Metallurgy of Titanium and its Alloys: Applications

H. K. D. H. Bhadeshia


In a conventional turbine compressor, the blades are manufactured separately from the disc, and then mechanically assembled together. The roots of the blades are located in machined slots on the compressor disc. There is therefore a discontinuity between each blade and the disc. Bladed discs of this kind can be heavy because of the need to support the weight of the blade root.

In an alternative scenario, the blades and the disc form a single metallic component with no discontinuities. The blades are joined to the disc at a metallic bond. This arrangement is called a 'blisk'. The joining process is linear friction welding, in which the blade is rubbed (oscillated) against the disc whilst a load applied normal to the junction, until a joint is achieved. The amplitude of the oscillation is typically 1-3 mm at a frequency of 25-125 Hz. The friction generates heat which locally softens the material at the weld interface, allowing some of it to be expelled as flash until the components are brought into their design geometry.

Because of the lack of mechanical fixtures, the blisk is considered to be a lighter variant of the bladed disc, with potential weight savings of some 30%.

The photograph shows a Ti-6Al-4V blisk manufactured for the Joint Strike Fighter project. The photograph has kindly been provided by Stephen Hill of Rolls-Royce.


Blings are a development from blisks, in which the blades are integral with a ring (disc with the centre missing). Brums are assemblies of blings, in the form of a drum.


An internal combustion engine generally uses a stoichiometric ratio of air to fuel. A turbocharger is a device to force more air into the engine, allowing a correspondingly greater quantity of fuel to be burned in each stroke. This boosts the power output of the engine.

The turbocharger consists of two components, a turbine which is driven by exhaust gases from the engine. This in turn drives an air pump which forces more air into the engine. The typical rate of spin is 100,000-150,000 rotations per minute. Because the turbocharger is driven by engine-exhaust gases, it gets very hot and needs to be oxidation resistant and strong.


Turbocharger made by Daido Steel of Japan, from TiAl intermetallic compound. Produced by precision casting.

The advantage of TiAl turbochargers is their low density, but they do cost more than equivalent chargers made from the nickel based superalloys. Both the oxidation resistance and the high-temperature strength can be improved by adding 7 at.% of niobium and small concentrations of silicon and nickel.

Aeroengine Fan Blades

A jet engine produces thrust by giving a large acceleration to a relatively small weight of air. During operations, the fan at the front rotates at about 7500 r.p.m. This sucks air into the engine. As it passes through the engine it is compressed, mixed with fuel and ignited. The resulting expansion causes the gas to exit the jet at a high velocity, giving the required thrust.

The diagram to the right shows the temperature, pressure and velocity distribution of the gas inside a typical large aeroengine.

aeroengine aeroengine

The entry end of a modern aeroengine is huge (compare against the size of the truck). The fan blades are therefore also very large.

Given the large size and the associated momentum when the engine is spinning, it is imperative that the weight of these blades should be as small as possible. They are therefore made of Ti-6Al-4V. To achieve a further reduction is weight, the blades are made hollow, in the case of Rolls-Royce using a diffusion bonding and superplastic forming process.

Given the energy associated with spinning fan blades, great care is taken to ensure that the failure of the blade during service is contained with the engine, i.e., does not lead to damage of the air frame. Movie showing an aeroengine containment test.


More photographs of fan blades.

Titanium Oxide

Titanium is found mostly as a binary oxide (rutile, TiO2) or a ilmenite (FeO.TiO2). The latter can be processed to give pure TiO2.

95% of the total use of titanium is in the form of its oxide TiO2, which because of it brilliant white colour has major applications in the paint (59%), paper (20%) and plastics (12%) industries. The opacity of the oxide is important in paint applications since a thin layer can hide the substrate.

Titanium in Medical Applications

Titanium alloys can have a combination of strength and biocompatibility which makes them suitable for medical applications. Ti-6Al-7Nb has an ultimate tensile strength of about 1000 MPa. Its metallurgy is similar to that of Ti-6Al-4V, with an α+β microstructure, but does not contain vanadium to make it more biocompatible. Vanadium oxide (VO2) generated by passivation is unstable, resulting in a release of vanadium into the body. Niobium is also currently cheaper than vanadium.

Ni-Ti alloys are some of the most relaible shape memory materials. A super-elastic version is used in orthodontic applications. The application of a suitable stress above the martensite-start temperature MS induces martensite, which in a thermoelastic alloy disappears on the removal of stress. The plastic strain at any stage is completely reversible, giving the effect of large elastic strains (about 8%). This is why the phenomenon is known as superelasticity. The alloy can therefore be used like a rubber band as an orthodontic retainer.

The photographs below are reproduced with the kind permission of Dr Hisashi Doi, of the Tokyo Medical and Dental University, Institute of Biomaterials and Biomedical Engineering.

Cast denture framework with a new high-strength Ti-6Al-7Nb alloy.
Orthodontic cast retainer made of super-elastic Ti-Ni alloy.


The following movies are reproduced for teaching purposes, with the kind permission of David Peacock of the Titanium Information Group.

The movies are all in AVI format.

Hydrogen TWIP Hydrogen in cracks Stainless nanofluid Aerogengine bearings 3rd edition
Ternary pearlite TRIP multipass welds Phosphorus Shear instabilities
Surface displacements Stressed duplex Superbainite bearing? Percolation

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