Titanium and its alloys

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Titanium and Its Alloys

(Ti)

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Titanium is a lustrous transition metal (lllB)

with a silver color, low density and high strength has and Z=22 and A=48

Pure titanium melts at 1670oC and has a density of 4.51 g /cm3. It is fairly abundant in nature, constituting about 1% of Earth’s crust (the 9th abundant element 0.86%).

It is highly resistant to corrosion in sea water, aqua regian and chlorine.

Ti

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Titanium is produced from TiO2 by the Kroll

process. The TiO2 is converted to TiCl4 (titanium

tetrachloride, also informally known as tickle), which is subsequently reduced to titanium metal by sodium or magnesium.

TiO2+2Cl2+C→TiCl4+CO2

TiCl4+4Na→Ti+4NaCl

Production

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The resultant titanium sponge is then

consolidated, alloyed as necessary and processed using vacuum arc melting.

Con…

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The principal ores of titanium are rutile, which

is 98% to 99% TiO2, and ilmenite, which is a combination of FeO and TiO2.

Rutile is preferred as an ore because of its higher Ti content.

In recovery of the metal from its ores, the TiO2 is converted to titanium tetrachloride (TiCl4) by reacting the compound with chlorine gas.

Ore

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This is followed by a sequence of distillation

steps to remove impurities. The highly concentrated TiCl4 is then reduced to

metallic titanium by reaction with magnesium; this is known as the Kroll process.

Sodium can also be used as a reducing agent. In either case, an inert atmosphere must be maintained to prevent O2, N2,or H2 from contaminating the Ti, owing to its chemical affinity for these gases.

Con…

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The resulting metal is used to cast ingots of

titanium and its alloys.

Con…

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Ti ore

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Ti is stiffer and stronger than aluminum Commercially pure titanium has density 4.51g/cm3

Ti’s coefficient of thermal expansion is relatively low among metals (Alloy Ti-6Al-4V 8.6*10-6(oc)-1)

It retains good strength at elevated temperatures Pure titanium is reactive which presents problems in

processing, especially in the molten state

Property

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Pure titanium has excellent resistance to

corrosion because it forms a thin adherent oxide coating (TiO2) and is used widely in the chemical industries.

Titanium alloys are considered biocompatible and bioactive.

Con…

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In the commercially pure state, Ti is used for

corrosion resistant components, such as marine components and prosthetic implants.

Titanium alloys are used as high- strength components in temperatures ranging 25oc -550oc, especially where its excellent strength to weight ratio is exploited. E.g aircraft and missile components.

These properties give rise to two principal application areas for titanium:-

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The crystal structure of titanium at ambient

temperature and pressure is close packed hexagonal (α) with a c/a ratio of 1.587.

Crystal structure

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Slip is possible on the pyramidal, prismatic

and basal planes in the close packed directions.

The coordination number and the atomic packing factor for the HCP crystalstructure are the same as for FCC: 12 and 0.74, respectively.

Con…

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At about 890oC, the titanium undergoes an

allotropic transformation to a body centered cubic β-phase which remains stable to the melting temperature.

Alloying of Ti All elements within the range 0.85–1.15 of the

atomic radius of titanium alloy substitution alloy and have a significant solubility in titanium.

Elements with an atomic radius less than 0.59 that of Ti occupy interstitial sites.

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The ease with which solutes dissolve in titanium makes

it difficult to design precipitation–hardened alloys. Titanium is allotropic, with the HCP crystal structure (α)

at low temperatures and a BCC structure (β) above 882°C.The four forms of titanium are:-

1. Commercially pure Ti: 99.5% Ti or 99.0% Ti2. Alpha Ti alloys: 5% Al-2.5% Sn3. Beta Ti alloys: 13% V-11% Cr-3% Al4. Alpha-beta Ti alloys: 6% Al-4% V

Alloys

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Thus, Al, O, N and Ga are all α–stabilizers.

Because they increase the temperature at which α transforms to β.

Mo, V, W and Ta are all β–stabilizers. Since they lower the transformation temperature, even causing β to be stable at room temperature.

Mn, Cr, and Fe produce a eutectoid reaction, reducing the temperature at which the transformation occurs and producing a two phase structure at room temperature.

The alloying elements can be categorized according to their effect on the stabilities of the

α and β phases.

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Molybdenum and vanadium have the largest influence on β stability and are common alloying elements. Tungsten is rarely added due to its high density. Cu forms TiCu2 which makes the alloys age hardening and heat treatable; such alloys are used as sheet materials. It is typically added in concentrations less than 2.5 wt% in commercial alloys.

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Hydrogen is the most important interstitial. Titanium is capable of absorbing up to 60 at% of

hydrogen, which can also be removed by annealing in a vacuum which provides a combination of high ductility, uniform properties, and good strength.

Hydrogen enters the tetrahedral holes which are larger in BCC than HCP since BCC Ti has three octahedral interstices per atom whereas HCP Ti has one per atom. Thus the solubility of hydrogen is larger in β.

Interstitials

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o The niobium is added for oxidation resistanceo The carbon to allow a greater temperature range over

which the alloy is a mixture of α+β, in order to facilitate thermo mechanical processing.

o This particular alloy is used in the manufacture of aero engine discs.

Specific–alloys

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Al reduces density, stabilizes and strengthens

α while vanadium provides a greater amount of the more ductile β phase for hot–working.

One difficulty with the β phase, which has a body centered cubic crystal structure is, it has a ductile–brittle transition temperature. The transition temperature tends to be above room temperature, with cleavage fracture dominating at ambient temperatures.

Con…

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Titanium can catch fire and cause severe

damage in circumstances where it rubs against other metals at elevated temperatures.

This is what limits its application in the harsh environment of aero engines, to regions where the temperature does not exceed 400 ◦C.

But the addition of chromium in concentrations exceeding 10 wt% helps improve the burn–resistance of titanium alloys.

Fire

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Consequences of a titanium fire in an aero

engine.

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Titanium and its alloys are relatively new

engineering materials that possess an extraordinary combination of properties. So they have a wide application in real life.

Some of them are:-In components which operate at elevated temperatureIn the aerospace industries

Application areas

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In the petroleum and chemical industriesCar suspension springs

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Heat Exchanger materialsDie-cast parts for automobiles, luggage, and electronic devicesbiomedical implants such as hip prostheses

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Marine componentsAirplane structures and Forgings of maximum strength for aircraft.

Con…

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Titanium and its alloys are similar in strength to

moderate-strength steel but weigh half as much as steel. The material exhibits very good resistance to corrosion,

has low thermal conductivity, is nonmagnetic, and has high-temperature strength.

Its modulus of elasticity is between those of steel and aluminum at 16.5 Mpsi (114 GPa).

Because of its many advantages over steel and aluminum, applications include: aerospace and military aircraft structures and components, marine hardware, chemical tanks and processing equipment, fluid handling systems, and human internal replacement devices.

Summary

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The disadvantages of titanium are:-

Its high cost compared to steel and aluminum andThe difficulty of machining it.

Drawback

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4 UR PATIENCE.

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