Oshida Y. Bioscience and Bioengineering of Titanium Materials
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chemical, biochemical, electrochemical, thermal, mechano-chemical, biological,
etc.), types of application, fabrication technologies, the surface/interface charac-
terization of implants, and new and advanced materials and technologies for
dental/medical titanium will be covered.
REFERENCES
[1-1] http://www.scescye.net/~woods/elements/titanium.
[1-2] http://en.wikipedia.org/wiki/Titanium.
[1-3] Lautenschlager EP, Monaghan P. Titanium and titanium alloys as dental
materials. Int Dent J 1993;43:245–253.
[1-4] Moore BK, Oshida Y. Materials science and technology in dentistry. In:
Encyclopedic handbook of biomaterials and bioengineering. Wise DL,
Trantolo DJ, Altobelli DE, Yaszemski MJ, Gresser JD, Schwartz ER, edi-
tors. New York: Marcel Dekker. 1995. pp. 1325–1430 (Chapter 48).
[1-5] Allen P. Titanium alloy development. Adv Mater Process 1996;154:35–37.
[1-6] Tuominen S, Wojcik C. Alloys for aerospace. Adv Mater Process 1995;
153:3–26.
[1-7] Ratner BD, Johnston AB, Lenk TJ. Biomaterials surfaces. J Biomed Mater
Res 1987;21:59–90.
[1-8] Loiacono C, Shuman D, Darby M, Luton JG. Lasers in dentistry. Gen Dent
1993;Sep/Oct:378–381.
[1-9] Singh J. The constitution and microstructure of laser surface-modified
metals. J Metals 1992;34:551–562.
[1-10] Froes FH. Titanium – Is the time now? J Metals 2004;56:30–31.
[1-11] Faller K, Froes FH. The use of titanium in family automobiles: current
trend. J Metals 2001;53:27–28.
[1-12] Kosaka Y, Fox SP, Faller K. Newly developed titanium alloy sheets for the
exhaust systems of motorcycles and automobiles. J Metals 2004;56:32–34.
[1-13] Kearns M. Titanium: alive, well and booming! Adv Mater Process
2005;163:63–64.
[1-14] Roncone K. A conversation with titanium suppliers and end users. J Metals
2005;57:11–13.
[1-15] Niionomi M, Hanawa T, Narushima T. Japanese research and development
on metallic biomedical, dental, and healthcare materials. J Metals 2005;57:
18–24.
[1-16] Oshida Y, Hashem A, Nishihara T, Yapchulay MV. Fractal dimension analy-
sis of mandibular bones – toward a morphological compatibility of
implants. J Bio-Med Mater Eng 1994;4:397–407.
[1-17] Kasemo B, Lausmaa J. Surface science aspects on inorganic biomaterials.
CRC Crit Rev Biocompat 1986;2:335–380.
[1-18] Smith DC. The biocompatibility of dental materials. In: Biocompatibility
of dental materials, Vol. 1: Characteristics of dental tissues and their
response to dental materials. Smith DC, Williams DF, editors. Boca Raton:
CRC Press Inc. 1982. pp. 1–37.
8 Bioscience and Bioengineering of Titanium Materials
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Chapter 2
Materials Classification
2.1. General 11
2.2. Medical/Dental Titanium and its Alloys 13
2.2.1 Commercially Pure Titanium (CpTi) 13
2.2.2 Ti-6Al-4V 16
2.2.3 Ti-6Al-7Nb 16
2.2.4 Ti-3Al-2.5V 16
2.2.5 Ti-5Al-3Mo-4Zr 16
2.2.6 Ti-5Al-2.5Fe 17
2.2.7 Ti-Ni 17
2.2.8 Ti-Cu 18
2.2.9 Ti-Mo 18
2.2.10 Ti-5Al-2Mo-2Fe (SP700) 18
2.2.11 Other Ti-Based Alloys 19
2.2.12 Intermetallic Alloys 19
References 20
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11
Chapter 2
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 strate-
gic 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 med-
ical applications (orthopedic implants, stunt, Harrington bar, and others) and den-
tal 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 seeds-
oriented 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: 885
o
C 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 high-
temperature performance. Alloying elements which decrease this phase transfor-
mation 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
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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 appli-
cations, too. Unlike beta alloys, alpha alloys cannot be strengthened by heat treat-
ment 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-4V
1
. Within the ⫹ type class, an alloy con-
taining 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 treat-
ment 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 ele-
ments, such as V, Mo, Nb, Ta, and Cr, which tend to reduce the temperature of the
↔ phase transformation (or simply -transus). They have excellent forgeabil-
ity over a wider range of forging temperature than alloys, and -alloy sheet is
cold formable in the solution-treated condition. Beta alloys have excellent work-
hardening 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
12 Bioscience and Bioengineering of Titanium Materials
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 W
x
and A
x
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 ele-
ments, conversion from w/o to a/o and from a/o to w/o is calculated as follows:
A
x
⫽ 100/{1⫹(X/Y)[(100/W
x
) ⫺ 1]} and A
y
⫽ 100 ⫺ A
x
W
x
⫽ 100/{1⫹(Y/X)[(100/A
x
) ⫺ 1]} and W
y
⫽ 100 ⫺ W
x
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of processing, properties, and potential applications. This group of alloys (includ-
ing 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 stabi-
lizers. Depending on the ratio of and phases, they can be furthermore sub-
grouped 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-8V-
5Fe, 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-1Cr-
1Fe-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 spe-
cific strengths and corrosion resistance, and the best biocompatibility character-
istics 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 Ti-
35Nb-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 spec-
ified <0.10 wt.% (wt.% or w/o) and H is also specified <0.015 wt.%. ASTM CpTi
Materials Classification 13
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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 hydro-
gen, the material would be susceptible to stress corrosion cracking and hydrogen
embrittlement [2-12].
14 Bioscience and Bioengineering of Titanium Materials
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 V, Mo, Nb, Ta, Cr
Typical materials Commercially pure Ti Ti-5Al-2.5Fe Ti-3Al-8V-6Cr-4Mo-4Zr
Ti-5Al-2.5Sn Ti-5Al-2Mo-2Fe Ti-4.5Al-3V-2Mo-2Fe
Ti-5Al-6Sn-2Zr-1Mo Ti-5Al-3Mo-4Zr Ti-5Al-2Sn-2Zr-4Mo-4Cr
Ti-6Al-2Sn-4Zr-2Mo Ti-5Al-2.5Fe Ti-6Al-6Fe-3Al
Ti-8Al-1Mo-1V 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
-Transus temperature Higher ← Lower
Specific density Lower → Higher
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 ←
Note: This chart does not include TiNi, Ti
3
Al (2), and TiAl () intermetallic type alloys.
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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 indus-
trial 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 lev-
els, 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 vari-
ous 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 inter-
esting, 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,
Materials Classification 15
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CpTi grade 2 is the least popular, although its corrosion resistance is somewhat supe-
rior 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 TiO
2
) 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 avail-
able 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 cus-
tom-made alloy designed for implants shows the same alpha/beta structures as
Ti-6Al-4V and exhibits equally good mechanical properties. The corrosion resis-
tance 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 pas-
sive layer. Highly stressed anchorage stems of different medical prostheses (includ-
ing 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
16 Bioscience and Bioengineering of Titanium Materials
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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 artifi-
cial 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 pros-
thesis 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 10
7
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 exhibit-
ing good damping capacity, NiTi alloy was developed. Buehler, who conducted
the bending tests on NiTi alloys, discovered that a large deformation was com-
pletely recovered by a slight heating to return to its original shape. This phenom-
enon 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 con-
ventional 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 recov-
ered 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 marten-
site normally forms during cooling, a stress-induced martensite is formed. This
martensite disappears when the stress is released, giving rise to a superelastic
Materials Classification 17
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