Aluminides applications

Bartolotta PA and Krause D L (1999), Titanium aluminide applications in the high speed civil transport . Gamma Titanium Aiuminides 1999, San Diego, TMS. 

There are two important titanium aluminides Tig A1 which has a hexagonal structure with a density of 4.20 g/cm and a melting point of 1600°C and Ti A1 which has a tetragonal structure with a density of 3.91 g/cm and a melting point of 1445°C. As do all aluminides, they have excellent high temperature oxidation resistance owing to the formation of a thin alumina layer on the surface. They have potential applications in aerospace structures. 

Reinforcement for metal-matrix composites with such metals as titanium, titanium aluminide, aluminum, magnesium, and copper. Applications are found mostly in advanced aerospace programs and include fan blades, drive shafts, and other components. 

As a conclusion, notice for all the nine metals of these groups, the large diffusion of the CsCl-type-based (binary and complex) compounds and the solid solutions formed with metals of various groups. Among these, several aluminides can be of interest, possibly also as additional components in high temperature applications. 

Titanium aluminides are ordered intermetallics and hence have lower diffusivity and high elastic modulus. These compounds are stronger than the conventional titanium alloys and are suitable for high temperature applications. But these compounds have low ductility due to the planarity of slip in these compounds. 

K. Natesan, Development of Aluminide Insulator Coatings for Eusion Reactor Application, Argonne National Laboratory Report ANL/PPP/TM-278, ITER/US/95/ lV-BL-03, 1995. 

The synthesis of several hundred materials, by both the SHS and VCS modes, has been reported in the literature. The types of compounds produced include carbides, borides, intermetallics, silicides, aluminides, composites, nitrides, hydrides, and oxides. The purpose of this section is to provide a description of the synthesized compounds, as well as the materials (i.e., powders, poreless materials, and functionally graded materials) and articles produced. The practical applications of experimental and technological methods are also described. 

NijAl and NiAl can be produced through a termite reaction between nickel and aluminium at a relatively low temperature. Nickel aluminides were produced between the zirconia and the metallic component to achieve the vacuum-tight joining. The morphology, composition, and density of the metal-zirconia joint have been optimized to minimize the GTE mismatch at the metal-ceranuc interface, and they are worth considering in detail as they could be used for sensor applications at temperatures up to 1000°C. 

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.)... 

The fundamentals of intermetallic oxidation will be briefly described followed by discussion of work on the Ni- and Fe aluminides, Ti aluminides, and refractory metal compounds. It will become clear that work is now focussed in two general areas (i) adding to the fundamental knowledge of oxidation mechanisms and (ii) developing approaches to allow intermetallics to reach the application stage e. g. coating, composite development, and life predictions. 

It is anticipated that studies, such as those above, will become more numerous as titanium aluminides approach the application stage. 

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. 

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. 

In industrial applications the environments usually contain more than one reactant. For example high temperature oxidation occurs in air by the combined attack of oxygen, nitrogen and quite frequently water vapour. However, most of the studies concerning the oxidation resistance are performed in dry oxygen or dry air. The oxidation behaviour of the intermetallic phases of theTi-Al system has recently received considerable attention. The influence of water vapour on the oxidation of titanium aluminides has not been studied intensively. There are only a few studies of the high temperature corrosion of titanium and its alloys. 

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