Part 111
Materials，FuelTechnology， and Fuel Systems
Materials
Temperature limitations are the most crucial limiting factors to gas turbine efficiencies. Figures 11-1a and 11-1b show how increased turbine inlet temperatures decrease both specific fuel and air consumption while increasing efficiency. Materials and alloys that can operate at high tempera-turesareverycostly-bothtobuyandtoworkon.Figure 11-1cshowsrelativerawmaterial costs.Thus， the cooling ofblades，nozzles， and combustor liners is an integral part of the total materials picture.
Since the design of turbomachinery is complex， and efficiency is directlyrelated to material performance， material selection is of prime importance.Gas and steam turbines exhibit similar problemareas， but these problem areas are ofdifferent magnitudes. Turbine components must operate under avariety ofstress， temperature， and corrosion conditions. Compressor blades operate at relatively low temperature but are highly stressed. The combustor operates at a relatively high temperature and low-stress conditions. Theturbine blades operate under extreme conditions ofstress， temperature， and corrosion. These conditions are more extreme in gas turbine than insteam turbine applications. As a result， the materials selection for individual components is based on varying criteria in both gas and steam turbines.
A design is only as efficient as the performance of the selected component materials. The combustor liner and turbine blades are the most critical com-ponents in existing high-performance， long-life gas turbines. The extremeconditions of stress， temperature， and corrosion make the gas turbine blade a materials challenge. Other turbine components present operational problemareas， but to a lesser degree. For thisreason， gas turbine blade metallurgy will be discussed for solutions to problem areas. Definition of potential solu-tions will also relate to other turbine components.
The interaction of stress， temperature， and corrosion yields a complex mechanism that cannot be predicted by existing technology. The required
Figure 11-1a. Specific air versus pressure ratio and turbine inlet temperatures
Figure 11-1b. Specific fuel consumption versus pressure ratio and turbine inlet temperature
Figure 11-1c. A comparison of raw material costs
material characteristics in a turbine blade for high performance and long lifeinclude limitedcreep， high-rupturestrength， resistance to corrosion， good fatiguestrength， lowcoefficient of thermal expansion， and high-thermal conductivity to reduce thermal strains. The failure mechanism of a turbine blade is related primarily to creep and corrosion and secondarily to thermal fatigue. Satisfying these design criteria for turbine blades will ensure high-performance， longlife， and minimal maintenance.
The development of new materials as well as cooling schemes has seen the rapidgrowthof theturbinefiring temperatureleading tohigh turbineefficiencies.The stage 1 blade must withstand the most severe combination of temperature， stress and environment; it is generally the limiting component in the machine. Figure 11-2 shows the trend of firing temperature and blade alloy capability.
Since1950， turbine bucket material temperature capability has advanced approximately 850 oF (472 oC)， approximately 20 oF/10 oC per year. The importance of this increase can be appreciated by noting that an increase of 100 oF (56 oC) in turbine firing temperature can provide a corresponding increase of 8-13% in output and 2-4% improvement in simple-cycle effi-ciency. Advances in alloys andprocessing， while expensive and time-consuming， provide significant incentives through increased power density and improved efficiency. Before discussing some of these materials in depth it is important to understand the general behavior of metals.
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