In aero-engines, the blade of the high pressure turbine was for a long time the highest of the high technology in the aero gas turbine, and despite the complexity of the modern fan blade, the challenge it provides does not reduce. The ability to run at increasingly high gas temperatures has resulted from a combination of material improvements and the development of more sophisticated arrangements for internal and external cooling (figure 1).
Figure 1. Schematic of a gas turbine engine.
A modern turbine blade alloy is complex in that it contains up to ten significant alloying elements, but its microstructure is very simple. The structure is analogous to an `Inca wall', which consisted of rectangular blocks of stone stacked in a regular array with narrow bands of cement to hold them together.
In the alloy case the `blocks' are an intermetallic compound with the approximate composition Ni3(Al,Ta), whereas the `cement' is a nickel solid solution containing chromium, tungsten and rhenium.
Superalloys have always contained phases of this type, but in recent years the titanium in the original intermetallic has been replaced by tantalum. This change gave improved high temperature strength, and also improved oxidation resistance. However, the biggest change has occurred in the nickel, where high levels of tungsten and rhenium are present. These elements are very effective in solution strengthening.
Since the 1950’s, the evolution from wrought to conventionally cast to directionally solidified to single crystal turbine blades has yielded a 250°C increase in allowable metal temperatures, and cooling developments have nearly doubled this in terms of turbine entry gas temperature. An important recent contribution has come from the alignment of the alloy grain in the single crystal blade, which has allowed the elastic properties of the material to be controlled more closely. These properties in turn control the natural vibration frequencies of the blade.
If metallurgical development can be exploited by reducing the cooling air quantity this is a potentially important performance enhancer, as for example, the Rolls-Royce Trent 800 engine uses 5% of compressor air to cool its row of high pressure turbine blades. The single crystal alloy, RR3000, is able to run about 35°C hotter than its predecessor. This may seem a small increase, but it has allowed the Trent intermediate pressure turbine blade to remain uncooled.