2600
(1316 C)°
2400 (1204 C)
°
2200
2000 (982 C)
°
1800
1600 (760 C)
°
1400
(538 C)°
1000
1950 1960 1970 1980 1990 2000
YEAR
Figure 1-... Firin. temperature increase with .lade material improvement.
sophisticated air-cooling technology to achieve the firing temperature cap-ability required for the new generation of gas turbines. The use of steam cooling to further increase combined-cycle efficiencies in combustors was introduced in the mid to late 1990s. Steam cooling in blades and nozzles will be introduced in commercial operation in the year 2002.
In the1980s， IN-738 blades were widely used.IN-738， was the acknow-ledged corrosion standard for the industry. Directionally Solidified (DS)blades， first used in aircraft engines more than 25 yearsago， were adapted for use in large airfoils in the early 1990s and were introduced in the large industrial turbines to produce advanced technology nozzles and blades. The directionally solidified blade has a grain structure that runs parallel to themajor axis of the part and contains no transverse grain boundaries， as in ordinary blades. The elimination of these transverse grain boundaries con-fers additional creep and rupture strength on the alloy， and the orientation of the grain structure provides a favorable modulus of elasticity in the longitudinal direction to enhance fatigue life. The use of directionally solid-ified blades results in a substantial increase in the creep life， or substantial increase in tolerable stress for a fixed life. This advantage is due to theelimination of transverse grain boundaries from theblades， the traditionalweak link in the microstructure. In addition to improved creeplife， the directionally solidified blades possess more than 10 times the strain control or thermal fatigue compared to equiaxed blades. The impact strength of thedirectionally solidified blades is also superior to that ofequiaxed， showing an advantage of more than 33%.
In the late1990s， single-crystal blades have been introduced in gas tur-bines. These blades offer additional， creep and fatigue benefits through theelimination of grain boundaries. In single-crystal material， all grain bound-aries are eliminated from the material structure and a single crystal with controlled orientation is produced in an airfoil shape. By eliminating allgrain boundaries and the associated grain boundary strengtheningadditives，a substantial increase in the melting point of the alloy can be achieved， thus providing a corresponding increase in high-temperature strength. The trans-verse creep and fatigue strength is increased， compared to equiaxed or DS structures. The advantage of single-crystal alloys compared to equiaxed and DS alloys in low-cycle fatigue (LCF) life is increased by about 10%.
Coatings
There are three basic types of coatings， thermal barrier coatings， diffusioncoatings， and plasma sprayed coatings. The advancements in coating have also been essential in ensuring that the blade base metal is protected at these high temperatures. Coatings ensure that the life of the blades are extendedand in many cases are used as sacrificial layer， which can be stripped andrecoated. Life of coatings depends on composition， thickness， and the stand-ard of evenness to which it has been deposited. The general type of coatings is very little different from the coatings used 10-15 years ago. These include various types of diffusion coatings such as Aluminide Coatings originally developed nearly 40 years ago. The thickness required is between 25-75 μm thick. The new aluminide coatings with Platinum increase the oxidationresistance， and also the corrosion resistance. The thermal barrier coatings have an insulation layer of 100-300 μmthick， and are based on Zr0 2-Y203 and can reduce metal temperatures by 120-300 0F (50-150 0C). This type ofcoating is used in combustion cans， transition pieces， nozzle guide vanes， and also blade platforms.
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