High-temperature applications, nickel aluminides

Since many years the intermetallic phases based on aluminides have been an important topic for research and development, because of their high melting points, low densities and excelent corrosion resistance at high temperatures. Especially, nickel aluminides have been of great interest as coating materials for several high temperature applications. 

FIGURE 5.7 Optical micrographs of the reaction products between Ni and Al. (a) Duplex phases of nickel aluminides formed after a treatment of 1000°C for 1 hour in vacuum (b) a cracked nickel aluminide layer formed after being treated at 640°C for 1 hour in vacuum and (c) a cross-section of a nickel and zirconia joint which was joined together through a nickel aluminide layer at a temperature of 680°C in vacuum. N nickel, R nickel aluminide, A aluminium, and C zirconia. (Reprinted from Mei, J. and Xiao, R, Joining metals to zirconia for high temperature applications, Scripta Materialia 40 (1999) 587-594, with permission from Elsevier Science.)... 

Aluminides based on the intermetallic phases Ni3Al and Fe3Al are considered both as structural materials and as coatings for high temperature applications [1-6]. Their excellent corrosion resistance is due to their forming a dense, protective alumina scale. Alumina, especially ot-Al203, shows low rate constants even at temperatures above 1000°C [7]. Unlike chromia, which is formed on conventional stainless steels and nickel base alloys, alumina does not evaporate above 1000°C [8] and it is even stable in oxygen deficient atmospheres. 

New structural intermetallic alloys for high-temperature applications are at the center of the present interest in intermetallics, which is still growing. A few developments, which are based on the classic phases NijAl, TijAl and TiAl, and which are known as the nickel aluminides and the titanium aluminides, are on the brink of commercialization, but even these developments are still at an early stage compared with other developments of advanced materials, e.g. the modern engineering ceramics. Much more experimental and theoretical work is necessary to solve the processing problems and to ad-... 

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