The susceptibility to hydrogen embrittlement has been investigated in two L12 superlattices, (Fe, Ni)3V and boron-modified sub-stoichiometric Ni3Al and in a B2 superlattice, FeCo-V. The relationship between ordering and hydrogen embrittlement is explained on the basis of characteristic dislocation structure in the ordered lattice and hydrogen-dislocation interactions

Fully reversed strain-controlled, tension-compression tests were performed at room temperature to study cyclic-deformation properties of Ni3Ge single crystals. The cyclic deformation is performed with the stress axis parallel to the [1 1.94 2.69] direction at a strain rate of 1.0 × 10-4 s-1 and at a total-strain amplitude between 5.0 × 10-4 and 1.5 × 10-3. Cyclic strain-hardening occurs to saturation over the range of the employed strain amplitudes. The cyclic-stress amplitude is higher in compression than in tension at a plastic-strain amplitude larger than 1 × 10-5 where screw dislocations are operative. This stress asymmetry becomes more prominent (i.e., the mean stress-amplitude becomes larger in magnitude) as the plastic-strain amplitude becomes larger. At a plastic-strain amplitude less than 1 × 10-5 where edge dislocations are operative, the stress amplitude is symmetric. A similar stress asymmetry is also observed for monotonic flow stress. The cyclic hardening is explained by considering an interaction between the screw dislocations.

Several experimental techniques have been implemented in order to compare the tribological response of Ni3Al, (Fe, Co)3V, and (Fe, Ni)3V to commercial engineering alloys. Abrasive and adhesive macrowear behavior during sliding contact has been determined. Microstructural data obtained from transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been correlated with macrowear results. In this correlation, consideration was given to the effects of wear on long-range-order and the presence (or absence) of dislocation subcell structures. The formation of dislocation subcell structures is partially related to the material's stacking fault energy (SFE) which has been the basis of recent wear theory. An experimentally verified wear mechanism based on inhomogeneous slip is defined.

The effect of DO3 transitions on the yield behavior of alloys near the Fe3 Al composition was investigated by tensile tests. Above 400°C, DO3 (Fe3 Al) ordered alloys exhibited an anomalous increase in yield strength with temperature with a peak value around 560 to 600°C which was not observed if the DO3 ordered alloys were also aged before testing. This yield behavior is explained by the precipitation of a (disordered solid solution) and the formation of B2 (FeAl) via the sequential reactions: DO3→α+DO3→α+B2.

The present paper deals with the occurence and magnitude of characteristic mechanical anomalies observed in many L12 intermetallic compounds and their relation to the phase stability of the crystal structure against other geometrically closed packed(GCP) phases. The dislocation configurations in the crystal structure and resultant plastic behavior are in close relation with anti-phase boundary(APB) and stacking fault(SF) energies in the compounds, while the relative magnitude of these energies on various crystallographic planes actually determines the phase stability of the L12 crystal structure. It is shown here, based on our series of investigations carried out over the past decade, that the general tendencies for particular deformation modes to occur can be systematically interpreted in terms of alloy chemistry of the compounds, including variables such as combination of component species, nonstoichiometry and ternary additions in the compounds, all of which affect the stability of the phase.

This paper examines the fundamentals of deformation behavior in ordered y′ (Ni3Al), β (NiAl) and β′ (Ni2AITi) phases, and specific two phase y/y′ and B/B′ alloys. The relative creep strengths of these ordered nickel-base alloys are discussed. Differences in creep behavior are explained in terms of the effects of temperature, crystallographic orientation and alloying on creep deformation mode. In particular, it is shown that trace additions of boron and carbon to y/y′ alloys, or deviations from stoichiometry in β′ -type alloys can exert a strong influence on creep behavior.

Gamma prime (Ni3Al), with its unusual temperature dependence of yield strength, is the strengthening phase in Ni-base superalloys. Many facets of γ/γ′ coherent precipitation strengthening have been well studied: coarsening kinetics, volume fraction vs. strength, yield strength vs. temperature, dislocation dynamics, etc.

This contribution deals with several important aspects of two phase behavior in coherent systems (for high temperature application) which are either relatively new or have received inadequate attention. The most desirable γ/γ′ misfit, in sense and magnitude, is dependent on the temperature of application. It must be recognized that the sense and size of misfit are not only a function of alloying, but temperature as well. Sufficient data are presented for various alloying additions to γ and γ′ to allow one to roughly compute the misfit at room temperature or 1000C. The evolution and significance of the ′′ morphology are discussed. Stability of particle size and shape are related to misfit, temperature and stress. It is demonstrated that for high temperature creep resistance, a fine rafted structure is the most desirable. Finally, some data are presented which demonstrate the dependence of γ′ strength on stoichiometry as well-as temperature.

A portion of this work was sponsored by AFOSR, with A. Rosenstein as contract monitor.

The aluminide intermetallic compounds are considered potential structural materials for aerospace applications. The B2 binary aluminide FeAl has a melting point in excess of 1500 K, is of simple cubic structure, exist over a wide range of composition with solubility for third elements and is potentially self-protecting in extreme environments. The B2 FeAl compound has been alloyed with 1 to 5 at % ternary additions of Si, Ti, Zr, Hf, Cr, Ni, Co, Nb, Ta, Mo, W, and Re. The alloys were prepared by blending a third elemental powder with pre-alloyed binary FeAl powder. Consolidation was by hot extrusion at 1250 K.

Annealing studies on the extruded rods showed that the third element addition can be classified into three categories based upon the amount of homogenization and the extent of solid solutioning. Constant strain rate compression tests were performed to determine the flow stress as a function of temperature and composition. The mechanical strength behavior was dependent upon the third element homogenization classification.

Thin surface films have been shown to enhance the ductility and decrease the flow stress of several body-centered cubic metals at temperatures T < 0.2 Tm. The origin of this effect lies in the large difference in the intrinsic mobilities of edge and screw dislocations in body-centered cubic crystals. B2 ordered intermetallic alloys, although simple cubic in structure, are based on the body-centered cubic structure and have dislocation core structures, dislocation mobilities and temperature and orientation dependent deformation qualitatively similar to that of bcc metals. This investigation was initiated to examine possible effects of surface films on the mechanical behavior of B2 ordered intermetallic alloys, using oxidized NiAl as the initial material for investigation. Experiments were performed on an impure non-stoichiometric (47.1 at.% Al) single crystal material with an axial orientation near [123]. Surface film softening was observed at room temperature in compression at a strain rate of 2 x 10-4 s-1. Flow stresses of the oxide coated crystals were as much as 20% lower than those of identically prepared uncoated crystals. The strains to fracture of coated specimens were larger than those of uncoated specimens and in a few instances the ductility enhancement was as much as four times. Of the several oxides examined, the largest softening effects were found for a thermally deposited delta-A1203 film formed at 1000 °C for 1 hour. The current results for NiAl are compared to results previously obtained for bcc metals. Experiments which could further enhance the film softening effects observed in B2 ordered intermetallic alloys are suggested.

The deformation behaviour of the ordered intermetallic (Fe, Ni)Al phase with B2 structure is studied as a function of composition and temperature with stress strain tests and creep tests.The alloy composition has been varied with respect to stoichiometry and the Fe/Ni ratio. The creep resistivity depends on the alloy composition in a complex manner: a deviation from the stoichiometric compositions leads to a reduction of the creep resistance, and there is a maximum of the creep resistance for the ternary alloys with about 10% Fe.

Ductile intermetallic Ni3 Al-B alloys have been processed through three rapid solidification techniques: melt spinning, gas atomization, and plasma deposition. Different thermal treatment was required for each rapidly solidified product to form samples for mechanical evaluation. Melt-spun ribbons were tested in ribbon form in the as-cast or annealed conditions, while atomized powders were tested after consolidation by hot isostatic pressing (HIP) or low pressure plasma deposition. Alloy strength, as well as tensile ductility, was found to depend strongly on processing technique, thermal treatment, sample geometry, and most importantly, alloy chemistry. Microstructural and fractographic observations indicate that grain boundary brittleness caused by material processing history plays the major role in determining the alloy's mechanical behavior.

The polycrystalline intermetallic alloys FeAl (50.9 to 58.2% Fe), NiAl (49.2 to 55.9% Ni) and CoAl (48.5 to 52.3% Co) have the B2 structure and are of interest for high temperature applications. We have used the PUCOT (piezoelectric ultrasonic composite oscillator technique) to measure mechanical damping or internal friction (Q−) and Young's modulus (E) as a function of temperature and composition for these materials. The modulus data for six CoAl alloys at temperatures up to 1300 K are presented. Examples are given of the strain amplitude dependence of Q−1 for four CoAl alloys. The curves showed the break away phenomenon and are interpreted in terms of a theory dealing with the pinning of dislocation lines and their eventual break away at large strain amplitudes. The dislocation density was calculated to be about 108 m−2. For all the compositions (Xl) of CoAl studied a single equation could be fitted to the data:

E (X1, T) = 210.8 + 2.2 (XI) -0.08T (1)

where T is in K and E in GPa.

The object of this paper is to describe a coordinated research and development program which has been pursued by an Air Force-Industry-University team for more than twelve years. The focus of our attention has been on the development, processing, and engine testing of alloys based on intermetallic compounds, specifically on the aluminides of titanium, iron, and nickel. The titanium aluminides, Ti3Al and TiAl, are the materials with which we have been working the longest and on which development has proceeded the furthest. This Symposium has provided the first opportunity to review the progress of the titanium aluminide development programs sponsored by the Air Force and some of the engine testing efforts undertaken by the engine manufacturers.

This paper provides a comprehensive review of current efforts on design of ductile polycrystalline Ni3 AI alloys. Microalloying has proven to be very effective in alleviating the grain-boundary emibrittlement problem. The ductility and fabricability of Ni3 Al (24 at. % Al) are dramatically improved by adding a few hundred parts per million of boron. The beneficial effect of boron is related to its unusual segregation behavior as predicted from the theory of grain-boundary cohesion developed by Rice, based on thermodynamic analyses. Alloy stoichiometry strongly influences grain-boundary chemistry, which, in turn, affects the boundary cohesion and overall ductility of Ni3 Al.

The solid-solution hardening of Ni3 AI depends on the substitutional behavior of alloying elements, atomic size misfit, and the degree of nonstoichiometry of the alloy. Hafnium additions are very effective in improving high-temperature properties of ternary Ni3Al (Al + Hf = 24 at. %) doped with boron. Alloying with <2% Hf substantially increases the yield stress and raises the peak-strength temperature. In addition, hafnium substantially improves creep properties and oxidation resistance. The Ni3 Al aluminides truly represent a new series of heat resistant materials which do not depend on chromium for oxidation resistance.

As part of NASA's Conservation of Strategic Aerospace Materials (COSAM) Program, a research effort is underway to explore the potential of the B2 aluminides as structural material alternatives for the strategic element containing superalloys currently used in gas turbine engines. Emphasis is being place on the equiatomic Fe and Ni aluminides. Although Co is a strategic material, the equiatomic Co aluminide is also being studied to gain a more complete understanding of these fourth period intermetallics. The research effort is a cooperative program involving in-house research at NASA Lewis plus several university grant programs. Research focuses on initial processing techniques such as ingot melting, powder metallurgy, and rapid solidification with and without additional thermomechanical processing; high temperature deformation - primarily compressive creep; compositional effects within the binary B2 aluminides; third-element alloying addition effects on high temperature strength and oxidation resistance; and near room temperature ductility as influenced by processing, alloying, and grain size. This paper will review the various programs now underway and present some of the highlights of research results.

A review of rapid solidification studies of high temperature ordered intermetallic compounds is presented. Emphasis is on the nickel - and iron - aluminides which are of potential interest as structural materials. The nickel-base aluminides which have been rapidly solidified exhibit changes in grain size, compositional segregation, and degree of long range order (as reflected in APB size and distribution) which markedly affect mechanical properties. Some experiments indicate the formation of a metastable L12 phase in rapidly solidified Fe-(Ni, Mn)-Al-C alloys, while other work observes only a metastable fcc phase in the same composition range. The metastable phases and/or microstructures in both nickel and iron aluminides are destroyed by annealing at temperatures >750K, with subsequent degradation of mechanical properties. Rapid solidification studies of several other intermetallic compounds are briefly noted.

This paper presents some of the results obtained in an on-going study of NiAl alloys for potential high temperature applications. Alloys were prepared by powder metallurgy techniques. Flow stress values at slow strain rates and high temperatures were measured. Some promising ternary alloying additions (Hf, Ta and Nb) have been identified. The mechanism of strengthening in alloys containing these additions appears to be a form of particle dislocation interaction. Interesting effects of grain size and stoichiometry in binary alloys are also presented.

Further development of Ni-base superalloys based on γ–γ′ structures offers little hope of substantial improvements in properties for gas turbine applications. However because of the substantial technology base which exists for nickel base alloys there are considerable advantages, in research time and money, in developing new nickel base alloys. Interest in the 1970's in directionally solidified composites indicated some possible new systems to replace γ–γ′ alloys. More recently there has been renewed interest in aluminides such as FeAl, CoAl and NiAl. These materials exhibit melting points which are substantially greater than for current γ–γ′ alloys. Problems of lack of ductility for these alloys may be resolved by alloying and novel processing routes such as rapid solidification.

In addition to NiAl there are a number of other nickel intermetallics which from limited experimental data are worth considering at least as second phases. Some of these compounds are Ni7Hf2, Ni7Zr2, Ni5Zr, Ni6A1Ta and Heusler alloys of the type NiAlx where x is a transition element. The solubilities and phase relationships and where available the mechanical property data in a number of nickel ternary systems will be reviewed in terms of their potential for development of new high temperature alloys.

Melt spinning was used to produce rapidly solidified ribbons of the B2 intermetallics NiAl and FeAl. Both Fe-40Al and Fe-45A1 possessed some bend ductility in the as spun condition. The bend ductility of Fe-40Al, Fe-45Al, and equiatomic NiAl increased with subsequent heat treatment. Heat treatment at approximately 0.85 Tm resulted in significant grain growth in equiatomic FeAl and in all of the NiAl compositions. Low bend ductility in both FeAl and NiAl generally coincided with intergranular failure, while increased bend ductility was characterized by increasing amounts of transgranular cleavage fracture.

The use of rapid solidification processing to produce a refined dispersion of second phase in Ti3Al is demonstrated. Laser surface melting produces a disordered solid solution of Ti and Al with a supersaturation of Er and 0. A refined dispersion of Er2O3 is formed on heat treatment at 973 K. This oxide is found to be stable at elevated temperatures up to 1173 K.