Nickel aluminide

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. 

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. 

Reactive Sputtering. Nanocomposite films of Ni3N/AlN, CoN/BN, and CoN/ Si3N4 were synthesized by reactive sputtering of a nickel aluminide, a cobalt boride,... 

Hafnium is an effective solid solution strengthener at higher temperatures for other alloys such as nickel aluminides (39,40). 

Mew Materials and Processes. New materials and processes include aligned eutectics, oxide and liber-reinforced superalloys, intermelalhc compounds and other ordered phases including titanium aluminidcs. nickel aluminides. and iron aluminidcs. 

Nickel aluminide (NiAl, Ni3AI), titanium aluminide (TiAl, Ti3AI), molybdenum disilicide (MoSi2)... 

Table 3.2. Gibbs free energy of formation of nickel aluminides (J cm 3) and the first phase to grow in reaction couples of the Ni-Al binary system at 350°C89... 

Table 3.3. Standard enthalphies (heats) of formation of nickel aluminides and their effective heats of formation calculated for the effective concentration at the interface corresponding to the composition (3.5 at.% Ni, 96.5 at.% Al) of the eutectic with the lowest melting point in the Ni-Al binary system.261 For all the intermetallic compounds, the limiting element is nickel...

The results of this kind have been obtained by R. Tarento and G. Blaise when studying the reactions in thin-film nickel-aluminium couples" (see also Ref. 263). Using an ingenious variant of ion mass spectrometry, they were able to examine the nickel aluminide layers as thin, as 5 nm. For comparison, the lattice spacings of the Ni-Al intermetallics lie in the range 0.3-0.7 nm.142 214... 

Intermet allies Nickel aluminide (NiAl, NijAl) Titanium aluminide (TiAl, TijAl) Molybdenum disilicide (MoSi )... 

Combustion synthesis of nickel aluminides in the VCS mode was first demonstrated during sintering of a Ni-Al mixture (Naiborodenko et al, 1968). Shortly thereafter, the SHS mode of combustion synthesis was reported for a variety of aluminides, including those of Ni, Zr, Ti, Cr, Co, Mo, and Cu (Naiborodenko et al, 1970, 1982 Naiborodenko and Itin, 1975a,b Maslov et al, 1976, 1979 Itin et al, 1980). 

Rabin, B. H., Bose, A., and German, R. M., Combustion synthesis of nickel aluminides. In Combustion and Plasma Synthesis of High-Temperamre Materials (Z. A. Munir and J. B. Holt, eds.). VCH PubUshers, New York, 1990, p. 114. 

Rogachev, A. S., Khomenko, I. O., Varma, A., Merzhanov, A. G., and Ponomarev, V. I., The mechanism of self-propagating high-temperature synthesis of nickel aluminides. Part II Crystal structure formation in a combustion wave. Int. J. SHS, 3,239 (1994a). 

Wenning, L. A., Lebrat, J.-P, and Varma, A, Some observations on unstable self-propagating high-temperature synthesis of nickel aluminides. J. Mater. Syn. Proc., 2, 125 (1994). 

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