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Titanium and its alloys

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Introduction Titanium appears on the periodic table as element 22, a fourth-row transition metal with an atomic weight of 47.88. An extremely reactive metal, titanium forms a tenacious oxide layer that contributes to its electrochemical passivity.

Titanium is the ninth most abundant element and is plentiful in the Earth's crust.

Not found in its free, pure metal form in nature but as oxides, i.e., ilmenite (FeTiO3) and rutile (TiO2)

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Ilminite ( FeTiO3 ) Lutile (TiO2 )

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History The element was discovered by Wilheim Gregor, a clergyman, who found the metal in a "black magnetic sand" in Cornwall in 1791.

Three years later, Klaproth found a rutile that was the oxide of a new metal he named titanium, after the Greek Titans. He recognized that this metal was identical to the material Gregor had discovered

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Extraction of titanium Titanium ore– rutile (TiO2) is converted into titanium sponge by KROLL’S PROCESS :

1) Passing Cl2 gas through charge the ore, resulting in colourless. titanium tetrachloride TiCl4.

2) TiCl4 is purified by fractional distillation.

3) The liquid form of TiCl4 is reacted with either Mg or Na under an inert (Ar)atmosphere to obtain titanium sponge while Mg or Na is recycled.

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Kroll’s process :

Tio2 + 2 cl2 + c = Ticl4 + co2

2Mg (l) + TiCl4(l) = 2MgCl2 (l) + Ti (s)

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Advantages

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Classification of titanium alloys ASTM International (the American Society for Testing and Materials) recognizes 4 grades of commercially pure titanium, or Ti, and 3 titaniumalloys (Ti-6Al-4V, Ti-6Al-4V Extra Low Interstitial [low components] and Ti-Al-Nb).

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Physical propertiesIt rapidly form a tenacious oxide that is responsible for the metal's biocompatibility.

At temperatures up to 882ºC, pure titanium exists as a hexagonal close-packed atomic structure (alpha phase).

Above that temperature, the structure is body-centered cubic (beta phase).

The metal melts at 1,665°C.

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Advantage of oxide layer

The metal oxidizes almost instantaneously in air to form a tenacious and stable oxide layer approximately 10 nanometers thick.

This oxide layer provides a highly biocompatible surface and a corrosion resistance similar to that of noble metals.

In addition, the oxide layer allows for bonding of fused porcelains, adhesive polymers or in the case of endosseous implants, plasma-sprayed or surface-nucleated apatite coatings.

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Mechanical propertiesThe characteristic trend of increasing strength with relatively constant modulus continues when comparing cp titanium with titanium alloys.

The elastic modulus of the alloys is slightly higher (113 MPa compared with 104 MPa of cp grade IV titanium), but the yield strength increases over 60% to 795 MPa for ELI (Extra Low Interstitial) alloys and 860 MPa for Ti-6Al-4V alloys.

Typically, fatigue strength limits are less than 50% of the ultimate tensile stress.

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Compared with Co-Cr-Mo alloys, titanium alloy is almost twice as strong and has half the elastic modulus.

Compared with 316L stainless steel, the Ti-6Al-4V alloy is roughly equal in strength, but again, it has half the modulus.

Strength is beneficial because materials better resist occlusal forces without fracture or failure.

Lower modulus is desirable because the implant biomaterial better transmits forces to the bone.

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Titanium grade 1 has the highest purity, lowest strength, and best room-temperature ductility of the four ASTM titanium unalloyed grades.

Grade 2 titanium is the main cp Ti used for industrial dental implant applications. The guaranteed minimum yield strength of 275 MPa for grade 2 is comparable to those of annealed austenitic stainless steels.

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Titanium grade 3 has 0.30 maximum iron content, which is lower than grade 4 (0.50 maximum).

Grade 4 has the highest strength of the unalloyed ASTM grades.

Grade 5, an ASTM titanium alloy (Ti-6Al-4V), is the most widely used titanium alloy in medical implants but not common in dental implants. The alloy is most commonly used in the annealed state.

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Density 4.5g/cm³ (considerably less than gold or Ni-Cr or Co-C r alloys)

Because of the light weight of the titanium and its strength-to-weight ratio, high ductility, and low thermal conductivity would permit design modifications in Ti restorations and removable prostheses, resulting in more functional and comfortable use.

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Corrosive behaviourTitanium is corrosive under mechanical stress , oxygen deficit, or at a low ph level.

Flouride reveals a high affinity to titanium, & Flouride ions can destabilize the oxide layer.

The self formed protective oxide film on titanium can be affected by excessive use of the commonest preventive agents in dentistry, prophylactic polishing and topical fluoride applications.

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Cp titanium Commercially pure Ti is available in four grades, which vary according to the oxygen (0.18 to 0.40 wt%) and iron (0.20 to 0.50 wt%) content.

These apparently slight concentration differences have a substantial effect on the physical and mechanical properties.

At room temperature, c.p. Ti has an HCP crystal lattice, which is denoted as the alpha (α) phase. On heating, an allotropic phase transformation occurs. At 883º C, a body-centered cubic (BCC) phase, which is denoted as the beta (β) phase, forms.

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Ti-6Al-4V

Several alloys of titanium are used in dentistry. Of these alloys, Ti 6Al-4V is the most widely used.

At room temperature, Ti-6Al-4V is a two-phase α+β alloy.

At approximately 975°C, an allotropic phase transformation takes place, transforming the microstructure to a single phase BCC β-alloy.

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Composed of 90% Ti , 6% Al , 4% V .

Aluminium decreases specific weight and improves elastic modulus

Vanadium decreases thermal conductivity and increases hardness.

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The alloys most commonly used for dental implants are of the alpha-beta variety.

Of these, the most common contains 6% aluminum and 4% vanadium (Ti-6Al-4V).

After heat treatment these alloys possess many favorable physical and mechanical properties that make them excellent implant materials.

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Phase transformationAllotropic transformation

Alpha phase HCP structure

βphaseBCC structure

882.3 ºC

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Titanium alloys of interest to dentistry exist in three forms: alpha, beta, and alpha-beta.

These types originate when pure titanium is heated, mixed with elements such as aluminum and vanadium in certain concentrations, and then cooled.

This treatment produces true solid solutions.

These added elements are said to act as ‘phase-condition stabilizers.’

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Aluminum has been called an alpha-phase condition stabilizer.

Aluminum also serves to increase the strength and decrease the weight of the alloy. Vanadium has been called a beta-phase stabilizer.

As aluminum or vanadium is added to Ti the temperature at which the alpha- to-beta transformation occurs changes to a range of temperatures.

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They are light, strong, and highly resistant to fatigue and corrosion.

Although they are stiffer than bone, their modulus of elasticity (stiffness) is closer to bone than any other important implant metal; the only exception is pure Ti.

This property leads to a more even distribution of stress at the critical bone-implant interface because the bone and implant will flex in a more similar fashion.

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Titanium castabilityThe two most important factors in casting titanium-based materials are its high melting point (= 1700º C for c.p. Ti) and chemical reactivity.

Because of the high melting point, special melting procedures, cooling cycles, mold material, and casting equipment to prevent metal contamination are required.

Titanium readily reacts with gaseous elements such as hydrogen, oxygen, and nitrogen, particularly at high temperatures (>600ºC).

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As a result, any manipulation of titanium at elevated temperatures must be performed in a well-controlled vacuum. Without a well-controlled vacuum, titanium surfaces will be contaminated with α case, an oxygen enriched and hardened surface layer, which can be as thick as 100 µm.

Because of the high affinity titanium has for hydrogen, oxygen, and nitrogen, standard crucibles and investment materials cannot be used.

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Investment materials must have oxides that are more stable than the very stable titanium oxide, and must also be able to withstand a temperature sufficient to melt titanium.. If this is not the case, oxygen is likely to diffuse into the molten metal.

Investment materials such phosphate-bonded silica and phosphate investment materials with added trace elements achieve this goal.

It has been shown that with magnesium oxide-based investments, internal porosity results.

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Advanced casting techniques, which combine centrifugal, vacuum, pressure, and gravity casting, new investment materials, and advanced melting techniques (e.g., electric arc melting) have been developed.

By alloying titanium, the melting temperature can be lowered to the same temperature as that of nickel-chromium and cobalt-chromium alloys. For example, the Ti-Pd and Ti-Cu alloys have melting points of 1350º C.

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Uses Today, Titanium And Titanium Alloys Are Used For :

The Fabrication of Prosthetic Joints,

Surgical Splints,

Stents And Fasteners,

Dental Implants,

Dental Crowns And Partial Denture Frameworks

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Artificial hip joints,

Artificial knee joints,

Bone plates,

Screws for fracture fixation,

Cardiac valve prostheses,

Pace - makers, and artificial hearts.

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Bone conduction hearing aids are anchored with titanium connected to the middle ear.

Shoulder and elbow joint implants.

In general stronger beta alloys are used in low modulus applications.

And alpha-beta alloys are used in high modulus applications ( bone plate )

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Ti-6Al-4V ( eli ) - alpha beta alloy – orthopedic

surgery.

Ti-5Al-2.5Fe - used for heart pacemaker electrodes

Ti-6Al-7Nb - copings

Ti-13Nb-13Zr – orthopedic implants

Ti-15Mo-3Nb - orthopedic implants

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Ti-6Al-4V has long been a main medical titanium alloy. However, for permanent implant applications the alloy has a possible toxic effect resulting from released vanadium and aluminum.

For this reason, vanadium- and aluminum-free alloyshave been introduced for implant applications, based on the Ti-6Al-4V implants.

These new alloys include Ti-6Al-7Nb (ASTM F1295), Ti-13Nb-13Zr (ASTM F1713), and Ti-12Mo-6Zr (ASTM F1813).

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The main physical properties of titanium responsible for the biocompatibility are:

low level of electronic conductivity,

high corrosion resistance,

thermodynamic state at physiological pH values,

low ion-formation tendency in aqueous

environments,

an iso-electric point of the oxide of 5–6.

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Applications in dentistry

Titanium and its alloys are also used for dentistry devices such as :

Implants ,

Crowns , bridges,

Overdentures , and

Dental implant prosthesis components (screw and

abutment).

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Dental implantsThere are three types of dental implant:

Osseo integrated ,

Mini -implant for orthodontic anchorage,

and zygomatic.

Each group needs different mechanical properties and must be made of cp Ti or a titanium alloy.

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Osseo integrated implant

The osseointegration of dental implants was initially defined by Branemark et al. as a direct bone-to-implant contact and later on defined on a more functional basis as a direct bone-to-implant contact under load.

In the past, osseointegrated endosseous dental implants have been made in a variety of shapes, including hollow baskets, blades, tripods, needles, disks, truncated cones, cylinders, and screws.

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Currently the most commonly used dental implant has a screw shape and is made of cp Ti or Ti-6Al-4V

The dental implants are available with diameters from 3.3 mm to 6.0 mm and lengths from 6 mm to 16 mm.

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Long term placement of implant revealed higher levels of the component elements ( especially vanadium ) can be detected in tissues locally and systemically.

Therefore vanadium and aluminium free alloys have been developed.

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Orthodontic mini-implant

Another dentistry implant is a temporary orthodontic mini-implant used generally to secure anchorage in contemporary orthodontic treatments.

This implant has a small diameter (1.2 mm to 2.0 mm) and the orthodontic load can deform the mini-implant.

the orthodontic implants are made with Ti-6Al-4V instead of cp Ti due to the alloy’s superior strength

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Zygomatic fixture The third dentistry implant group is the zygomatic implants, which are made of cp Ti.

The zygomatic implant developed by Branemark as been used as posterior anchorage for implant supported prostheses in patients with atrophic maxillae.

The zygomatic implant has a diameter equal to 4–5 mm and 30–53 mm length.

It penetrates the maxilla at the second premolar region as close to the alveolar crest as possible

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Titanium in complete denture

Complete denture with titanium framework ( Journal of Oral Rehabilitation 27; 131 – 135 )

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Titanium in RPD

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Titanium in maxillofacial prosthetics

Cranial prosthesis with titanium mesh

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Titanium ceramicBiological advantages of titanium comparing it to metal coping are :

1) good biocompatability due to formation of oxide layer.2) low specific weight.3)low corrosion activity.4) low thermal conductivity5)forms a bacteriostatic gel in combination with gingival fluid.

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Fabrication of a titanium dental coping using powder metallurgy

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Titanium powder was pressed at 300 to 900 MPa, milled to the right shape, and sintered at 1,200°C for a 2-hour holding time.

The copings were silvery bright in appearance and showed sharp edges.

The titanium in the sintered copings was ductile, dense (97% to 99%+), and free from open porosity after sintering.

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IMPROVING THE TITANIUM CERAMIC BOND

Firing porcelain in a reduced argon atmosphere significantly improved titanium-ceramic bonding for machined and as-cast titanium.

The sputter-coated gold layer on titanium provided improved titanium-ceramic bonding only when combined with firing porcelain in reduced argon atmosphere.

When porcelain was fired in vacuum in the presence of the gold layer, the titanium-ceramic bonding was weakened in as-cast titanium and was not affected in machined titanium.

(J Prosthet Dent 2003;90:10-17.)

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Sputter coating and electroplating coating of chromium forms protective oxides .

Surface treatment using either airborne-particle abrasion or bonding agent alone enhanced the bond strength of cast commercially pure titanium to low-fusing porcelain.

The combination of airborne-particle abrasion and bonding agent provided the greatest improvement in titanium-ceramic bond strength.

(J Prosthet Dent 2005;94:350-6.)

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According to study conducted on various titanium alloys by Koichi Akagi, and Yosbizo Okamoto,

10Ti-Ir is considered to sufficiently fulfill the conditions of a titanium metal/ceramic alloy that can be cast in the ordinary atmosphere using a high frequency centrifugal casting machine and a ceramic crucible.

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Cytotoxicity of titanium

According to Li etal elemental metal powders of Ti and Nb showed substantial cytotoxicity, but the bulk Ti and Nb showed biocompatibility.

( J Dent Res 89(5):493-497, 2010 )

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