mo Ti-6Al-2Sn-4Zr-2Mo Ti-8Al-1Mo-1V
-Transus temperature Specific density Room temperature strength Room temperature toughness Modulus of elasticity Machinability Age hardenability Heat resistance Weldability High-temperature strength Heat-treatability Plastic formability Strain-rate sensitivity Superplastic formability Creep resistance
V, Mo, Nb, Ta, Cr Ti-5Al-2.5Fe Ti-3Al-8V-6Cr-4Mo-4Zr Ti-5Al-2Mo-2Fe Ti-4.5Al-3V-2Mo-2Fe Ti-5Al-3Mo-4Zr Ti-5Al-2Sn-2Zr-4Mo-4Cr Ti-5Al-2.5Fe Ti-6Al-6Fe-3Al Ti-6Al-7Nb Ti-10V-2Fe-3Al Ti-6Al-4V Ti-13V-11Cr-3Al Ti-6Al-6V-2Sn Ti-15V-3Cr-3Al-3Sn Ti-6Al-2Sn-4Zr-6Mo Ti-35V-15Cr Ti-8Mo-8V-2Fe-3Sn Ti-11.5Mo-6Zr-4.5Sn Ti-30Mo, Ti-40Mo Ti-13Nb-13Zr Ti-25Pd-5Cr Ti-20Cr-0.2Sn Ti-30Ta Higher ← Lower Lower → Higher → → ← ← → ← ← ← → → → → ←
Note: This chart does not include TiNi, Ti3Al (2), and TiAl () intermetallic type alloys.
grades 1–4 (unalloyed titanium) allow a hydrogen content up to 0.015 wt.% (i.e., 15 ppm). It was reported that if CpTi contains more than 250 ppm of hydrogen, the material would be susceptible to stress corrosion cracking and hydrogen embrittlement [2-12].
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Grade 1: Grade 1 CpTi is the lowest strength unalloyed titanium with a slightly lower residual content. Both oxygen and iron residuals improve the impact strength. Oxygen acts as an interstitial strengthener, maintaining a single -phase hexagonal closed-packed microstructure. Iron acts as a second phase BCC grain refiner, offering moderate strengthening capabilities. The lower residual content makes grade 1 the lowest strength CpTi grade, but it has the highest ductility, with an excellent cold formability. Grade 2: The grade 2 is the most frequently selected titanium grade in industrial service, having well-balanced properties of both strength and ductility. The strength levels are very similar to those of common stainless steel and its ductility allows for good cold formability. Grade 3: CpTi grade 3 possesses a slightly higher strength due to its slightly higher residual content (primarily oxygen, and also nitrogen) with slightly lower ductility. Grade 4: The grade 4 is the highest strength grade of the CpTi series, so that grade 4 serves mainly in the aerospace/aircraft industry. For all CpTi grades 1–4, the 0.2% off-set equivalent-yield strength (0.2YS) and the ultimate tensile strength (UTS) appear well correlated to oxygen contents [2-13]. Through linear regression analysis, with the oxygen content expressed by [O], it was found that 0.2YS = 1336.2 ⫻ [O] ⫺66.7 in MPa with r (correlation coefficient) of 0.9865, and UTS = 1351.5 ⫻ [O] ⫺3.7 in MPa with r of 0.9946. By far the most widely used of the CpTi grades is grade 2 [2-13]. From this base, the other grades have been developed for better formability or higher strength levels, significantly increasing corrosion resistance at higher temperatures, and/or improving corrosion resistance at lower pH (or higher acidity) levels [2-14]. There is a distinct difference in using CpTi between industry and medicine. Among various applications of CpTi materials in dental and medical fields, the dental CpTi implant is the most frequently and widely employed. Information on CpTi grade selection for dental application indicates that, despite the popularity of grade 2 in industry [2-11], grades 3 and 4 are equally selected in most of the 20 major dental implant systems. Only two companies use grades 1 and 2 [2-15–2-18]. More interesting, however, is their availabilities. In preparing CpTi grades 1–4 for corrosion resistance comparisons, Hernandez [2-15] searched 14 material suppliers, but found that only 2 out of the 14 suppliers were able to provide all four grades. Grades 2 and 4 were available without any problems, whereas CpTi grade 3 is the least popular and difficult to obtain; 2 out of the 14 suppliers were able to provide CpTi grade 3. After conducting electrochemical corrosion testing, using 37°C Ringer’s solution as an electrolyte, it was found that CpTi grade 3 exhibited the least corrosion resistance. Regardless of material availability [2-15, 2-16], this supported the unpopularity of CpTi grade 3. Hence, despite the popularity and availability of grade 2 in industries,
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CpTi grade 2 is the least popular, although its corrosion resistance is somewhat superior to grade 3, which is the worst grade in terms of corrosion resistance among the four grades. 2.2.2 Ti-6Al-4V This alloy belongs to the ⫹ phase alloy group and is particularly popular because of its high corrosion resistance and the reputed low toxicity of ions released from the surface due to dense and protective passive oxide (which is mainly TiO2) film formation. Ti-6Al-4V (which is, in some articles, marked as Ti-6/4) exhibits good mechanical and excellent tissue compatibility properties, which make it well suited for biomedical applications where a bone anchorage is required, particularly for implant applications [2-18]. Ti-6Al-4V ELI is also available and employed in the medical area [2-19]. 2.2.3 Ti-6Al-7Nb As a result of searches for vanadium-free Ti-6Al-4V equivalent alloys, this alloy was developed to enhance the wear resistance [2-20] and castability [2-21]. The optimal composition was found to be Ti-6Al-7Nb (or simply Ti-6/7). This custom-made alloy designed for implants shows the same alpha/beta structures as Ti-6Al-4V and exhibits equally good mechanical properties. The corrosion resistance of Ti-6Al-7Nb in sodium chloride solution was evaluated to be equivalent to that of pure titanium and Ti-6Al-4V, due to formation of a very dense and stable passive layer. Highly stressed anchorage stems of different medical prostheses (including hip, knee, and wrist joints) have been made from hot-forged Ti-6Al-7Nb. The surfaces of these Ti-6Al-7Nb prostheses were further hardened by means of a very hard 3–5 m thick titanium nitride coating or by oxygen diffusion hardening to a depth of 30 m, in order to enhance the biotribological properties [2-22]. 2.2.4 Ti-3Al-2.5V Ti-3Al-2.5V alloy possesses an excellent ductility and cold formability, allowing it to be cold-worked by standard tube-making processes and bent for installation. It is easily welded, and may be heat-treatable to a wide range of strengths and ductility [2-23]. 2.2.5 Ti-5Al-3Mo-4Zr A newly developed titanium for surgical implant application, Ti-5Al-3Mo-4Zr (or simply referred to Ti-5/3/4), was evaluated, and its properties were compared with conventional biomaterials. Mechanical properties and metallic ion elution were also examined. It was reported that the new alloy is advantageous over Ti-6Al-4V
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ELI because it does not contain bio-hazardous alloying elements (e.g., vanadium), and has superior mechanical properties to stainless steel. Corrosion abrasive wear resistance is also improved in Ti-5Al-3Mo-4Zr. As a result, newly designed artificial hip joints were fabricated with this alloy [2-23, 2-24]. 2.2.6 Ti-5Al-2.5Fe In the course of developing a vanadium-free titanium-based alloy, the another new alloy Ti-5Al-2.5Fe (which is an ⫹ phase alloy) has been developed to avoid the presence of toxic vanadium which is an alloying element in the Ti-6Al-4V alloy and may increase to more than 15% in the phase. Hip prostheses and hip prosthesis heads were fabricated from the Ti-5Al-2.5Fe alloy with a grain size of 20 m or less. The frictional biotribological behavior of a hip prosthesis head in contact with an ultrahigh molecular weight polyethylene (UHMWPE) cup was investigated. It was shown that (i) the frictional behavior of a head coated with an oxide layer about 1–3 m thick produced by induction heating of the surface equal to that of a head fabricated from alumina ceramics, and (ii) the oxide layer was still present after 107 cycles in 0.9% NaCl solution, indicating excellent corrosion resistance of this alloy [2-25]. 2.2.7 Ti-Ni Historically, when the US Navy searched for a new submarine material exhibiting good damping capacity, NiTi alloy was developed. Buehler, who conducted the bending tests on NiTi alloys, discovered that a large deformation was completely recovered by a slight heating to return to its original shape. This phenomenon is later referred to as a SME. Since the test was performed and the material was developed at the US Naval Ordnance Laboratory, this unique material is called as NITINOL (NIckel-TItanium Naval Ordnance Laboratory) [2-26]. This material is also often called and marked as NiTi, TiNi or Nitinol. In this book, since the Ni element contains more than 50 wt.% (weight percentage), the term “NiTi” is used according to the ordinal way to describe alloy systems in the conventional metallurgy. It is well known that this alloy exhibits unique phenomena such as SME as well as SE. By SME, a material first undergoes a martensitic transformation. After deformation in the martensitic condition, the apparently permanent strain is recovered when the specimen is heated to cause the reverse martensitic transformation. Upon cooling, it does not return to its deformed shape. When SME alloys are deformed in the temperature regime a little above the temperature at which martensite normally forms during cooling, a stress-induced martensite is formed. This martensite disappears when the stress is released, giving rise to a superelastic
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stress–strain loop with some stress hysteresis. This property applies to the parent phase undergoing a stress-induced martensitic transformation. The other unique property associated with NiTi is SE. When SME alloys are deformed, a superelastic alloy deforms reversibly to very high strains up to 10% by the creation of stressinduced phase. When the load is removed, the new phase becomes unstable and the material regains its original shape. Unlike SME alloy, no change in temperature is needed for the alloy to recover its initial shape. There is a term called pseudoelasticity, which is a more generic term that encompasses both superelastic and rubberlike behavior [2-27]. Because of the uniqueness of SME and the SE associated with NiTi, there have been many applications in both the industrial and dental/medical fields, including dental and orthopedic implants, artificial heart valve, stunt, endodontic files, orthodontic archwires, etc. [2-28–2-33]. Superelastic devices take advantages of their large, reversible deformation and their applications are not limited to medical/dental fields, but can be found in parabola antenna used at the space shuttle, frames for eyeglasses, and sporting goods such as fishing line. 2.2.8 Ti-Cu Aiming for developing an alloy for dental casting with better mechanical properties than unalloyed CpTi, Ti-Cu (Cu: 0.5–10 wt.%) was cast in an argon-arc melting furnace. It was found that (i) the mean tensile strength was significantly higher than for cast CpTi, and (ii) increases of 30% in the tensile strength and yield strengths of 40% over CpTi were obtained for the Ti-5Cu alloy [2-34]. 2.2.9 Ti-Mo TiMo alloy is widely used as an orthodontic archwire for performing an orthodontic mechanotherapy [2-35]. When NiTi wire, TMA (-phase Ti-Mo) wire, and austenitic stainless steel wire were compared, it was found that (i) TMA showed the greatest plastic strain and springback, followed by NiTi and stainless steel, (ii) NiTi wire showed the highest stored energy value in bending-torsion, followed by TMA and stainless steel, and (iii) the highest spring ratio (stiffness) in bendingtorsion was found in stainless steel, followed by TMA and NiTi [2-36]. Lin et al. [2-37] studied alloying effect of Fe on the Ti–Mo alloy, and found that Ti–Mo with iron contents in the range of 2–5 wt.% appeared to have a great potential for use as an implant material. 2.2.10 Ti-5Al-2Mo-2Fe (SP700) Ti-6Al-4V is the most widely used titanium alloy. However, Ti-6Al-4V still suffers from limited applications in non-aerospace fields compared to CpTi, aluminum
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alloys, and steels. The reasons for this include (1) poor hot-workability, with a normal hot-working temperature range that is too high and too narrow, (2) poor coldworkability that virtually prohibits cold-working, (3) poor hardenability that is frequently inconsistent, and (4) a high superplastic forming temperature of around 900oC that results in rapid die wear. To overcome these disadvantages associated with Ti-6Al-4V, a new beta-rich alpha-beta duplex phase titanium alloy has been developed (called SP700, since the newly developed Ti alloy remarkably exhibits high superplastic capability at operating temperatures around 700oC). It was reported that SP700 provides excellent hot- and cold-workability, as well as greatly improved superplastic forming characteristics (i.e., the strain rate sensitivity exponent, m-value, is about 1.0, indicating that flow is close to that of a Newtonian viscous flow), high strength, and high toughness. Dental denture bases have been fabricated using this SP700 alloy by superplastic forming technique [2-38]. 2.2.11 Other Ti-based alloys An interesting study was done by Brème et al. [2-39], using isoelastic porous sintered Ti-30Ta alloy and titanium wire loop, to accomplish the mechanical compatibility in endosseous dental implant systems. Their mechanical properties were optimized by the production parameter such as sintering and diffusion bonding. The functionality was tested after insertion into an artificial jaw, which had properties corresponding to the natural mandibular. It was reported that the elastic properties of both implants are similar to the properties of the bone, and the implant has a safe anchorage bone ingrowth [2-39]. Qazi et al. [2-6] developed metastable -Ti alloy (mainly Ti-35Nb-7Zr-5Ta) and solution-treated, followed by aging at 482oC. It was reported that (i) the heattreated Ti-35Nb-7Zr-5Ta alloys exhibited 0.2% off-set yield strength of 1300 MPa with 8% elongation, due to ⫹ and ⫹⫹-phase precipitations, and (ii) this enhanced strength makes the alloy candidates for bone plates and screws. 2.2.12 Intermetallic Alloys Although not ordinary alloys, titanium aluminide-based alloys, including TiAl (-phase) and Ti3Al (2 phase), are used for high temperature applications. The titanium aluminide-based alloys provide an inherently low-density material and exhibit excellent creep-rupture properties [2-13, 2-40]. TiAl amorphous alloy provides high strength, linear elastic behavior, and the infinite fatigue life necessary for high device reliability. Tregilgas [2-41] developed amorphatized TiAl alloys for a material for the digital micromirror device chip. In order to enhance the wear resistance of -TiAl (Ti-47Al-2Nb) alloy, the surface of this alloy was modified in a nitrogen-ion plasma atmosphere [2-42].
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[2-34] Kikuchi M, Takada Y, Kiyosue S, Yoda M, Woldu M, Cai Z, Okuno O, Okabe T. Mechanical properties and microstructures of cast Ti-Cu alloy. Biomaterials 2003;19:174–181. [2-35] Ho WF, Ju CP, Chern Lin, JH. Structure and properties of cast binary Ti-Mo alloys. Biomaterials 1999;20:2115–2122. [2-36] Drake SR, Wayne DM, Powers JM, Asgar K. Mechanical properties of orthodontic wires in tension, bending, and torsion. Am J Orthod 1982;82: 206–210. [2-37] Lin DJ, Chern Lin JH, Ju CP. Structure and properties of Ti-7.5Mo-xFe alloys. Biomaterials 2002;23:1723–1730. [2-38] Ouchi C, Minakawa K, Takahashi K, Ogawa A, Ishikawa M. Development of -rich - titanium alloy: SP-700. NKK Technical Review 1992;65:61–67. [2-39] Brème J, Biehl V, Schulte W, D’Hoedt B, Donath K. Development and functionality of isoelastic dental implants of titanium alloys. Biomaterials 1993;14:887–892. [2-40] Jha SK, Larsen JM, Rosenberger AH. The role of competing mechanisms in the fatigue-life variability of a titanium and gamma-TiAl alloy. J Metals 2005;57:50–54. [2-41] Tregilgas J. Amorphous hinge material. Adv Mater Processes 2005;163: 46–49. [2-42] Thongtem S, McNallan M, Thongtem T. Surface modifications of -TiAl alloys by nitrogen plasma deposition. Adv Technol Mater Mater Processing 2004;6:170–175.
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Materials Classification 2.1. 2.2.
General Medical/Dental Titanium and its Alloys 2.2.1 Commercially Pure Titanium (CpTi) 2.2.2 Ti-6Al-4V 2.2.3 Ti-6Al-7Nb 2.2.4 Ti-3Al-2.5V 2.2.5 Ti-5Al-3Mo-4Zr 2.2.6 Ti-5Al-2.5Fe 2.2.7 Ti-Ni 2.2.8 Ti-Cu 2.2.9 Ti-Mo 2.2.10 Ti-5Al-2Mo-2Fe (SP700) 2.2.11 Other Ti-Based Alloys 2.2.12 Intermetallic Alloys References
11 13 13 16 16 16 16 17 17 18 18 18 19 19 20
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Materials Classification Materials research, development, and application should include needs-oriented and seeds-oriented approaches. A good example of needs-oriented materials R&D is the search for a light-weight structural material (particularly demanded by the aerospace industries). This has accelerated the development of titanium alloys. On the other hand, NiTi provides a typical example of seeds-oriented material. Opposing its initial specific aim for development (which was a strategic search for submarine structural materials with a relatively low damping capacity), its unique shape-memory effect (SME), as well as superelasticity (SE) properties, needed a successful application. This was realized when used for medical applications (orthopedic implants, stunt, Harrington bar, and others) and dental applications (blade-type implants, orthodontic wires, splints, as well as endodontic files). Recently owing to advanced quantitative metallography, as well as molecular/ atomic dynamics and designing concepts, these two needs-oriented and seedsoriented concepts have been integrated systematically to develop nano-scale materials designs and manufacturing technologies. 2.1. GENERAL
Basically, titanium and titanium-based alloys can be classified into type (HCP: hexagonal closed-packed crystalline structure), near type, (⫹) type, and type (BCC: body centered-cubic crystalline structure) alloy groups. Alloying elements added to titanium are divided into two groups: alpha () stabilizers and beta () stabilizers. Elements, such as Al, Sn, Ga, Zr, and interstitial elements (either singly of C, O, and N or in combination), dissolve into the titanium matrix, and are strong solid solution strengtheners which produce little change at the transformation temperature (-transus: 885oC for pure Ti) from the HCP () to the BCC () structure of pure titanium when heating, and from BCC to HCP when cooling. Hence they are known as -stabilizers and exhibit good hightemperature performance. Alloying elements which decrease this phase transformation temperature are referred to as -stabilizers. Generally, -stabilizing elements are the transition metals, such as V, Mo, Nb, Ta, and Cr, providing much friability [2-1]. Besides these alloying elements, Fe, Cu, Ni, Si, and B are frequently added to Ti-based alloys for improving mechanical strength, chemical 11
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stability, castability, and/or grain refining. By increasing the -phase portion, it is generally recognized that (1) -transus temperature increases, (2) creep strength as well as high temperature strengths enhance, (3) flow stress increases, and (4) weldability improves. By increasing the -phase portion, it is known that (1) room temperature strength increases, (2) heat treatment and forming capabilities enhance, and (3) strain-rate sensitivity increases, so that superplastic forming is more favorably applicable. When any titanium material possesses either one of the aforementioned types as a major constitutional phase, such titanium material is named after the type of phase. Alpha alloys generally have creep resistance superior to that of beta alloys, and are preferred for high-temperature applications. The absence of a ductile–brittle transition (a feature of beta alloys) makes alpha alloys suitable for cryogenic applications, too. Unlike beta alloys, alpha alloys cannot be strengthened by heat treatment because the alpha structure is a stable phase. Alpha⫹beta alloys have compositions of a mixture of and phases and may contain between 10 and 50% phase at room temperature. The most common type within this group alloy is Ti-6Al-4V1. Within the ⫹ type class, an alloy containing much more alpha than beta is often called a near-alpha alloy. Generally, when strengthening is needed, the alloys are rapidly cooled (i.e., quenched) from a temperature high in the alpha-beta range above the -transus. This solution treatment is followed by an intermediate-temperature treatment (aging) to produce an appropriate mixture of alpha and transformed beta products. Beta alloys. Beta titanium has a wider solubility for alloying elements without precipitating any intermetallic compounds. Beta alloys contain transition elements, such as V, Mo, Nb, Ta, and Cr, which tend to reduce the temperature of the ↔ phase transformation (or simply -transus). They have excellent forgeability over a wider range of forging temperature than alloys, and -alloy sheet is cold formable in the solution-treated condition. Beta alloys have excellent workhardening and heat-treatment capabilities. A common thermal treatment involves solution treatment followed by aging at temperatures ranging from 450 to 650°C [2-2]. Beta alloys are one of the most promising groups of titanium alloys in terms 1
Ti-6Al-4V indicates the Ti-based alloy added with 6 wt.% (or w/o) of Al and 4 wt.% of V. The text uses this alloy description, otherwise if atomic % (a/o) is used, both a/o and w/o will be indicated. For conversion between w/o and a/o, let Wx and Ax be wt.% and atomic % of element X in the X–Y binary alloy, where X and Y are atomic weights of respective elements. For each of the respective elements, conversion from w/o to a/o and from a/o to w/o is calculated as follows: Ax ⫽ 100/{1⫹(X/Y)[(100/Wx) ⫺ 1]} and Ay ⫽ 100 ⫺ Ax Wx ⫽ 100/{1⫹(Y/X)[(100/Ax) ⫺ 1]} and Wy ⫽ 100 ⫺ Wx
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of processing, properties, and potential applications. This group of alloys (including beta, metastable beta, and beta-rich alpha/beta compositions) represents the highest range of strength, fatigue resistance, and environmental resistance among all titanium materials. Of course, in these alloys, the single -phase and single phase regions are separated by a two-phase (⫹) region. As a result, the alloys utilize multi-component elements and are composed of mixtures of and stabilizers. Depending on the ratio of and phases, they can be furthermore subgrouped into near () and near () alloys. Typical beta group Ti alloys include (1) beta type (Ti-35V-15Cr, Ti-40Mo, Ti-13V-11Cr-3Al, Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-30Mo), metastable beta type (Ti-6V-5.7Fe-2.7Al, Ti-12V-11Cr-3Al, Ti-1Al-8V5Fe, Ti-12Mo-6Zr-2Fe,Ti-4.5Fe-6.8Mo-1.5Al,Ti-15V-1Mo-0.5Nb-3Al-3Sn-0.5Zr, Ti-3Al-8V-4Mo-4Zr, Ti-15Mo, Ti-8V-8Mo-2Fe-3Al, Ti-15Mo-2.6Nb-3Al-0.2Si, Ti-15V-3Cr-3Sn-3Al, Ti-11.5Mo-6Zr-4.5Sn, Ti-10V-2Fe-3Al, Ti-5V-5Mo-1Cr1Fe-5Al, Ti-5Al-2Sn-4Mo-4Cr, Ti-4.5Al-3V-2Mo-2Fe), and beta-rich group (Ti-5Al-2Sn-2Cr-4Mo-4Zr-1Fe, Ti-13Nb-13Zr, Ti-4.5Al-3V-2Mo-2Fe) [2-3–2-6]. Table 2.1 summarizes the above discussion. 2.2. MEDICAL/DENTAL TITANIUM AND ITS ALLOYS
Currently, pure titanium and ⫹ type Ti-6Al-4V ELI (Extra Low level of Interstitial content) alloys are widely used as structural and/or functional biomaterials for the replacement of hard tissues in devices such as artificial total hip or knee replacements and dental implants, since they exhibit excellent specific strengths and corrosion resistance, and the best biocompatibility characteristics among metallic biomaterials. They are used more than any other titanium biomaterials; however, other new titanium alloys for biomedical applications have now been included in ASTM standardizations [2-7, 2-8]. For example, -type Ti-15Mo [2-9] has been registered in ASTM standardizations, and - type Ti35Nb-7Zr-5Ta [2-10] and ⫹ type Ti-3Al-2.5V [2-11] are in the process of being registered. Among several dozen commercially available alloys, the following are typical titanium materials which are utilized or experimentally and clinically tried in both the medical and dental fields. 2.2.1 Commercially pure titanium (CpTi) Under the category of “unalloyed grades” of ASTM specification, there are five materials classified in this group; they include ASTM grade 1 (99.5%Ti), grade 2 (99.3%Ti), grade 3 (99.2%Ti), grade 4 (99.0%Ti), and grade 7 (99.4%Ti). Although each material contains slightly different levels of N, Fe, and O, C is specified <0.10 wt.% (wt.% or w/o) and H is also specified <0.015 wt.%. ASTM CpTi
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Table 2.1. Three major types of titanium materials and influencing effects of major alloying elements. Type/material property
and near
⫹
and near
-Stabilizing elements Al, Sn, Ga, Zr, C, O, N -Stabilizing elements Typical materials Commercially pure Ti Ti-5Al-2.5Sn Ti-5Al-6Sn-2Zr-1Mo Ti-6Al-2Sn-4Zr-2Mo Ti-8Al-1Mo-1V
-Transus temperature Specific density Room temperature strength Room temperature toughness Modulus of elasticity Machinability Age hardenability Heat resistance Weldability High-temperature strength Heat-treatability Plastic formability Strain-rate sensitivity Superplastic formability Creep resistance
V, Mo, Nb, Ta, Cr Ti-5Al-2.5Fe Ti-3Al-8V-6Cr-4Mo-4Zr Ti-5Al-2Mo-2Fe Ti-4.5Al-3V-2Mo-2Fe Ti-5Al-3Mo-4Zr Ti-5Al-2Sn-2Zr-4Mo-4Cr Ti-5Al-2.5Fe Ti-6Al-6Fe-3Al Ti-6Al-7Nb Ti-10V-2Fe-3Al Ti-6Al-4V Ti-13V-11Cr-3Al Ti-6Al-6V-2Sn Ti-15V-3Cr-3Al-3Sn Ti-6Al-2Sn-4Zr-6Mo Ti-35V-15Cr Ti-8Mo-8V-2Fe-3Sn Ti-11.5Mo-6Zr-4.5Sn Ti-30Mo, Ti-40Mo Ti-13Nb-13Zr Ti-25Pd-5Cr Ti-20Cr-0.2Sn Ti-30Ta Higher ← Lower Lower → Higher → → ← ← → ← ← ← → → → → ←
Note: This chart does not include TiNi, Ti3Al (2), and TiAl () intermetallic type alloys.
grades 1–4 (unalloyed titanium) allow a hydrogen content up to 0.015 wt.% (i.e., 15 ppm). It was reported that if CpTi contains more than 250 ppm of hydrogen, the material would be susceptible to stress corrosion cracking and hydrogen embrittlement [2-12].
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Grade 1: Grade 1 CpTi is the lowest strength unalloyed titanium with a slightly lower residual content. Both oxygen and iron residuals improve the impact strength. Oxygen acts as an interstitial strengthener, maintaining a single -phase hexagonal closed-packed microstructure. Iron acts as a second phase BCC grain refiner, offering moderate strengthening capabilities. The lower residual content makes grade 1 the lowest strength CpTi grade, but it has the highest ductility, with an excellent cold formability. Grade 2: The grade 2 is the most frequently selected titanium grade in industrial service, having well-balanced properties of both strength and ductility. The strength levels are very similar to those of common stainless steel and its ductility allows for good cold formability. Grade 3: CpTi grade 3 possesses a slightly higher strength due to its slightly higher residual content (primarily oxygen, and also nitrogen) with slightly lower ductility. Grade 4: The grade 4 is the highest strength grade of the CpTi series, so that grade 4 serves mainly in the aerospace/aircraft industry. For all CpTi grades 1–4, the 0.2% off-set equivalent-yield strength (0.2YS) and the ultimate tensile strength (UTS) appear well correlated to oxygen contents [2-13]. Through linear regression analysis, with the oxygen content expressed by [O], it was found that 0.2YS = 1336.2 ⫻ [O] ⫺66.7 in MPa with r (correlation coefficient) of 0.9865, and UTS = 1351.5 ⫻ [O] ⫺3.7 in MPa with r of 0.9946. By far the most widely used of the CpTi grades is grade 2 [2-13]. From this base, the other grades have been developed for better formability or higher strength levels, significantly increasing corrosion resistance at higher temperatures, and/or improving corrosion resistance at lower pH (or higher acidity) levels [2-14]. There is a distinct difference in using CpTi between industry and medicine. Among various applications of CpTi materials in dental and medical fields, the dental CpTi implant is the most frequently and widely employed. Information on CpTi grade selection for dental application indicates that, despite the popularity of grade 2 in industry [2-11], grades 3 and 4 are equally selected in most of the 20 major dental implant systems. Only two companies use grades 1 and 2 [2-15–2-18]. More interesting, however, is their availabilities. In preparing CpTi grades 1–4 for corrosion resistance comparisons, Hernandez [2-15] searched 14 material suppliers, but found that only 2 out of the 14 suppliers were able to provide all four grades. Grades 2 and 4 were available without any problems, whereas CpTi grade 3 is the least popular and difficult to obtain; 2 out of the 14 suppliers were able to provide CpTi grade 3. After conducting electrochemical corrosion testing, using 37°C Ringer’s solution as an electrolyte, it was found that CpTi grade 3 exhibited the least corrosion resistance. Regardless of material availability [2-15, 2-16], this supported the unpopularity of CpTi grade 3. Hence, despite the popularity and availability of grade 2 in industries,
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CpTi grade 2 is the least popular, although its corrosion resistance is somewhat superior to grade 3, which is the worst grade in terms of corrosion resistance among the four grades. 2.2.2 Ti-6Al-4V This alloy belongs to the ⫹ phase alloy group and is particularly popular because of its high corrosion resistance and the reputed low toxicity of ions released from the surface due to dense and protective passive oxide (which is mainly TiO2) film formation. Ti-6Al-4V (which is, in some articles, marked as Ti-6/4) exhibits good mechanical and excellent tissue compatibility properties, which make it well suited for biomedical applications where a bone anchorage is required, particularly for implant applications [2-18]. Ti-6Al-4V ELI is also available and employed in the medical area [2-19]. 2.2.3 Ti-6Al-7Nb As a result of searches for vanadium-free Ti-6Al-4V equivalent alloys, this alloy was developed to enhance the wear resistance [2-20] and castability [2-21]. The optimal composition was found to be Ti-6Al-7Nb (or simply Ti-6/7). This custom-made alloy designed for implants shows the same alpha/beta structures as Ti-6Al-4V and exhibits equally good mechanical properties. The corrosion resistance of Ti-6Al-7Nb in sodium chloride solution was evaluated to be equivalent to that of pure titanium and Ti-6Al-4V, due to formation of a very dense and stable passive layer. Highly stressed anchorage stems of different medical prostheses (including hip, knee, and wrist joints) have been made from hot-forged Ti-6Al-7Nb. The surfaces of these Ti-6Al-7Nb prostheses were further hardened by means of a very hard 3–5 m thick titanium nitride coating or by oxygen diffusion hardening to a depth of 30 m, in order to enhance the biotribological properties [2-22]. 2.2.4 Ti-3Al-2.5V Ti-3Al-2.5V alloy possesses an excellent ductility and cold formability, allowing it to be cold-worked by standard tube-making processes and bent for installation. It is easily welded, and may be heat-treatable to a wide range of strengths and ductility [2-23]. 2.2.5 Ti-5Al-3Mo-4Zr A newly developed titanium for surgical implant application, Ti-5Al-3Mo-4Zr (or simply referred to Ti-5/3/4), was evaluated, and its properties were compared with conventional biomaterials. Mechanical properties and metallic ion elution were also examined. It was reported that the new alloy is advantageous over Ti-6Al-4V
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ELI because it does not contain bio-hazardous alloying elements (e.g., vanadium), and has superior mechanical properties to stainless steel. Corrosion abrasive wear resistance is also improved in Ti-5Al-3Mo-4Zr. As a result, newly designed artificial hip joints were fabricated with this alloy [2-23, 2-24]. 2.2.6 Ti-5Al-2.5Fe In the course of developing a vanadium-free titanium-based alloy, the another new alloy Ti-5Al-2.5Fe (which is an ⫹ phase alloy) has been developed to avoid the presence of toxic vanadium which is an alloying element in the Ti-6Al-4V alloy and may increase to more than 15% in the phase. Hip prostheses and hip prosthesis heads were fabricated from the Ti-5Al-2.5Fe alloy with a grain size of 20 m or less. The frictional biotribological behavior of a hip prosthesis head in contact with an ultrahigh molecular weight polyethylene (UHMWPE) cup was investigated. It was shown that (i) the frictional behavior of a head coated with an oxide layer about 1–3 m thick produced by induction heating of the surface equal to that of a head fabricated from alumina ceramics, and (ii) the oxide layer was still present after 107 cycles in 0.9% NaCl solution, indicating excellent corrosion resistance of this alloy [2-25]. 2.2.7 Ti-Ni Historically, when the US Navy searched for a new submarine material exhibiting good damping capacity, NiTi alloy was developed. Buehler, who conducted the bending tests on NiTi alloys, discovered that a large deformation was completely recovered by a slight heating to return to its original shape. This phenomenon is later referred to as a SME. Since the test was performed and the material was developed at the US Naval Ordnance Laboratory, this unique material is called as NITINOL (NIckel-TItanium Naval Ordnance Laboratory) [2-26]. This material is also often called and marked as NiTi, TiNi or Nitinol. In this book, since the Ni element contains more than 50 wt.% (weight percentage), the term “NiTi” is used according to the ordinal way to describe alloy systems in the conventional metallurgy. It is well known that this alloy exhibits unique phenomena such as SME as well as SE. By SME, a material first undergoes a martensitic transformation. After deformation in the martensitic condition, the apparently permanent strain is recovered when the specimen is heated to cause the reverse martensitic transformation. Upon cooling, it does not return to its deformed shape. When SME alloys are deformed in the temperature regime a little above the temperature at which martensite normally forms during cooling, a stress-induced martensite is formed. This martensite disappears when the stress is released, giving rise to a superelastic
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stress–strain loop with some stress hysteresis. This property applies to the parent phase undergoing a stress-induced martensitic transformation. The other unique property associated with NiTi is SE. When SME alloys are deformed, a superelastic alloy deforms reversibly to very high strains up to 10% by the creation of stressinduced phase. When the load is removed, the new phase becomes unstable and the material regains its original shape. Unlike SME alloy, no change in temperature is needed for the alloy to recover its initial shape. There is a term called pseudoelasticity, which is a more generic term that encompasses both superelastic and rubberlike behavior [2-27]. Because of the uniqueness of SME and the SE associated with NiTi, there have been many applications in both the industrial and dental/medical fields, including dental and orthopedic implants, artificial heart valve, stunt, endodontic files, orthodontic archwires, etc. [2-28–2-33]. Superelastic devices take advantages of their large, reversible deformation and their applications are not limited to medical/dental fields, but can be found in parabola antenna used at the space shuttle, frames for eyeglasses, and sporting goods such as fishing line. 2.2.8 Ti-Cu Aiming for developing an alloy for dental casting with better mechanical properties than unalloyed CpTi, Ti-Cu (Cu: 0.5–10 wt.%) was cast in an argon-arc melting furnace. It was found that (i) the mean tensile strength was significantly higher than for cast CpTi, and (ii) increases of 30% in the tensile strength and yield strengths of 40% over CpTi were obtained for the Ti-5Cu alloy [2-34]. 2.2.9 Ti-Mo TiMo alloy is widely used as an orthodontic archwire for performing an orthodontic mechanotherapy [2-35]. When NiTi wire, TMA (-phase Ti-Mo) wire, and austenitic stainless steel wire were compared, it was found that (i) TMA showed the greatest plastic strain and springback, followed by NiTi and stainless steel, (ii) NiTi wire showed the highest stored energy value in bending-torsion, followed by TMA and stainless steel, and (iii) the highest spring ratio (stiffness) in bendingtorsion was found in stainless steel, followed by TMA and NiTi [2-36]. Lin et al. [2-37] studied alloying effect of Fe on the Ti–Mo alloy, and found that Ti–Mo with iron contents in the range of 2–5 wt.% appeared to have a great potential for use as an implant material. 2.2.10 Ti-5Al-2Mo-2Fe (SP700) Ti-6Al-4V is the most widely used titanium alloy. However, Ti-6Al-4V still suffers from limited applications in non-aerospace fields compared to CpTi, aluminum
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alloys, and steels. The reasons for this include (1) poor hot-workability, with a normal hot-working temperature range that is too high and too narrow, (2) poor coldworkability that virtually prohibits cold-working, (3) poor hardenability that is frequently inconsistent, and (4) a high superplastic forming temperature of around 900oC that results in rapid die wear. To overcome these disadvantages associated with Ti-6Al-4V, a new beta-rich alpha-beta duplex phase titanium alloy has been developed (called SP700, since the newly developed Ti alloy remarkably exhibits high superplastic capability at operating temperatures around 700oC). It was reported that SP700 provides excellent hot- and cold-workability, as well as greatly improved superplastic forming characteristics (i.e., the strain rate sensitivity exponent, m-value, is about 1.0, indicating that flow is close to that of a Newtonian viscous flow), high strength, and high toughness. Dental denture bases have been fabricated using this SP700 alloy by superplastic forming technique [2-38]. 2.2.11 Other Ti-based alloys An interesting study was done by Brème et al. [2-39], using isoelastic porous sintered Ti-30Ta alloy and titanium wire loop, to accomplish the mechanical compatibility in endosseous dental implant systems. Their mechanical properties were optimized by the production parameter such as sintering and diffusion bonding. The functionality was tested after insertion into an artificial jaw, which had properties corresponding to the natural mandibular. It was reported that the elastic properties of both implants are similar to the properties of the bone, and the implant has a safe anchorage bone ingrowth [2-39]. Qazi et al. [2-6] developed metastable -Ti alloy (mainly Ti-35Nb-7Zr-5Ta) and solution-treated, followed by aging at 482oC. It was reported that (i) the heattreated Ti-35Nb-7Zr-5Ta alloys exhibited 0.2% off-set yield strength of 1300 MPa with 8% elongation, due to ⫹ and ⫹⫹-phase precipitations, and (ii) this enhanced strength makes the alloy candidates for bone plates and screws. 2.2.12 Intermetallic Alloys Although not ordinary alloys, titanium aluminide-based alloys, including TiAl (-phase) and Ti3Al (2 phase), are used for high temperature applications. The titanium aluminide-based alloys provide an inherently low-density material and exhibit excellent creep-rupture properties [2-13, 2-40]. TiAl amorphous alloy provides high strength, linear elastic behavior, and the infinite fatigue life necessary for high device reliability. Tregilgas [2-41] developed amorphatized TiAl alloys for a material for the digital micromirror device chip. In order to enhance the wear resistance of -TiAl (Ti-47Al-2Nb) alloy, the surface of this alloy was modified in a nitrogen-ion plasma atmosphere [2-42].
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REFERENCES
[2-1] Collings EW. The physical metallurgy of titanium alloys. Columbus, OH: The American Society for Metals, 1984. pp. 3–5. [2-2] Ivansishin OM, Markovsky PE. Enhancing the mechanical properties of titanium alloys with rapid heat treatment. J Metals 1996;48:48–52. [2-3] Eylon D, Vassel A, Combres Y, Boyer RR, Bania PJ, Schutz RW. Issues in the development of beta titanium alloys. J Metals 1994;46:14–15. [2-4] Niiomi M. Recent metallic materials for biomedical applications. TMS Metallurgical and Materials Transactions 2002;33A;477–486. [2-5] Jha SK, Ravichandran KS. High-cycle fatigue resistance in beta-titanium alloys. J Metals 2000;52:30–35. [2-6] Qazi JI, Marquardt B, Rack HJ. High-strength metastable beta-titanium alloys for biomedical applications. J Metals 2004;56:49–51. [2-7] Niiomi M. Fatigue performance and cyto-toxicity of low rigidity titanium alloy, Ti-29Nb-13Ta-4.6Zr. Biomaterials 2003;24:2673–2683. [2-8] Niiomi M, Kuroda D, Fukunaga K, Morinaga M, Kato Y, Yashiro T, Suzuki A. Corrosion wear fracture of new type biomedical titanium alloys. Mat Sci Eng A 1999;A263:193–199. [2-9] ASTM designation F2066-01. Standard specification for wrought titanium15 molybdenum alloy for surgical implant applications. Philadelphia PA: American Society for Testing and Materials. 2001. pp. 1605–1608. [2-10] ASTM designation draft #3. Standard specification for wrought titanium35Niobium-7Zirconium-5Tantalum alloy for surgical implant applications (UNS R58350). Philadelphia, PA: American Society for Testing and Materials. 2000. [2-11] American Society for Testing and Materials designation draft #6. Standard specification for wrought titanium-3Aluminum-2.5Vanadium alloy for surgical implant applications (UNS R58350). Philadelphia PA, ASTM, 2000. [2-12] Paton NE, Williams JC. Effect of hydrogen on titanium and its alloys. In: American society for metals. Hydrogen in metals. 1974. pp. 409–432. [2-13] Titanium and Titanium Alloys – Source Book. Donachie MJ, editor. Metals Park, OH: American Society for Metals. 1982. [2-14] Mountford JA Jr. Titanium alloys for the CPI (Chemical Process Industry). Adv Mater Processes 2001;159:55–58. [2-15] Hernandez FE. Corrosion behavior for commercially pure titanium with different grades and from different material suppliers. Indiana University Master Degree Thesis, 2003. [2-16] Lim WC. Comparison of electrochemical corrosion behavior among four grades of commercially pure titanium. Indiana University Master Degree Thesis, 2001. [2-17] Binon PP. Implants and components: entering the new millennium. Int J Oral Maxillofac Implants 2000;15:76–94.
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