NiAl Intermetallic Phases

Next consider NisAl designated as LI2 or cP4. The L designates an alloy, but unless one were familiar with Strukturbericht notation, it would be necessary to look up the stiucture in a catalog of Strukturbericht crystal structures such as the Naval Research Laboratory online catalog of crystal structures (www.nrl.navy.mil/lattice/). The Pearson symbol is perhaps more informative, telling us we have a primitive cubic crystal with four atoms per unit cell, and with a little imagination, one could guess that A1 atoms must occupy the comers while Ni atoms sit on the six faces. 

what about AlaNi Would not it be the reverse of NisAl One might think so, but the notation DOn or 0PI6 tells us that it has a much more complicated structure. Perhaps it might help if we knew the space group was Pinna, or not. We can see immediately that the unit cell is orthorhombic and contains 16 atoms, but one would have to consult a catalog to get the actual structure from the Strukturbericht notation. 

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The NiAl intermetallic phases are of particular importance because of their use in strengthening Ni-based alloys, especially the superalloys used in high-temperature gas turbine blades (the Ni-Al phase diagram is shovm in Figure 12.12). Nickel aluminide (Ni-Al) is the simplest of these compounds. It is described by the Strukturbericht symbol B2, which tells us it has the CsCl structure, and the Pearson symbol cP2 tells us we have a cubic lattice with two atoms per unit cell. Therefore NiAl has Ni atoms on the comers of a cube and an A1 atom in the center (or vice versa). (Note this is not a bcc structure because the atom in the center is not the same as those on the comers.) The width of this phase in the phase diagram tells us that some solid solution is possible within the phase. 

A model for the nano-structural evolution of Raney-type nickel catalysts (widely used in hydrogenation reactions) from the constituent intermetallic phases present in nickel-aluminium precursor alloys is presented here. Nano-porous nickel catalysts are prepared via a caustic leaching process where the NiAl alloy powder (typically 50-50 at.%) is immersed in concentrated NaOH solution in order to leach away the aluminium present to leave a highly-porous nickel catalyst (often referred to as spongy nickel). 

Firstly, a kinetic Monte Carlo (kMC) [8] for the nano-structural evolution of so-called spongy nickel from the constituent intermetallic phases present in nickel-aluminium precursor alloys is described. Experimental data concerning nano-porous nickel catalyst powder used in this paper are derived from leached NiAl alloy powder produced via a spray-atomization route rather than the conventional cast-and-crushed route. 

It should be noted that a different way to produce stainless steels with high hardness is by precipitation hardening. Such steels have a low carbon content and contain in addition to chromium a few wt% of Ni and Cu. The hardening is caused hy Cu precipitates. Others use precipitation hardening hy intermetallic phases such as NiTi, TiAl, or NiAl. 

Two intermetallic phases are of interest for high-temperature applications in the Ni-Al system NiAl (fi) and Ni3Al ( /). The more Al-rich phases, NiAl3 and Ni2Al3 have relatively low melting points (Fig. 6-9) and poor mechanical properties, and thus are of limited interest. The NisAl3 phase is stable only to ca. 700 °C and is also very brittle. 

In addition, some nanocoatings with a specified composition have oxidation performance different in other ways from that of their conventionally coarsegrained alloy counterparts with the same composition. For example, it has been reported that there is a long period of typical phase transformation of the TGO alumina during the oxidation of NiAl intermetallics at lOOO C or below, from metastable y and 0 to the most stable In contrast, the... 

Cottrell, A.H., 1996, Point defects in Al-Ni-Cu alloys based on the NiAl phase, Intermetallics, 4 1 Leapman, R.D., and Silcox, J.,1979, Orientation dependence of core edges in electron energy loss spectra from anysotropic materials, Phys. Rev. Lett., 42 1361. 

NiAl is the best known example of the intermetallics with a cubic B2 structure (Fig. 1), which form one of the largest groups of intermetallics (Baker and Mun-roe, 1990). The physical and mechanical properties of NiAl have recently been reviewed in detail (Miracle, 1993). As is illustrated by the phase diagram in Fig. 20,... 

Koiwa, 1992 Bakker et al., 1992). Only recently a model, which is based on a combination of two mechanisms, has been proposed for describing the composition dependence of diffusion in B2 phases (Kao and Chang, 1993). It should be noted that the understanding of the basic diffusion processes for other intermetallics is still less than for NiAl and other B2 phases (Wever, 1992). Diffusion studies of multi-component systems are rare, and with respect to NiAl base alloys only data for the ternary phase (Ni,Fe)Al are available from a systematic study of the system Ni-Al-Fe (Cheng and Dayananda, 1979 Dayananda, 1992). Recently, the effect of Cr on diffusion in NiAl has been studied (Hopfe et al., 1993). 

Figure 25. Temperature dependence of creep resistance (in compression with 10 s secondary strain rate) for various single-phase intermetallic alloys NiAl, CoAl and related alloys with a B2 structure (Jung et al., 1987 Sauthoff, 1989), NijTiAl with an L2, structure (Strutt and Polvani, 1973), two NijAl variants with Ll structures, i.e., an advanced alu-minide (+)(Schneibel et at, 1986) and NijAl Fe(x) (Nicholls and Rawlings, 1977), FcjAIC with an L l structure (Jung and Sauthoff, 1989 b), and A Nb with a DOjj structure (Sauthoff, 1990a, b Reip, 1991).

Similar effects have been observed in other intermetallic NiAl-base alloys with less regular distributions of hexagonal C14 Laves phases (Machon, 1992 Sauthoff, 1993 a), and have been discussed by Sauthoff (1991b). In those alloys with coarse phase distributions the observed secondary creep rates follow a rule of mixtures at a first approximation, and additional strengthening effects are only observed for alloys with fine phase distributions. From this it is concluded that particulate and nonparticulate intermetallic alloys creep in similar ways and can be described by the same constitutive equations as conventional multiphase alloys. 

As in the case of conventional disordered alloys, NiAl alloys can be strengthened appreciably by second phases. Apart from the effects on strength, second phases may also be beneficial for ductility and toughness, as has been discussed with respect to NiAl-based intermetallic alloys (Noebe et al., 1991 Clemens and Bildstein, 1992). In any case, the effects of second phases depend on the properties of the respective phases and on the phase distribution. This is illustrated in the following sections by regarding various NiAl alloy systems which have been studied in some more detail. The creep behavior of NiAl alloys with strengthening second phases is addressed in Sec. 4.3.4. 

Besides their fascinating bonding properties and unusual reactivity, these intermetallic complexes are of potential interest as single source precursors for the thin film deposition (MOCVD process) of alloys such as p-CoGa [137], CuA and ot/p-CuAl [138], 9-CuE2 (E = Al, Ga) and Cui cAE phases [139]. Also, such molecular entities may be useful molecular precursors for nanoparticles synthesis in solution, as reported for a-/p-NiAl nanoparticles [140]. 

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