24.1 Introduction

In the last chapter we saw how a basic knowledge of the mechanisms of creep was an important aid to the development of materials with good creep properties. An impressive example is in the development of materials for the high-pressure stage of a modern aircraft gas turbine (see Figure 1.3). Here we examine the properties such materials must have, the way in which the present generation of materials has evolved, and the likely direction of their future development.

The ideal thermodynamic efficiency of a heat engine is given by

T1–T2T1=1–T2T1

Figure 24.1 shows the variation in efficiency of a turbofan engine plotted as a function of the turbine inlet temperature. In 1950 a typical aeroengine operated at 700°C. The incentive then to increase the inlet temperature was strong, because of the steepness of the fuel-consumption curve at that temperature. By 1975 a typical engine (the RB211, for instance) operated at 1350°C, with a 50% saving in fuel per unit power output over the 1950 engines. But is it worth raising the temperature further?

The shallowness of the consumption curve at 1400°C suggests that it might not be profitable; but there is a second factor: power-to-weight ratio. Figure 24.2 shows a typical plot of the power output of a particular engine against turbine inlet temperature. This increases linearly with the temperature. If the turbine could both run at a higher temperature and be made of a lighter material there would be a double gain, with important financial benefits of increased payload.

24.2 Properties Required of a Turbine Blade

Let us first examine the development of turbine-blade materials to meet the challenge of increasing engine temperatures. Although so far we have been stressing the need for excellent creep properties, a turbine-blade alloy must satisfy other criteria too. They are listed in Table 24.1.

The first—creep—is our interest here. The second—resistance to oxidation—is the subject of Chapter 25. Toughness and fatigue resistance (Chapters 14 and 18) are obviously important: blades must be tough enough to withstand the impact of birds and hailstones; and changes in the power level of the engine produce mechanical and thermal stresses which—if the blade material is wrongly chosen—cause thermal fatigue. The alloy composition and structure must remain stable at high temperature—precipitate particles can dissolve away if the alloy is overheated and the creep properties will then degrade significantly. Finally, the density must be as low as possible—not so much because of blade weight but because of the need for stronger and hence heavier turbine discs to take the radial load.

These requirements severely limit our choice of creep-resistant materials. For example, ceramics, with their high softening temperatures and low densities, are ruled out for aeroengines because they are far too brittle (they are under evaluation for use in land-based turbines, where the risks and consequences of sudden failure are less severe—see the following). Cermets offer no great advantage because their metallic matrices soften at much too low a temperature. The materials that best fill present needs are the nickel-based super-alloys.

24.3 Nickel-Based Super-Alloys

The alloy used for turbine blades in the high-pressure stage of an aircraft turbofan engine is a classic example of a material designed to be resistant to dislocation (power-law) creep at high stresses and temperatures. At take-off, the blade is subjected to stresses approaching 250 MN m^–2, and the design specification requires that this stress shall be supported for 30 h at 850°C without more than a 0.1% irreversible creep strain. In order to meet these stringent requirements, an alloy based on nickel has evolved with the rather mind-blowing specification given in Table 24.2.