Aluminides

Behm J M and Morse M D 1994 Spectroscopy of]et-cooled AIMn and trends in the electronic structure of the 3d transition metal aluminides J. Chem. Rhys. 101 6500... 

Fig. 5. Comparison of crack growth rates of titanium aluminides, composites, and IN-100 tests at 650°C, R = 0.1, v = 0.2 Hz except for the composite...

Table 15. Properties of Titanium Aluminides, Titanium-Base Conventional Alloys, and SuperaHoys ...

The market for lighter weight P/M materials such as aluminum and titanium aluminides is expected to grow, especially for uses in automobiles. 

P/M processing of titanium aluminides results in more consistent product quaHty than the conventional casting process, and offers novel alloy/microstmcture possibiHties and improved ductiHty. Processing trends include use of high (1200—1350°C) temperature sintering to improve mechanical properties of steel and stainless steel parts. 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Aluminide and sUicide cementation coatings such as TaAl on tantalum and MoSi2 on molybdenum oxidize at slow rates and possess some inherent self-repair characteristics. Fine cracks that appear and are common to these coatings can be tolerated because stable, protective oxides form within the cracks and seal them. Thermal cycling, however, accelerates faUure because of thermal expansion mismatch that ultimately dismpts the protective oxide coating. 

Intermetallics also represent an ideal system for study of shock-induced solid state chemical synthesis processes. The materials are technologically important such that a large body of literature on their properties is available. Aluminides are a well known class of intermetallics, and nickel aluminides are of particular interest. Reactants of nickel and aluminum give a mixture with powders of significantly different shock impedances, which should lead to large differential particle velocities at constant pressure. Such localized motion should act to mix the reactants. The mixture also involves a low shock viscosity, deformable material, aluminum, with a harder, high shock viscosity material, nickel, which will not flow as well as the aluminum. 

Products of nickel aluminides are also observed along the axis of the sample over a diameter of about 1 mm where the numerical simulations show radial focusing of the pressure to values approaching 50 GPa for times of about 200 ns. 

The shock-modified composite nickel-aluminide particles showed behavior in the DTA experiment qualitatively different from that of the mixed-powder system. The composite particles showed essentially the same behavior as the starting mixture. As shown in Fig. 8.5 no preinitiation event was observed, and temperatures for endothermic and exothermic events corresponded with the unshocked powder. The observations of a preinitiation event in the shock-modified mixed powders, the lack of such an event in the composite powders, and EDX (electron dispersive x-ray analysis) observations of substantial mixing of shock-modified powders as shown in Fig. 8.6 clearly show the first-order influence of mixing in shock-induced solid state chemistry. 

The initial studies on nickel-aluminide synthesis defined a number of important issues in shock-induced solid state synthesis. This work was extended to the influence of powder particle morphology in recent work of Thadhani and... 

The response of titanium-aluminum powder mixtures in a 3 1 molar ratio was investigated under the same shock-loading conditions as in the nickel aluminides. Such mixtures are especially interesting in that the shock impedances of the materials are approximately equal and both are relatively hard and difficult to deform. In addition to any chemical differences, such materials should prove to be difficult to mix with the shock conditions. 

It was observed, under conditions when the nickel-aluminide mixtures of the same ratio were fully reacted, that the titanium aluminides were essentially unreacted reactions were only localized. Because the products were of such small size, it was difficult to identify them, but they were thought to be TiAlj or ordered superstructures TiQAl23 or TigAl24. No further studies have been carried out on these samples. 

Botton, G.A., Guo, G.Y., Temmerman, Szotek, Z., Humphreys, C.J., Wang, Y., Stocks, G.M., Nicholson, D.M.C., Shelton, W.A., 1996b, EELS studies of B2-type transition metal aluminides Experiments and theory, in Materials Theory, Simulations, and Parallel Algorithms, E. Kaxiras, J. Joannopoulos, P. Vashishta, R.K. Kalia, eds. Materials Research Society, Pittsburg, Vol. 408., 567. 

The main protective method against atmospheric catastrophic attack is surface coatings of silicides, and aluminides. ... 

When the coating metal halide is formed in situ, the overall reaction represents the transfer of coating metal from a source where it is at high activity (e.g. the pure metal powder, = 1) to the surface of the substrate where is kept less than 1 by diffusion. The formation of carbides or intermetallic compounds such as aluminides or silicides as part of the coating reaction may provide an additional driving force for the process. 

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