Low and high temperature tensile tests were performed on single crystals of pure Ni3Al and Ni3Al+B in order to determine the effect of B additions on the ductility and fracture behavior. Tests were carried out in air at a constant strain rate of 1.3 × 10−3 s−1. The orientations tested were [001] for which the yield stress in tension is always greater than in compression, and those for which the tension/compression asymmetry is zero ([T=C]) for each particular composition. At room temperature, the results show a positive effect of B additions on both the fracture stress and on the ductility. The ductility at 800K appears to decrease monotonically with B additions. The largest ductilities are found for [T=C] at room temperature where an improvement of about 26% (resolved strain) for an addition of 0.2 at% B was obtained. However, the most dramatic increase in ductility occurs for the [001] oriented samples at room temperature where a 55% improvement was measured over that of pure Ni3Al. Fracture surfaces show a combination of massive slip, some cleavage, and heavily dimpled areas. These observations show that B additions not only increase the ductility of polycrystalline Ni3Al, as has been previously observed by many investigators, but also that of the already-ductile single crystalline material, indicating that a “bulk effect” should be added to the grain boundary strengthening effect of B when explaining the improvement in ductility of polycrystalline Ni3Al due to B additions.

The ductility of preoxidized Ni3Al (Ni-23Al-0.5Hf-0.2B, at.%) specimens with various grain sizes (17∼193 μm) was evaluated by means of tensile tests at 600 and 760°C in vacuum. It was found that the preoxidation does not affect the ductility of the finest-grained material at either temperature, whereas it causes severe embrittlement in the largest-grained material, especially at 760°C. A continuous, thin Al-rich oxide layer, which forms on the fine-grained samples, protects the underlying alloy from oxygen penetration, preventing any loss of ductility, whereas the nickel-rich oxide which forms on the large-grained samples allows oxygen to penetrate along grain boundaries, causing severe embrittlement. The grain boundaries act as short-circuit paths for rapid diffusion of aluminum atoms from the bulk to the surfaces, and this is responsible for the difference in oxidation behavior between fine- and large-grained materials. The embrittlement of large-grained samples can be eliminated through control of oxide formation on Ni3Al surfaces.

Binary polycrystalline alloys of B2 NiAl, ranging from 47 to 51.5 at% Al, were prepared by casting and extrusion. These were tested in tension at temperatures ranging from 300 K to 873 K. Significant tensile ductility is observed in the near-stoichiometric alloy at room temperature as well as at 473 K in spite of the presence of only <100> dislocations. This anomaly appears to be related to the recrystallized texture, to the lower yield strength and to the presence of residual dislocations at grain boundaries in this alloy.

Microhardness vs temperature and elastic moduli have been measured for a suite of intermetallic compounds that melt above 1400°C. Binary intermetallics were selected to represent a variety of crystal structures and yet have optimal combinations of high melting temperature (Tm) and low specific gravity. Some deliberately two-phase alloys were prepared in which one phase is a terminal-phase metal and the other an intermetallic compound.

Binary compounds can be described by two patterns. In those where plasticity is difficult, hardness decreased slowly with temperature up to Tm/2, the decrease being no more than that normally shown by the elastic moduli. In those compounds where single crystal plasticity is known (or at least plausible), microhardness decreases more rapidly than do elastic moduli, presumably due to thermally activated slip.

Selected area electron channeling patterns were used to identify the cleavage planes in a polycrystalline Al3Ti-base alloy having the L12 structure. In order to do this unambiguously in the scanning electron microscope (SEM), one needs to know that the cleavage facet from which any given channeling pattern is obtained is indeed normal to the electron beam. We accomplished this by utilizing a recently-developed technique in which an optical microscope with a short depth of focus is inserted in the SEM column and used to measure the elevations of several points on the cleavage facets. By appropriately tilting and rotating the sample, and using the optical microscope to measure elevations, it was possible to orient the facets normal to the beam. The cleavage planes in a cast and extruded alloy having an equiaxed grain structure were compared with those in a directionally-solidified (DS) alloy of the same composition. Of the eight cleavage facets examined in the DS material, six were of the {110} type and two were of the {111} type. Of the six facets examined in the cast and extruded material, two each were of the {110} and {111} types, and one each were of the {100} and {013} types. Although it cannot be said that all possible cleavage planes have been identified in this alloy, the availability of several low-strength cleavage planes apparently exacerbates its brittleness.

Rapid solidification processing (RSP) of TiAl-base alloys often results in the formation of metastable crystalline products (e.g., Ti3Al) A key parameter controlling both the solidification pathway and the stability of the metastable structure is alloy chemistry. This study focuses upon the influence of minor alloy additions on the microstructure and properties of Ti52Al48 alloys produced by RSP. While alloys having minor Ta addi ions (<1 at.%) maintain near-equilibrium solidification structures similar to the binary alloy, the addition of Cr at a level of 5 at.% results in a nearly 100 vol.% metastable Ti3Al structure containing a fine dispersion of TiCr2 grain boundary precipitates. These precipitates limit grain growth during elevated temperature exposure, maintaining the fine (TiAl) grain size created during decomposition of the metastable product structure. Mechanical property evaluation of the consolidated RSP material indicates a greatly reduced flow stress for the fine grained Cr modified alloys. A preliminary analysis attributes this to the reduction in grain size and enhanced elevated temperature diffusion rates for the Cr containing TiAl-base alloys.

The deformation behavior of two alloys whose compositions (Ti-25Al-9Nb and Ti-11Al-23Nb in at.%) correspond to alpha-two and beta phases in the Ti-24Al-11Nb alloy has been investigated. Results from both compression and tensile tests over a range of temperatures from 27° to 650°C indicate that the yield stress as well as the strain and strain-rate hardening characteristics of the Ti-24-11 alloy are controlled by the Ti3Al-base alpha-two phase. In contrast to the Alpha-two alloy, the Beta alloy has a flow stress which is very sensitive to temperature and strain rate at 650°C, suggesting the onset of high temperature creep processes.

The effect of the Ti/Al ratio and third elements (Cr, Hf and Nb) on the mechanical properties of TiAl has been studied. Solution hardening in these elements is so large that, e.g., the strength doubles by the addition of 3 at.% Cr. Ductility to fracture strain has been improved up to 2.7 % and 2.7 % by the Cr addition and the use of high purity base metals, respectively. Parameters relating to the improvement of ductility have been discussed. Namely, the improvement of ductility was caused by the decreases of the grain size, axial ratio and impurity. A hypothesis has been proposed that the ductility improvement was related with the excess shrinkage of volume of a unit cell due to the addition of the third element which implies the strengthening of chemical bonds and vice versa. The effect of twins and a lamellar structure with the plate-like Ti3Al and the matrix on the ductility have been discussed.

The tetragonal compound Al3Zr can be transformed into the cubic Ll2 structure by additions of Cr, Fe, Ni or Cu. This result is interpreted in terms of Pettifor's structure map. The transformation is accompanied by substantial softening. The significant porosity of cast buttons which develops during the homogenization required to produce the Ll2 structure is attributed to the Kirkendall effect. Al-5.5Fe-25Zr (at. %) specimens with the Ll2 structure can be deformed at room temperature in compression, and their yield stress has been determined as a function of temperature. Al-Fe-Zr is nevertheless a brittle material. This is illustrated for Al-Fe-V-Zr Ll2-structure compound, the fracture toughness of which is only slightly higher than that of sintered SiC. Evidence is presented to suggest that Ll2 formation is accompanied by an increase in toughness. Significant improvements in the mechanical properties of Al3Zr-type alloys may possibly be achieved by improved processing.

In both single and polycrystalline Ni3Al, no strain rate dependence of 0.2% flow stress is observed in a temperature range where anomalous positive temperature dependence of strength is exhibited. A distinct strain rate dependence is observed, however, in single crystals when the test temperature is raised beyond the peak in flow stress. It is shown that (111) type slip involving diffusional process is responsible for the strain rate sensitivity at near [001] orientation, while (001) type slip which is governed by the Peierls mechanism at orientations away from [001]. In polycrystalline compound with a fine grain size an apparent loss of the positive temperature dependence of strength is observed at a slower strain rate due to grain boundary sliding.

Structural intermetallic materials have undergone an evolutionary process whereby some of the materials could provide major payoffs in gas turbine engines. This maturation of select intermetallic systems has provided significant hope for making still greater advances in turbine performance through further developments in other intermetallic materials. The same maturation process has highlighted specific limitations and requirements which are key to the utilization of intermetallic systems. This paper briefly reviews some critical ground rules for high temperature intermetallics development and identifies research directions being pursued by the Air Force for advancing intermetallic materials.

Advanced aero-propulsion engine designs now being considered for implementation require structural materials with high temperature strength and creep properties above 1300°C, in excess of the range where nickel base superalloys are now being used. A number of ordered single phase compounds having melting points above 1500°C have been identified which both hold promise for engine applications and are representative of a number of different crystal structures such as B2, C15 (Laves), A15, C1 and DO19. Elevated temperature characterization has been conducted on these compounds which includes ductile/brittle transition temperature determination, minimum creep rate analysis, elastic modulus, tensile strength and cyclic oxidation testing. The results of these tests have led to insight in the processing methods required for these compounds and selection of compounds based on elemental constituents and crystal structure.

Binary TiAl alloys containing 45–60 at.% Al were studied in detail in order to understand the fundamental deformation behavior and to establish a guideline for advanced alloy design. Because the formation of this compound involves a peritectic reaction, all alloys were processed by the melt spinning technique to minimize segregation. Microstructural and mechanical characterizations were carried out on both the as-solidified and consolidated materials. The results are presented to address the issues of (1) the equilibrium phase relationship, (2) the rapid solidification microstructure, (3) the room temperature deformation behavior, (4) the brittle/ductile transition, (5) the temperature dependence of yield stress, and (6) the implication on alloying effects. Several examples of ternary alloys are given.

The ternary TiAl-Nb (Llo) alloys stabilized at 1000°C for a week were prepared into miniature specimens for compressive deformation tests. The specimens were deformed in uniaxial compression at room temperature as well as various high temperatures. The yield stress and fracture strain were determined with respect to Nb concentration, and as a function of temperature to investigate positive temperature dependence.

Dislocation structures and other deformation structures of the deformed alloys were studied by TEM.

Structures and mechanical properties of TiAl-base alloys have been studied, in which Zr, Nb, V or Mn is added as the third element and the Al content is changed. Behavior of the third element in the TiAl-base alloys and effects of various factors on ductility have been discussed. It has been concluded that the Ti-48.4 at% Al-1.0 at% Mn alloy is the optimum composition for obtaining good room temperature ductility.

Alloys with large volume fractions of the intermetallic phase Ni3Si have unique aqueous corrosion resistance that provides the driving force for an alloy design program to improve their generally poor room temperature mechanical properties. The target of the alloy design program at Oak Ridge National Laboratory was to produce wrought alloys with at least 10% room temperature ductility while retaining good aqueous corrosion resistance. In this paper several important findings will be discussed. These include macro and micro alloying (B, C, Be) to produce superplastic wrought alloys with high strength and high room-temperature ductility. Some of the alloys developed are strong enough to be competitive with medium temperature structural alloys such as Inconel® 718.

This paper reviews the several powder processing techniques that have been applied to the fabrication of both monolithic aluminides and their composites. Attention is given to the active research at Rensselaer on reactive sintering and reactive hot isostatic pressing of elemental powders, hot pressing of prealloyed powders, and molding of powder-fiber-binder slurries. The application of one or more of these techniques to the following monolithic intermetallics is described: NiAl, Ni3Al, TiAl, NbAl3, Ta2Al, and TaAl3. Progress has occurred in fabrication of dense compacts, including some composites, from these aluminides.

The mechanical alloying behavior of elemental powders in the Nb-Si, Ta-Si, and Nb-Ta-Si systems was examined via X-ray diffraction. The line compounds NbSi2 and TaSi2 form as crystalline compounds rather than amorphous products, but Nb5Si3 and Ta5Si3, although chemically analogous, respond very differently to mechanical milling. The Ta5Si3 composition goes directly from elemental powders to an amorphous product, whereas Nb5Si3 forms as a crystalline compound. The Nb5Si3 compound consists of both the tetragonal room-temperature α phase (c/a = 1.8) and the tetragonal high-temperature β phase (c/a = 0.5). Substituting increasing amounts of Ta for Nb in Nb5Si3 initially stabilizes the α-Nb5Si3 structure preferentially, and subsequently inhibits the formation of a crystalline compound.

Ni3Al and NiAl pre-alloyed powders produced by argon gas atomization are investigated by light and transmission electron microscopy concerning their structure, chemical and phase composition. For example, Ni3Al powder consists of a L12 matrix (γ′ -phase) with precipitates whose Al content is 6% higher than in the matrix. Selected single powder particles were isothermally deformed in a modified high resolution dilatometer to investigate the creep behavior. From these experiments HIP parameters are predicted and compared with HIP experiments at different pressures and temperatures. HIP diagrams were calculated according to Ashby's model.

An attempt has been made to determine the γ/γ′ phase boundaries with particular emphasis on the γ′ solvus in ternary Ni-Al-X systems with X being one of the VA or VIA transition metal elements. It is shown that the γ′ solvus in each ternary system can be obtained as a surface with respect to composition and temperature by employing differential thermal analysis (DTA) as a key experimental technique with support from energy dispersive X-ray spectroscopy combined with scanning elecron microscope(SEM-EDX). Also shown is that the DTA results often provide useful information on the phase relation in the vicinity of γ and γ′ phase fields. In the Ni-Al-V ternary system, for example, the pseudo-binary constitution between Ni3Al and Ni3V is suggested. The results presented here are systematic and consistent and therefore are reliable as compared with the previous results reported fragmentarily.