Turbine of the times

Due to the extraordinary temperature and forces they must withstand, turbine blades in the hot sections of jet engines are manufactured from a class of materials known as superalloys.

Although such superalloys have been successfully used for more than 50 years due to their outstanding mechanical properties, materials scientists are still working to improve their characteristics, enabling design engineers to develop jet engines that can operate at even higher temperatures and hence use fuel more efficiently. Even as they do so, a new class of intermetallic materials is emerging that may replace superalloys entirely.

Superalloys used in today’s aero-engine blades are metallic alloys based on nickel, the high-temperature strength of which is enhanced by adding aluminium at the melt-processing stage. As the alloys are cooled from high temperatures, a microstructure is created that consists of two main equilibrium phases – a gamma phase, as the matrix, and a gamma-prime phase as the precipitate. It is the precipitate that is largely responsible for the elevated temperature strength of the material, its resistance to creep deformation and its high oxidation resistance.

According to Dr Howard Stone, assistant director of research at the University of Cambridge’s Department of Materials Science and Metallurgy, the strength of most metals decreases as their temperature is increased, due to the effect of thermal dislocations in the metal, which cause plastic and creep deformation. However, because of the ordered crystal structure created in the nickel aluminium precipitates of superalloys, they actually get stronger with temperature, up to a point.

To improve their thermal and structural properties, many of the nickel-based superalloys being developed have more than 10 additional elements. These are added in carefully controlled quantities at the melt-processing stage to improve the creep, rupture, oxidation and hot corrosion resistance of the blade.

’In these alloys, molybdenum is added to strengthen the nickel matrix further, titanium and tantalum are added to strengthen the precipitates, the use of chromium can provide a higher oxidation and corrosion resistance, while tungsten can be used to strengthen both of the two phases that are present,’ said Dr Stone.

Critically, however, it is the use of rhenium, a very heavy slow-diffusing element, that has been used in the latest generation of alloys, at concentrations of up to six per cent, to strengthen the nickel matrix and dramatically reduce its creep resistance.

Adding additional elements to the composition of superalloys does not come without its own set of challenges, however. High levels of refractory elements such as rhenium can make the alloys prone to the formation of what are known as deleterious topologically close packed (TCP) phases, which can rob the material of the strength that the alloying elements were originally intended to provide.

This has led materials scientists to include yet more elements, such as ruthenium, or reduce the concentrations of others, such as chromium, in an attempt to control the formation of such phases. The effect is that the creep properties of the alloys can be significantly improved, especially at high temperatures.

The use of ruthenium also has a commercial implication too. ’Despite the technical fact that ruthenium-containing superalloys have some clear performance advantages, the element itself is both expensive and in limited supply. So it remains to be seen whether superalloys that use it actually become commercially viable,’ said Dr Stone.

As important as the formulation of the superalloy materials are, the hollow turbine blades only survive the extreme temperatures found in the aero engine because of the exacting manufacturing techniques that are used to make them.

Engineers have taken the process to its limit, literally growing blades from single crystals, completely eliminating all of the grain boundary diffusion effects, providing an even greater creep resistance than before

Dr Howard Stone

Under high-temperature conditions, the creep life of a blade is one of its most important characteristics. Hence the use of older polycrystalline superalloys – in which grain boundaries in the superalloy acted as fast diffusion paths – was superseded by the use of directionally solidified materials, where the grain boundaries created in the manufacturing process are all aligned parallel to the loading axis.

’In the latest method used to manufacture the blades, engineers have taken the process to its limit, literally growing blades from single crystals, completely eliminating all of the grain boundary diffusion effects, providing an even greater creep resistance than before,’ explained Dr Stone.

The modern single-crystal manufacturing stage used to enhance the performance of the superalloys has also resulted in changes to their formulation.

In previous generations of polycrystalline blades, for example, the grain boundaries found in the materials were strengthened by the inclusion of boron and carbon. Once the single-crystal manufacturing process came into play, however, these elements were both unnecessary and detrimental to the properties of the single crystal and were removed from the mix of elements used.

Even so, due to the fact that they must operate in an environment far above their melting point, an external oxidation-resistant coating must be applied to the blades to enable them to withstand the high temperatures in the engine. The oxidation-resistant coating, or bond coat, also provides a layer on which a further ceramic thermal barrier coating can adhere. Next-generation blades made from nickel-based superalloys will still require such coatings, as they give an additional benefit in temperature capability.

While developments on the materials science front continue unabated, and as advanced as the process for the creation and manufacture of the nickel-based superalloys has become, it now appears that the first of a new generation of materials are emerging that may well replace nickel-based superalloys in some aero-engine applications.

’Under investigation for decades, but only now beginning to emerge as serious contenders for the throne of the nickel-based superalloys, these titanium aluminides are modern intermetallic materials that don’t have any nickel in them at all,’ said Stone.

What they do have are excellent mechanical properties and oxidation and corrosion resistance at elevated temperatures, which make them a possible replacement for traditional nickel-based superalloy components in aircraft turbine engines.

Advantages aside, the principal difficulty with them is that they are difficult to cast and have little ductility and damage tolerance. Whereas a nickel-based superalloy, like most structural alloys, can show a significant amount of ductility and plastic deformation before it breaks, the newer titanium aluminides by comparison are relatively brittle.

’Despite that fact, turbine blades manufactured from titanium aluminide do have some advantages. Most importantly, they are about half as dense as the nickel-alloy blades currently used in jet engines. As such, their use could remove a lot of weight out of the back end of an aero engine, improving its specific thrust,’ explained Stone.

It would appear that blades made from such materials could soon replace the lowpressure blades currently found in the hot sections of aero engines.

However, it still appears likely that the domain of the high-pressure turbine blade is one that will, for the short term at least, still remain the domain of its nickel-based cousins.