We have investigated the lattice location of Ta in Ni3Al using Rutherford backscattering with channeling, and nuclear reaction analysis. An 8 MeV 4He ion beam was directed along the < 100> crystallographic axis of the Ni75Al24Ta single crystal. A silicon surface barrier detector was used to analyze 4He ions backscattered from Ni and Ta atoms. Neutrons generated from Al by the 27Al(4He,n)30P reaction were detected by a large volume liquid scintillator placed outside of the scattering chamber. Essentially all of the Ta atoms were found to be substitutional, as determined by the Ta channeling minimum yield. A comparison of the width of the channeling angular scan for Al, Ni and Ta indicated that the Ta atoms are predominantly distributed on the Ni sites. This result is in conflict with expectations based on the ternary phase diagram.

Intermetallic compounds are usually so brittle that they simply cannot be efabricated into useful shape components. Recent developments of physical and metallurgical principles for the structural materials have led to the conclusion that the deformability of several intermetallic systems can be substantially improved. Among them, in this lecture, the nature of grain boundary structure in L12-type intermetallics is first discussed.

Then, the possibilities and instances to overcome the brittleness are shown. Polycrystalline L12 -type Ni3Al, even prepared from high purity metals, exhibits brittle intergranular fracture. On the other hand, single crystals of Ni3Al are ductile even at room temperature. Thus, it is said that the grain boundaries of this alloy are intrinsically weak. Our recent analyses showed that there exist several configurations of atomic bonds, resulting in the heterogeneous electronic environments which can be regarded as “cavities”. The drastic ductility improvement of Ni3Al alloy by a small boron addition, which was found in 1979, can be interpreted as segregated boron atoms at grain boundaries to modify the electronic environments of boundaries and to reinforce boundary cohesion, thus suppressing intergranular fracture. From a quite similar point of view, modification of boundary structures can be achieved by substituting proper elements for the constituents of compounds or by selecting atom combinations of compounds. Thus, the ductility of Ni3Al could be improved by substituting a few percent of Mn or Fe for Al. Also, by selecting atom combination, several L12-type alloys such as Co3Ti, Cu3Pd, Ni3Mn and Ni3Fe have been found as ductile intermetallics.

Many L12 ordered alloys including the Ni3Al intermetallic system are noted for their anomalous temperature dependence of strength. It is also generally accepted that this dependence is due to a thermally assisted cross-slip, work hardening based model [1,2]. An alternative antiphase boundary (APB) based model has long been dismissed by the research community because prior calculations of APB energy, and some measurements, have shown that the appropriate APB energy of Ni3Al should remain constant with temperature [3]. These aspects will be reviewed briefly and will serve as a basis for a presentation of some more recent results. These will include the strain rate insensitivity of strength versus temperature in the increasing strength temperature region, a result that is, in our view, rather contradictory to the thermally assisted cross-slip model. Some very recent calculations of equilibrium APB energies will also be reviewed in the context of the strength dependence issue. These results show that the equilibrium APB energy increases with temperature. The question of what role, if any, equilibrium APB plays in the deformation and strengthening process will be discussed.

Since writing on this subject two years ago [1], a number of developments have occurred, particularly in relation to the mechanical properties of the L12 nickel aluminide Ni3Al. Some elucidate the nature of the yield strength and the extraordinarily beneficial effect of boron on low-temperature ductility. Some others expose, at least in part, the nature of the marked reduction in ductility at elevated temperatures. Another considers the mechanisms dominating creep deformation. Also during this period, contradictions have appeared: the relationship between the yield strength and the grain size, d, at room temperature has been contested, and opposing views of grain refinement on ductility have been reported.

This paper reviews these developments. Although broadly directed at intermetallic compounds, the discussion is specific to Ni3Al. The hope is that the knowledge and understanding gained about this compound will benefit the class as a whole.

The positive temperature dependence of yield stress in certain L12 and B2 alloys, e.g. Ni3Al and β′-CuZn, is analyzed on the basis of the nature of dislocation dissociations predicted by anisotropic elasticity theory. In the case of Ni3Al, the torque due to the tangential component of the elastic interaction between two superpartials is a major driving force for either the cross-slip pinning model or the force couplet model. The corresponding torque term is relatively unimportant in the B2 structure. The core transformation of individual superpartials may be important for the anomalous increase of yield stress in β′-CuZn.

It is important for a number of reasons to have a basic understanding of the mechanisms by which ternary additions strengthen Ni3Al. First of all, since the basic strength-controlling mechanisms in pure Ni3Al are different from those in pure metals and substitutional solid solutions, it is expected that the mechanisms of solid solution strengthening will also be different in Ni3Al. Secondly, such an understanding will provide valuable insights into the properties of nickel-base superalloys in which Ni3Al is a key constituent. In addition, since new alloys based on an ordered Ni3Al matrix are being developed, it is important to understand the strengthening mechanisms in such alloys. In the present study, flow stress measurements have been performed on single crystals of Ni3Al containing additions of Hf and Ta, and on binary Ni rich, Ni3A1 used as a reference alloy. The data have been collected over a wide range of temperatures, for different orientations within the unit triangle, and as a function of the sense of the applied uniaxial stress. The effect of such additions on the critical resolved shear stress (CRSS) for octahedral slip has been determined and combined with previous data. An attempt is then made to clarify whether a lattice parameter/modulus mismatch effect or a dislocation core effect is the dominant mechanism for the strength increase with compositional changes. It appears that a lattice parameter/modulus mismatch is the dominant mechanism for orientations in which the tension/compression flow stress asymmetry disappears.

Recent studies suggested that additions of either Fe or Mn to polycrystalline Ni3Al without boron, result in a ductile alloy. In the present study the effects of Fe and Mn additions on the strength and ductility of Ni3Al were investigated at 25°C, 600°C in air and at 600°C in vacuum. Metallographic examination showed that with 4 at.% solute a single phase structure existed but at higher solute levels the alloys were two phase. Single phase materials tested in bending showed virtually no plastic strain and low bend strengths at all temperatures. The two phase materials tested at 25°C and 600°C in vacuum showed marked ductility. However, the same alloys tested at 600°C in air exhibited brittle failure. These results suggest that Fe and Mn have no effect on the high temperature dynamic environmental embrittlement known to occur in Ni3Al.

Single crystalline Mn3Sn was chosen as a model material for a study of the deformation machanism of DO19 ordered alloys. Single crystals were grown by the Bridgman method in an argon atmosphere. Compression test specimens of six different orientations were cut from the bulk single crystals by electric discharge machining and polished for slip trace analysis. Compression tests were carried out in an Instron testing machine at a strain rate of 1.2 X 10−3 s−1 at temperatures ranging from 77 OK to 1180 oK. Tests at temperatures above 400 oK were performed in an argon atmosphere while those of lower temperatures were performed in air.

Preliminary results were obtained and analyzed in order to determine the temperature and orientation dependence of the operating slip systems and the yield stress. Basal slip was observed to be the dominant slip mechanism. Slip lines were observed on specimen surfaces compressed at low temperatures but such observations were difficult on specimens deformed at temperatures above 1000 OK due to surface contamination.

An anomalous positive temperature dependence of the critical resolved shear stress for basal slip was found, similar to that reported for octahedral slip in many intermetallic compounds with the Ll2 structure.

The sensitivity of iron-based LRO alloys to strain rate at elevated temperatures was investigated. The tensile results indicate that the iron-based LRO alloys exhibit a positive temperature dependence of yield behavior, but are insensitive to variations in strain rate between 10-5 and 100 sec-1. These observations, along with SEM and TEM results, suggest that previous proposed diffusion-controlled mechanisms are not responsible for the increase in yield strength with increasing temperature.

It is now well established that microalloying additions of B to Ni3Al drastically reduce low temperature grain boundary fracture and consequently increase the ductility of this intermetallic compound. One possible explanation for such effects involves the relationship between boron segregation to grain boundaries and free surfaces, and the resulting effect of such segregation on the cohesive energy of the grain boundaries. This study involves the extension of these concepts to an alloy based on Ni3Si. Auger spectroscopy has been carried out on fractured grain boundaries, grain interiors, and free surfaces to determine how B segregates in Ni3(Si,Ti). The consequences of the segregation of B on the cohesive energy of grain boundaries in Ni3Si based alloys are discussed.

The fatigue behavior of intermetallic compounds is reviewed. The effects of long range order, stoichiometry, test temperature and test environment on crack initiations high cycle fatigue lives and crack growth rates are emphasized. In the case of Ni3Al+B stoichiometry affects high cycle lives largely through the influence of aluminum on ductility and notch sensitivity. High cycle fatigue behavior of Fe3Al is dependent upon stoichiometry and temperature in a complex way which is connected with the formation of superlattice dislocations and with phase changes during high temperature exposure. Oxygen and hydrogen are shown to be detrimental to high cycle fatigue and crack growth in several compounds.

This paper examines the possibilities for obtaining a high creep resistance in intermetallic phases. In a first section, the creep mechanisms are discussed with respect to the effects of stress, temperature and composition in the light of recent experimental results on the creep behaviour of the B2 phase (Fe,Ni) Al between 650 °C and 1100 °C. The familiar models for describing the creep of conventional disordered alloys apply to intermetallic phases, too, and describe the observed behaviour in a quantitative way. In the second section the creep of various ordered bcc phases - (Fe,Ni)Al and (Co,Ni)Al with B2 structure and the Heusler-type Ni2AlTi with L21 structure - and ordered fcc phases - Ni3Al with L12 structure, Fe3AlC and Ni3AlC with E21 structure and NbAl3 with DO22 structure - are examined. Large increases in creep strength are possible by the transition to phases with more complex structures as well as by appropriate alloying. Much further work is necessary for making more reliable predictions on the obtainable creep strengths.

In this paper, firstly, the results of deformation experiments which have been made on Al3Ti and Al3Ti-base alloys are reviewed. Secondly, the possibility of the improvement of ductility of Al3Ti by alloying processes is discussed, and then possible alloying elements for this purpose are suggested. Thirdly, the oxidation resistance of Al3Ti is described and then possible future technological applications of Al3Ti are briefly discussed.

The role of substitutional element alloying of single phase γ' has become of primary interest to alloy designers who would like to exploit its low density and excellent oxidation resistance. Current γ' alloys have not shown sufficient strength to be useful in a creep limited environment. In order to maximize the potential of single phase γ' alloys and to more fully understand the creep strengthening mechanisms in two phase Ni-base superalloys, it has become necessary to clarify the role of Al-substitution elements. Ta is a potent strengthening element in γ' as well as imparting beneficial surface stability to superalloys; its effect on the creep properties of Ni3Al is the subject of this paper. The 1300°C isotherm of the Ni-Al-Ta system was determined in order to establish the γ' single phase field. Comrpositions were fabricated having chemistries which systematically varied both the Al:Ta ratio at Ni=75% and Ni:(AI+Ta) ratio at Ta=6%. Creep tests were conducted on <001> oriented single crystals at 760, 871 and 982°C. Electron microscopy was used to characterize the nature of slip deformation, confirm phase purity and to determine the existence of tetragonal distortions in these crystals. In this manner the strengthening due to Ta was examined in the absence of grain boundary effects. These γ' mono—crystals did not display classical creep response. Incubation creep was observed in all of the specimens tested. Surprisingly, the maximum incubation time was found to occur in the high ratio Ni:(Al+Ta) compounds, where less than 0.5% creep strain was obtained after 200 hours at stress. After incubation, either tertiary creep leading to failure, or apparently classic primary, secondary and tertiary creep ensued. In addition extremely long elongations, to 85%, were measured.

A nickel aluminide with the composition Ni-23.5 Al-O.5 Hf-O.2 B (at. %) was creep-tested in tension at constant load (initial stress 250 MPa, temperature 1033 K). The creep rate reaches a minimum at a strain of approximately 0.3%. Its increase at larger strains is partly caused by grain boundary cavitation. Cavity size distributions corresponding to different creep times and strains were determined metallographically and evaluated in order to obtain the cavity growth rate as a function of cavity size. Contrary to a newly developed model for crack-like diffusive growth the cavity tip velocity increases with cavity size. This discrepancy is attributed to cavity coalescence. Sintering at elevated temperatures (1473 K) reduces the diameter and increases the thickness of the disc-shaped cavities. Sintering of such cavities might be exploited to measure surface diffusivities.

Ductile Ni3Al-B type intermetallic alloys show a unique fatigue crack growth behavior at elevated temperatures. A crack propagation mechanism has been investigated in an experimental P/M alloy by testing the alloys with different fatigue frequencies at 400°C. The Ni3Al-B intermetallic alloy shows a substantial time-dependence of fatigue cracl growth rate when tested in air. Under a given cyclic stress intensity, an order of magnitude difference of crack growth rate was observed by decreasing the fatigue frequency. However, such a time-dependence did not occur when the alloy was tested in vacuum. It is concluded that “dynamic” embrittlement in an oxidation environment is the major factor controlling the fatigue crack growth in ordered Ni3Al-B alloys at elevated temperatures.

A Ni3AI + B base composition and derivatives containing Hf, Zr, Nb, or Nb + Hf substitutions for Al were processed by powder metallurgy, consolidated by compaction + extrusion, and evaluated for mechanical properties. The mechanical property evaluation consisted of tensile tests performed in air and vacuum, and fatigue crack growth tests performed in air. Microstructures, test specimen fracture morphologies, and tensile and fatigue deformation features were also studied. Some of the substitutions improved certain properties, but none improved ductility. The discussion considers property-fracture morphology-deformation structure relationships.

The microstructure of extruded rods of iron-rich FeAl (B2-structure), as characterized by TEM, SEM, optical microscopy and x-ray diffractometry, consisted of elongated grains with a <111> fibre texture containing a high dislocation density. Numerous oxide particles were found, mostly in lines which reflected the matrix flow during extrusion. In addition, some large inclusions were present. Tensile testing of annealed, relatively dislocation-free specimens as a function of increasing temperature found increasing ductility up to 900K, above which a ductility drop occurred accompanied by a change in fracture mode, from transgranular cleavage to intergranular fracture. The yield strength, which was independent of temperature up to 800K (at ∼500MPa), also decreased rapidly as diffusion became more important. The predominant slip vector changed from <111> to <100> around 700K

Studies have been conducted of the mechanical properties of Fe3Al alloys containing 24 to 30 at.% Al, to which 0.5 wt% TiB2 was added for grain refinement. In tensile tests conducted at room temperature, it has been found that, as the aluminum content is increased, the yield strength decreases sharply from 760 to 310 MPa. The decrease in yield strength is accompanied by a four-fold increase in room-temperature ductility. Ordered iron aluminides (containing no disordered α phase) showed a clear increase in yield strength with temperature above 300°C. Their strength reached a maximum around 600°C, above which it decreased sharply. All these results will be discussed and correlated with stability of superlattice dislocations as a function of aluminum content.

Hot compression testing has been used to examine the effects of Mo and Ti additions on the yield strength and rate of work hardening for cast alloys near Fe3Al. A few powder processed materials have also been studied. Significant improvements in high temperature compressive properties on alloying have been related to increases in the DO3→B2 transition temperature and the associated changes in the nature of the dislocations involved in the deformation processes.