Physcial Metallurgy and Microstructure of Superalloys
نویسنده
چکیده
When significant resistance to loading under static, fatigue and creep conditions is required, the superalloys have emerged as the materials of choice for high temperature applications. This is particularly true when operating temperatures are beyond about 800C. This is the case for gas turbines used for (i) jet propulsion, e.g. the 100,000 lb thrust engines used for powering large two-engined aeroplanes such as the Boeing 777, or the smaller 1,000 lb engines used for helicopter applications and (ii) electricity generation, e.g. the 250 MW gas-fired industrial gas turbine which can generate enough power to satisfy a large city, or a smaller 3 MW gas-fired generator suitable for back-up capability. However, as high temperature materials which are relatively expensive, the superalloys should be employed only after consideration of others which are available. When weight-saving is important, titanium alloys are used in preference to the superalloys but their poor oxidation resistance restricts their application to below about 700C. For some electricity-generating power plant applications which rely upon superheated steam at about 565C, high strength creep resistant ferritic steels are preferred on account of their lower cost. The superalloys come into their own particularly in extreme environments, when resistance to oxidation and corrosion is required; for this purpose, they are often used in combination with protective coatings.
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