The high-temperature oxidation behavior of several ordered alloys in the Ni-Al and Fe-Al systems is reviewed with special emphasis on Ni3Al and NiAl. Ordering influences oxidation through its effect on the activities of the alloy components and by changing the point defect concentration in an alloy. Three categories of Ni-Al alloys are distinguished based on Al content and oxidation behavior. A characteristic feature of the oxidation of high-aluminum Ni-Al and Fe-Al alloys is the formation of voids in the substrate at the oxide-metal interface. The mechanism of void formation and its suppression by minor additions of oxygen-active elements is discussed. A brief description of the effect of preoxidation on the reactions of Ni3Al-base alloys in SO2/O2 environments is also included.

Experiment and theory of diffusion in Ll(2) alloys are briefly reviewed. Current work on volume and grain boundary diffusion in Ni(3)Al alloys are presented and discussed.

The oxidation of single crystal β-NiAl has been studied primarily using electron microscopy. Oriented metastable Al2O3 phases form during transient oxidation at 800°C. Specific orientation relationships exist on all metal orientations studied and are a result of the small mismatch along aligned close-packed directions in the cation sublattices of the metal and oxide. Transformation of the metastable Al2O3, phases at 1100°C results in an oxide morphology described as the “lacey” structure of α-Al2O3 scales. This structure results from impingement of oriented patchei α a-Al2O3 as the transformation initiates and moves radially parallel to the sugfhce. Scale growth occurs by diffusion along high angle grain boundaries. A drastic reduction in oxidation rate accompanies the change in oxide morphology.

Ni-Mo alloys containing 26–28 wt. pct. Mo are known for their excellent corrosion properties in reducing environments. When exposed to temperatures in the range 600°C to 800°C, however, these alloys undergo a longrrange ordering reaction from disordered fcc lattice to ordered Dla superlattice. This ordering phenomenon has been found to have detrimental effects on corrosion properties. Also, alloys based on the Ni-16Mo-16Cr system undergo a long range ordering reaction from disordered fcc lattice to ordered Pt2Mo-type superlattice in the temperature range of 400° to 600°C. However, in contrast to the case of Ni-Mo alloys, ordering in the Ni-Mo-Cr alloys has some beneficial effects on the aqueous corrosion properties. This difference in behavior between the Ni-Mo alloys and the Ni-Mo-Cr alloys is discussed interms of the ordering characteristics determined from transmission electron microscopy and diffraction, X-ray microanalysis in the scanning transmission electron microscope and Auger electron spectroscopy.

If the ductile Ni3Al-based aluminides are to achieve their full potential as engineering materials, they must demonstrate a capability for being joined by conventional welding processes. The goal of this study was to identify some of the factors controllinq weldability of the ductile aluminides. The alloys selected for this initial study were boron-doped Ni3 Al and Ni3 Al containing 10 at. % Fe. Sheet stock was used to make full penetration autoqenous welds by the electron beam and gas tungsten arc processes. The main process variables were travel speed and preheat. The aswelded coupons were examined visually and by normal metalloqraphic methods. Although sound welds were made, initial results indicated weldments of these alloys are susceptible to cracking. Weldability of the ductile Ni3Al-based aluminides was found to be affected by alloy composition and welding speed.

Thermal conductivity, electrical resistivity, Seebeck coefficient and thermal expansion data were obtained on well-annealed Ni3Al containing 24 and 25 at. % Al. The results span the temperature range 300 to 1000 K. The expansion coefficients did not vary with composition and increased with temperature, reaching values of aIout 17 × 10−6 K−1 at 1000 K. The thermal conductivity and electrical resistivity changed rapidly with composition, and the thermal conductivity of 24 at. % Al is as much as 30% lower than that for stoichiometric Ni3A1. The electronic Lorenz function of Ni3Al was obtained by subtracting the estimated phonon conductivity component and found to be within about 5% of the Sommerfeld prediction from 300 to 1000 K. The electrical resistivity results for stoichiometric Ni 3Al are influenced by the loss of ferromagnetic order at lower temperatures and are not adequately described by the Bloch-Grüneisen equation.

Several samples of the ordered intermetallic compound with the Ll2 structure and chemical composition NiO.75-xAl0.25-xFe2x (doped with 0.2 at. % B) have been studied over an Fe compositional range 0 ≤ 2x ≤ 0.20. Stoichiometric Ni3Al is known to be a strongly exchange-enhanced-material, forming a giant moment system when Fe impurities are introduced in trace amounts. For the greater Fe concentrations employed in this work, more conventional ferromagnetic systems are obtained. Well below the ferromagnetic Curie temperature Tc the samples exhibit a spontaneous moment in small magnetic fields and at higher fields the isothermal magnetization data show a clear tendency toward saturation. Magnetic investigations using vibrating-sample techniques permitted measurements over the temperature interval from 4.2 K to 800 K in fields up to 5.0 T. The Curie temperature was found to increase from ∼50 K for the Fe-free sample in the series to 410 K for the limiting concentration of 12 at. % Fe in the Ll2 structure. Samples with higher Fe content up to 20 at. % contain a second, disordered fcc phase with Tc ≃ 715 K. Peaks in the temperature dependence of the lattice parameter observed in x-ray studies yield Tc values in reasonable agreement with the magnetic measurements.

Compounds of silicon and nickel have been formed by implanting silicon ions into nickel at various fluences and temperatures. Temperature during implantation is a major factor controlling implanted ion concentrations and phase developments. The ordered phase Ni3Si, which forms during implantation at 600°C, is replaced by Ni5Si2 during implantation at 400°C or below. In contrast to aluminum implantation of nickel, crystalline phases form even at high fluences (>1 × 1018 ions/cm2) and at low temperatures (<200°C) in silicon implanted nickel.

Shape-memory behavior is a complicated phenomenon which Intimately relates macroscopic strain recovery with martensitic phase transformation. A vital step In the theoretical understanding of this effect is a clear picture of a superficially rather simpler phenomenon: martensitic pseudoelasticity. Within the martensftic phase, a number of ordered alloys exhibit deformation strain recovery which Is reminiscent of rubber elasticity. The deformation Is observed to occur via coherent motion of parallel twin boundaries. In this case, where long range elastic accomodatlon forces play no role, the origin of the restoring force on the twin boundaries is still unclear. In this work, we present a quantitative theory which generalizes previous suggestions in the literature and unifies this phenomenon with elastic mechanical untwinning. Our description is based on the concept of a “pseudotwin” originally proposed by Laves and later elucidated by Cahn. Here, the motion of a twin boundary generates a new metastable crystallographic structure (the pseudo-twin) of locally higher free energy. This notion not only provides a source for a “volume” restoring force but leads to a natural descriptioni of observed stabilization effects. Specific experiments to test our description will be proposed.

Nickel-based alloys with over 7 percent silicon have excellent corrosion resistance to sulfuric acid, oleum and sulfate-ion-containing environments. This alloy is characterized by the presence of intermetallic Ni3Si (beta) and Ni5Si2 (gamma) phases in Ni-Si solid solution (alpha) matrix. The corrosion resistance is provided by the coarse intermetallic phases. Size of intermetallic phases is dependent on the processing condition and the heat treatment. The corrosion resistance can be changed by heat treatment.

Results of corrosion tests are explained in terms of microstructural features.