The alloy Ni−20 at. % Mo is FCC and disordered at high temperatures, but below 868°C forms a superlattice of the D1a type. The disordered structure can be retained by quenching, and subsequent aging below 868°C can develop a variety of ordered domain structures. In the disordered condition, the yield stength at 25°C is typically 140– The tensile properties were measured for two ordered domain sizes: 24 nm and 3,000 nm. For the 24 nm domain size, the yield strength at 25°C was 862 MPa (125,000 psi). This decreased with increasing test temperature to about 690 MPa (100,000 psi) at 800°C, then decreased greatly to about 210 MPa (30,000 psi) at 1000°C. In the ordered condition, the alloy had a very low ductility (1% elongation or less) from 25°C to the disordering temperature of 868°C, above which it increased considerably (e.g., 30% elongation). The alloy with the 3,000 nm domain size showed similar behavior, although the strength was less. (The ordered alloy with 24 nm domain size had a yield strength-temperature curve similar to that of Waspaloy.) These results are correlated with fractography of the tensile samples.

The available experimental data for the Ni3X, LI2 intermetallics indicate a strong effect of chemistry on the tendency for brittle intergranular fracture. Combining the data for the binary compounds with the effects of macroalloying with × substituents and microalloying with boron, the resistance to intergranular fracture varies as Fe ~ Mn > Al ~ Ga > Si > Ge. Two models have been proposed to explain these chemistry effects. The basis for both models is that electronic charge localization is responsible for the brittle behavior of the ordered intermetallics. The models differ in the measure of charge redistribution; one uses valency differences whereas the other uses electronegativity differences.

This paper provides a comprehensive review of the recent work on tensile ductility and fracture behavior of Ni3AI alloys tested at ambient and elevated temperatures. Polycrystalline Ni3Al is intrinsically brittle along grain boundaries, and the brittleness has been attributed to the large difference in valency, electronegativity, and atom size between nickel and aluminum atoms. Alloying with B, Mn, Fe, and Be significantly increases the ductility and reduces the propensity for intergranular fracture in Ni3 Al alloys. Boron is found to be most effective in improving room-temperature ductility of Ni3Al with <24.5 at. % Al.

The tensile ductility of Ni3Al alloys depends strongly on test environments at elevated temperatures, with much lower ductilities observed in air than in vacuum. The loss in ductility is accompanied by a change in fracture mode from transgranular to intergranular. This embrittlement is due to a dynamic effect involving simultaneously high localized stress, elevated temperature, and gaseous oxygen. The embrittlement can be alleviated by control of grain shape or alloying with chromium additions. All the results are discussed in terms of localized stress concentration and grain-boundary cohesive strength.

The existing work on structure and mechanical behavior of intermetallic alloys containing dispersed second phases — dispersoids — is reviewed. The dispersoids considered in the review are those given the conventional definition as inert, stable, insoluble phases in the matrix, such as oxides. The only detailed mechanistic studies of dispersoids in intermetallics have been carried out on the model material Cu3Au, and these did not include elevated temperature, eg. creep, mechanical behavior. A number of investigations on dispersoids in the potentially important elevated temperature materials — the aluminides — are reviewed. The materials discussed include FeAI, Fe3AI, NiAI, Ni3AI, and Ti3Al. Many of these studies are preliminary in nature and much more research anddevelopment is needed to allow for the design of intermetallic-dispersoid materials with improved fabricability and elevated temperature performance.

The B2 aluminides are currently being investigated for potential high temperature structural applications. Although they are not being as actively pursued as the titanium aluminides or the L12 nickel aluminide, the B2 aluminides are very attractive from density gonsiderations. Several recent reviews of the potential for aluminides are available in literature [e.g Ref. 1,2]. Table I is a comparison of the titanium, nickel and iron aluminides of interest and shows that B2 NiAl and FeAl have the major advantage of lower densities than Ni3Al and Fe3Al. In addition, the melting point of NiAl is over 200K higher than convetitional nickel based superalloys. Hence, although low density is the prime driving force, at least in NiAl a temperature advantage is also possible. Both of these aluminides have the advantage of containing very inexpensive elements. In fact, the thrust towards the B2 aluminides evolved from a program aimed at conserving strategic aerospace materials at NASA Lewis Research Center. A recent thrust at NASA Lewis Research Center has been to consider these aluminides as matrix materials for fiber reinforced composite systems.

This paper presents a review of recent research conducted at the Air Force Wright Aeronautical Laboratories (AFWAL)/Materials Laboratory to develop iron-aluminides as elevated temperature structural materials. The research consisted of investigations on the microstructure, tensile behavior, deformation, and fracture mechanisms of the DO3 and B2 ironaluminides. Binary Fe-Al alloys with a wide range of Al contents, solidsolution ternary alloys and precipitation- and dispersion-strengthened two-phase alloys have been investigated. It is shown that iron-aluminides have a potential to be structural materials at least up to 650°C.

In the widespread search for structural materials for high temperatures, it is useful to first apply basic physical considerations to eliminate the least promising candidates. Certain basic quantities are indicators of some of the several properties that are demanded of hightemperature materials. Those properties are high strength and specific strength, high stiffness and specific stiffness, and low creep and thermal expansion coefficient. The basic quantities (structure-insensitive in that they vary little with processing history and consequent microstructural changes) which are chosen to measure these properties are melting temperature Tm, specific gravity ρ, and elastic moduli Cij. Fortunately the most readily available of these structure-insensitive properties, Tm and ρ, are the most useful. As Tm increases, the general trend is for stiffness and strength to increase and thermal expansion and creep to decrease. For space and aircraft applications, knowledge of ρ allows relative values of specific stiffness and specific strength to be estimated from the ratio Tm/ρ. Data for more than 280 intermetallic compounds have been collected and subdivided by crystal class so as to allow the most promising compounds and groups of compounds to be selected. The Ll2 structures tend to lie in the least desirable regions of Tm and ρ and therefore are not the first m candidates for maximizing high-strength at elevated temperatures. At present we have no simple structure-independent indicators of ductility and oxidation resistance--two other properties that are needed in hightemperature structural materials.

Multicomponent nickel base intermetallics with the L12 structure were evaluated as high temperature structural materials. The compounds were based on the γ' composition of PWA 1480, a high strength single crystal nickel base superalloy. The best balance of properties in the compound was achieved with <111> oriented single crystals but no significant advantage could be demonstrated over the precipitation hardened superalloys. Insufficient impact resistance was a major deficiency of the L12 compounds. Other nickel base intermetallics were also evaluated but showed little advantage over superalloys.

The tensile properties of SiC fiber, Ti Al+Nb and SiC/Ti3Al+Nb composite have been determined from 300 to 1365K. The Composite results compared favorably to rule-of-mixtures (ROM) predictions in the intermediate temperature regime of 475 to 700K. Deviations from ROM are discussed. Composite tensile results were compared on a strength/density basis to wrought superalloys and found to be superior. Fiber-matrix compatibility was characterized for the composite at 1250 and 1365K for 1 to 100 hours.

Grain boundary segregation, and its effects on strength and ductility, play an especially important role in the performance of intermetallic alloys based on Ni3Al. The effect of a given segregant on grain boundary strength appears to be determined to some extent by its tendency to segregate to free surfaces. In this paper, we briefly review the available information on grain boundary and surface segregation in boron doped Ni3Al.

The high temperature oxidation of metal silicides is briefly reviewed followed by a description of recent experiments on the oxidation and mixed oxidant corrosion of Ni- and Fe-silicides. The oxidation kinetics are related to the structure and composition of the oxide films formed under various exposure conditions. Finally, the effects of pre-formed SiO2 films on minimizing the corrosion of Ni- and Fe-base alloys in sulfur-bearing gases are presented.

As part of a development effort on nickel aluminides based on Ni3Al as structural materials for fossil energy applications, oxidation/sulfidation studies are being conducted at Argonne National Laboratory on materials that are being developed at Oak Ridge National Laboratory. Sheet samples of nickel aluminide, containing 23.5 at, % Al, 0.5 at. % Hf, and 0.2 at. % B were tested in an annealed condition and after preoxidation treatments. Continuous weight-change measurements were made at 875°C by a thermogravimetric technique in exposure atmospheres of air, a low-p02 gas mixture, and low-p02 gas mixtures with several levels of sulfur. getailed analyses of the corhosion product scale layers were conducted using a scanning electron microscope equipped with an energy dispersive x-ray analyzer and an electron microprobe. The air-exposed specimens developed predominantly nickel oxide; the specimen exposed to a low-p02 environment developed an aluminum oxide scale. As the sulfur content of the gas mixture increased, the alumina scale exhibited spallation and the alloy tended to form nickel sulfide as the reaction phase. The results indicated that the sulfidation reaction in nickel aluminide specimens (both bare and preoxidized) was determined by the rate of transport of nickel from the substrate through the scale to the gas/alumina scale interface, mechanical integrity of the scale, and the H2S concentration in the exposure environment.

The reaction chemistry of the Fe-Al and Ni-Al systems was studied using differential scanning calorimetry (DSC). Experimentally measured heats of reaction and compositions of reaction products were characterized in terms of sample heating rates, reaction temperature, shape and size of aluminum powder, and sample preparation. Standard enthalpies of formation were derived from DSC results. Reaction products were identified by x-ray powder diffraction techniques.

During deformation of bcc metals and bcc-based ordered alloys, conditions of elastic and plastic constraint associated with the presence of thin adherent surface films can be responsible for introducing increased densities of mobile dislocations in the metal, resulting in enhanced ductility and reduced yield and flow stresses of the film-coated materials. In the present paper, surface film effects were investigated as a function of temperature and crystallographic orientation for single crystal β-NiAl. Appreciable temperature-dependent and orientation-dependent surface film effects were observed, as were significant effects of film adherence on the observation of surface film softening.

Melt spinning has been carried out on binary TiAl alloys at three Ti/Al ratios. Antiphase domains were observed in one ribbon specimen, but no significant disordering was induced by the rapid solidification as indicated by X-ray and electron diffraction analyses. Bending tests of both the ribbons and the consolidated counterparts showed a decrease in ductility with increasing Al concentration. This compositional effect can be correlated with the TiAl tetragonality (the c/a ratio) as well as the grain structure.

Ductile ordered intermetallic alloys of nickel aluminum or nickel aluminum chromium have been developed at the Oak Ridge National Laboratory (ORNL) by optimized additions of boron. These alloys show excellent elevated temperature mechanical properties and corrosion properties. However, in order for the alloys to find use in various applications, they should be fabricable by either the well established or innovative processing technologies. The present paper will discuss the details of fabrication technology being pursued at ORNL. The processes being investigated include powder consolidation by extrusion, powder consolidation by capping, isothermal forging of powder compacted material, twin-roller casting to thin sheet followed by cold-rolling, direct casting rod from liquid, extrusion of billets made by argon-induction melting and electroslag remelting processes, injection molding of powders, and hot isostatic pressing of powders. Relative merits of each process will be discussed. Mechanical properties data on products made by various processes will also be presented and compared.

The rolling anisotropy of Ni3Al single crystals was studied. A single crystal sheet in the (011) plane showed remarkable anisotropy. Rolling the sheet in the [100] direction was simple but was almost impossible in the [011] direction. Substantial anisotropy was not observed in the (111) and (001) sheets. The texture of the rolled (011) and (111) sheets were {011}<011>. It is concluded that the rolling anisotropy of single crystal sheets is determined by the presence of active slip system related to compressive strain normal to the sheet plane, and tensile strain parallel to the rolling direction.

It has been shown that it is possible to obtain significant room temperature tensile ductility in FeAl alloys using iron-rich deviations from stoichiometry. A comparison of the room temperature tensile and compressive behaviors of Fe−50at% Al and Fe−40at% Al shows that FeAl is brittle at higher Al contents because it fractures along grain boundaries before general yielding. Lower aluminium contents reduce the yield stress substantially and hence some ductility is observed before fracture.

Addition of boron results in measurable improvements in ductility of Fe−40at% Al and is accompanied by an increase in transgranular tearing on the fracture surface, suggesting a grain boundary strengthening mechanism.

Increasing the cooling rate following annealing at 1273 K results in a large increase in the yield strength and a corresponding decrease in ductility.

High temperature mechanical properties of various Zr and Cr strengthened single phase Ni3Al are investigated, with emphasis on the ability of each element to elevate Tp, the temperature corresponding to the peak yield strength. It is observed that Zr is a very effective strengthener, more so below Tp than above it, while a combination of Cr and Zr is capable of shifting Tp to a higher temperature. The combination results in an effective improvement of the rupture strength of Ni3Al. The strengthening mechanisms of each element will be discussed in this paper.