Single-crystal NiAl

A number of studies have introduced the reactive element into the aluminides by ion implantation. The author considers this approach to be of limited value since, as discussed by Pint and Hobbs [27], the high local concentration of reactive element can stabilize the transition aluminas and, at high temperatures, has a short-lived effect. More importantly, from a fundamental standpoint, the implantation process can have a profound effect on the nature of the exposed surface. For example, Schumann [28] has shown that Y implantation into single-crystal NiAl results in a 45 nm thick, finegrained crystalline region which is disordered. 

In this paper results from two sets of experiments are described. In the first part results from a study of the early stages of oxidation are presented. These experiments have been performed using single-crystal NiAl, because of the epitaxial aspects, which are important for the growth of transition aluminas. In the second part results from segregation measurements on Y and Zr doped polycrystalline NiAl are reported. 

NiAl single crystals were cut by spark erosion with the surface normal aligned parallel to the (001) pole within 1° as measured by Laue diffraction. The composition of the NiAl single crystal is 50.0 at % A1 doped with 0.01 wt % Y. Wet chemistry revealed 6 ppm S impurity in the sample.The single crystal NiAl samples were oxidized at 950 °C for 0.1 h or 50 h respectively in air. 

Platinum improves scale adhesion at 1000-1200 C in cast NiAl alloys (e.g. Pint et al., 1998a see also Fig. 6-11) and is used in commercial Pt-Aluminide coatings (Leh-nert and Meinhardt 1972 Smith and Boone, 1990 Warnes and Punola, 1997). Interestingly, at 950 °C a Pt layer approximately 50 nm thick deposited on single-crystal NiAl did not improve scale adhesion and, in fact, inhibited the 0 to a phase transformation (Roux et al, 1993). 

Fig. 2. Yield stress as a function of temperature for NiAl, Ni Al, and several commercial superaUoys where <001> is the paraHel-to-tensile axis for single crystals and (° ) are data points for NiAl + (2). See text. To convert MPa to psi, multiply by 145.

At room temperature, NiAl deforms almost exclusively by (100) dislocations [4, 9, 10] and the availability of only 3 independent slip systems is thought to be responsible for the limited ductility of polycrystalline NiAl. Only when single crystals are compressed along the (100) direction ( hard orientation), secondary (111) dislocations can be activated [3, 5]. Their mobility appears to be limited by the screw orientation [5] and yield stresses as high as 2 GPa are reported below 50K [5]. However, (110) dislocations are responsible for the increased plasticity in hard oriented crystals above 600K [3, 7]. The competition between (111) and (110) dislocations as secondary slip systems therefore appears to be one of the key issues to explain the observed deformation behaviour of NiAl. 

D. Goldberg, G. Sauthoff. Effect of ageing at 673 K on the compressive behaviour of <110> oriented ("soft") NiAl single crystals and polycrystals with and without Ti additions, Intermetallics 4 143 (1996)... 

Structure sensitivity of 1,3-butadiene hydrogenation was recently investigated by Rupprechter et al. [59] on well-defined Pd/Al203/NiAl(l 1 0) single crystals. 

M. I. Weaver, M. J. Kaufman, and R. D. Noeb, The Effects of Alloy Purity on the Mechanical Behavior of Soft Oriented NiAl Single Crystals, Scripta Met. Mat., 29,1113 (1993). 

NiAl single crystal with embedded Re wires... 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

This experimental assembly is much more complex than the preceding one. The oxide surfaces are ultrathin alumina films grown on NiAl(l 1 0) single crystals, in the preparation chamber following a standard procedure [16]. The alumina films are characterized in situ by AES and LEED. The metal clusters are prepared by vacuum condensation at RT of a metal atoms beam generated by an electron bombardment evaporator calibrated by a quartz microbalance. Metal atoms condense only on the sample through an aperture placed closed to it. After preparation the sample is transferred in the reaction chamber. The characterization of the metal clusters is based on STM observations of deposits performed in the same conditions in another UHV chamber [16]. 

In the present study we have used a thin, well-ordered atomically flat alumina film grown on a NiAl(llO) single crystal surface as a model support [27]. The atomic arrangement within line defects of this film have recently been investigated [28]. In addilion, Ihere exists a proposed structure of the film based on X-ray diffraction data which is controversially debated at the moment [29]. The structure and size of the oxide-supported metal particles were controlled utilizing nucleation and growth of vapor deposited metal atoms under ultrahigh vacuum conditions. 

C.2.1. PdlAl20s Preparation and Structural Properties. To prepare a thin well-ordered AI2O3 model support, a NiAl(l 10) alloy single crystal was oxidized in 10 mbar of O2 at 523 K (290). The structure of the alumina film was examined by a variety of techniques (see Reference (101) and references cited therein), and recently it was even possible to image its atomic structure by STM at 4K (Fig. 19) (215). The alumina film was only approximately 0.5 nm thick and hydroxyl-free, and one should also keep in mind that its exact structure may deviate from those of bulk aluminas (101,215,292,293). Its properties are certainly influenced by the observed line defects (antiphase domain boundaries and reflection domain boundaries). 

In this section the experimental results are presented, according to the two sets of NiAl samples. First the results from the NiAl single crystal oxidized at 950 °C are shown. 

Specimens were manufactured from single crystalline [i-NiAl with nearly stoichiometric composition (Al-content from 49 to 50at.%).The single crystals were prepared by the Bridgman method. Specimens for bend tests at RT were doped with about 0.1 at.% Fe and exhibited an enhanced RT ductility [23]. 

Fig. 6. SEM micrograph of the side face of a bending bar after 3-point-bending at RT showing regions of spalled scale (bright spots) under tensile (top) and compressive (bottom) load (Fe-doped NiAl single crystal, tQ% = 21 h, Tm — 1243 K, NF - neutral fibre).

This work was supported by the Deutsche Forschungsgemeinschaft. The authors would like to thank Th. Hutzler, W. Loser and G. Vaerst for providing NiAl single crystals and performing initial bend tests, and J. Edelmann for his assistance in the... 

The elastic behavior of NiAl has been studied repeatedly and the elastic moduli have been determined experimentally for polycrystals and single crystals as a function of composition and temperature (Wasilewski, 1966 Rusovic and Warlimont, 1977, 1979 Harmouche and Wolfenden, 1985, 1987). The elastic behavior has also been studied theoretically by quantum-mechanical, ab initio calculations, and the resulting elastic moduli are in close agreement with the experimental values (Yoo et al. 1990 Fu and Yoo, 1992b Freeman etal., 1992 Yoo and Fu, 1993). 

Model catalysts (Section 2.3) permit the use of the same techniques of examination as single crystals. Palladium particles formed on alumina-coated NiAl( 100) adsorbed ethene in both the tt- and a-forms, but the latter was favoured with increase in particle size this could indeed be the basis for the weak size dependence noted previously (Section 7.2.2) in alkenehydrogenation. Hydrogen adsorbed more strongly on small particles, and in a now familiar way shifted the adsorbed states of ethene towards the r-form this reacted with weakly-held hydrogen atoms to form ethane. ... 

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