Dislocation properties revealed by transmission electron microscopy in several families of intermetallic alloys and limitations of such analyses are reviewed. The domain covered includes dislocation processes not necessarily related to mechanical properties, although some emphasis is made on this aspect.

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The mechanistic understanding of yield and flow strengths and brittle fracture behavior of ordered transition-metal aluminides has been critically assessed on the basis of quantum mechanical total-energy calculations, atomistic simulation modeling, and anisotropic elasticity theory of dislocations and cracks. The bonding mechanism is described by the combination of charge transfer and strong p-d hybridization effects. The ground state elastic constants, various shear fault energies, and cleavage energies are calculated for aluminides of cubic (L1₂ and B2) and tetragonal (L1₀ and D0₂₂) structures. The orientation dependence of Peierls stress at low temperatures is estimated based on the anisotropic coupling effect of non-glide stresses on the dislocation core. The anomalous yield behavior is analyzed by means of symmetry considerations and the interaction torque effect on the mobility of superdisloations subjected to a generalized applied stress. The ideal cleavage strength is determined by the surface electronic structure calculation, and the critical stress-intensity factor for Model-I crack is obtained using the calculated cleavage energy and elastic constants. In tetragonal aluminides, the twin-slip conjugate relationship makes an important contribution to the strain compatibility for localized plasticity at a crack tip. The boron ductilizing effect in Ni₃Al and the hydrogen embrittlement effect in FeAl are briefly discussed in terms of the present results.

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We review here recent experimental and theoretical work aimed at characterizing and understanding the deformation and fracture behavior of L1₂ trialuminides, with emphasis mainly on Al₃Ti-base alloys. We also review recent work on the binary compound Al₃Sc, which is a model L1₂ trialuminide that is being studied for comparison with first-principles quantum mechanical calculations. The topics covered in this review include: alloy-element effects and phase stability; dislocation structures; mechanical properties; cleavage fracture behavior; and first-principles calculations of elastic constants, fault energies, and ideal cleavage strengths. We discuss various possible reasons for the brittleness of these alloys, and summarize our current understanding of the rather unusual phenomenon of brittle cleavage in relatively soft materials having the high-symmetry L1₂ structure.

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The recent discovery of cubic L1₂ trialuminides formed by alloying normally tetragonal Al₃Ti with Cr, Mn and Co, when combined with previous work with Fe, Ni, Cu and Zn as cubic stabilizing elements, makes it possible to examine changes in properties and structure as a function of the stabilizing element. in particular, the improvement in mechanical properties with the decreasing atomic number of the stabilizing element, Zn to Cr, can be related to decreased bond strengths as shown by changes in the lattice constant and elastic moduli. As expected, there are corresponding changes in the nature of the dislocations carrying the plastic deformation. Recent attempts to understand the fracture behavior of the cubic forms are discussed. Determinations of the cyclic oxidation resistance and the coefficient of thermal expansion for the alloys with best mechanical properties, Al₆₇Cr₈Ti₂₅ and Al₆₇Mn₈Ti₂₅, are also presented.

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In the development of high temperature intermetallics, it has become evident that it is essential to consider the strong influence of materials processing. Among the fundamental data needed for effective processing are the relevant phase diagrams, the characteristic diffusivities and possible solidification reaction pathways. In the Ti-Al system recent advances in the clarification of the phase diagram have had a direct impact on the analysis of phase stability and crystal growth processes. Building on the binary phase equilibria, it has been possible to develop new insight into the Ti-Al-Nb ternary system and the identification of ternary intermetallic phase reactions. Similarly, diffusion couple studies have allowed for an analysis of reaction rates that is a necessary bases for an effective microstructural control and design strategy especially in the case of intermetallic matrix composites.

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The structure and the compressive mechanical properties of the Ti-rich TiAl (approximately Ti-47at%Al) compact produced by vacuum hot pressing of PREPed powder were investigated and compared with those of the compact having approximately stoichiometric TiAl composition (Stoi-TiAl compact) in the previous study. Ti-rich TiAl compact showed ultra-fine microduplex structure (average grain diameter 2 μm) consisting of equiaxed γ grain and the grain which was composed of (γ+α₂) lamellar structure. Such a Ti-rich TiAl compact indicated higher strength and better ductility than those of the Stoi-TiAl compact at room temperature. However, at high-temperature such as 1 223 K, flow stress of the Ti-rich TiAl was lower than that of the Stoi-TiAl compact. During the deformation of the Ti-rich TiAl compact at 1 223 K, dynamic recrystallization (DR) which introduced a certain extent of grain refinement was occurred. This compact exhibited larger m value than 0.3 at 1 223 K, and was presumed to be deformed by superplastic flow closely related to DR.

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Gamma-base titanium-aluminides were produced by reactive hot isostatic pressing (RHIP) of cold extruded titanium-aluminium elemental powder mixtures. Various RHIP temperatures as well as additional heat treatments were applied in order to obtain different microstructures. Compression samples were tested between room temperature and 900°C. Changes in microstructure during compression such as microcracking, inhomogeneous deformation and dynamic recrystallization were investigated. The measured properties are discussed with respect to microstructure.

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The effects of Cr additions to TiAl-base alloys have been investigated, using both consolidated rapid-solidification materials and wrought ingot materials. The composition ranges studied are 0-4 at% Cr and 44-54 at%Al. It was found that Cr additions do not affect the deformation behavior of single-phase γ alloys. However, they significantly enhance the plasticity of Al-lean duplex alloys, which contain grains of single-phase γ and grains of lamellar γ/α₂. Plastic elongations of [til.gif] 4% were measured in wrought Ti-46Al-2Cr at room temperature. Other Cr effects on the microstructure, phase stability, and site occupancy were characterized and correlated to the observed mechanical behavior. It was concluded that the ductilization effect of Cr is partially due to its ability to occupy Al lattice sites and modify the Ti-Al bond. It is also partially due to its ability to promote twin formation, by modifying the Al partitioning and therefore the γ/α₂ volume ratio in the lamellar regions.

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Of all the intermetallics that belong to Ni-Nb-Al system, (Ni₃Al+Ni₃Nb), (NiAl+NiAlNb), (NiAl+Ni₃Al), NiAl and Ni₃Al were produced by mechanical alloying (MA) process with a view to refining microstructures. MA was carried out in the attritor mill using pure nickel, pure niobium powder and NiAl prealloyed powder as starting materials. MA powders were consolidated using the vacuum hot pressing.
Sintered aluminides showed fine grain structures with average grain diameter under 5 μm and relative density above 96%. The combination of high strength and large fracture strain of more than 10% at room temperature was confirmed by compression test for each aluminide except for both (NiAl+NiAlNb) and (Ni₃Al+Ni₃Nb). At high temperatures, these aluminides exhibited a steady state deformation and no fracture during the testing. The strain rate sensitivity exponent (m value) of more than 0.3 was obtained for alloys except NiAl. Especially the highest value of m (>0.6) was observed for (Ni₃Al+Ni₃Nb) at 1 173 K. Such large m value more than 0.3 is believed to be associated with the development of superplasticity.

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Recent work on the ordered Fe-Al based intermetallics (B2-FeAl and D0₃-Fe₃Al) for potential high temperature applications is reviewed with emphasis on improvements in mechanical properties achievable by processing and compositional control. Constitution and microstructure as well as the role of crystalline defects, including vacancies, dislocations, tubes, antiphase boundaries and grain boundaries are discussed. The influence and behaviour of these defects with respect to strength, work hardening, ductility and fracture are analysed. Proposed mechanisms for deformation and recent investigations on creep and fatigue of these materials are also reviewed. Current literature on ternary additions, multi-phase alloys and non-equilibrium processing is discussed.

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We have grown TiAl crystals of a nearly stoichiometric composition composed of a single lamellar grain as well as those composed of two lamellar grains and studied their deformation behaviour at room temperature. The deformation behaviour of lamellar grains depends strongly on their lamellar orientation. Yield stress is high for uniaxial loading perpendicular or parallel to the lamellar boundaries (the hard mode of deformation), while it is low for uniaxial loading at an intermediate angle (the easy mode of deformation). The tensile elongation for the easy mode of deformation can be as large as 20% at room temperature. By utilizing such easy mode of deformation, TiAl can be rolled up to 59% reduction in thickness at room temperature. Microstructures resulted from annealing of cold-rolled TiAl have been found to depend on the amount of reduction in thickness. This paper summarizes the results of a systematic study on the deformation and recrystallization of TiAl crystals with the lamellar structure.

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One of the authors has reported that unidirectional solidification by a floating zone method (FZ-UDS) is effective in improving the room temperature ductility of polycrystalline Ni₃Al without ductility enhancing elements (Acta Metall. Mater., 38 (1990), 2667). More details on the microstructure and mechanical properties of the stoichiometric Ni₃Al grown by FZ-UDS were investigated in this paper. The solidification structure was dependent on the growth rate. The alloys showed a columnar grained and single phase structure with weak ‹100›, ‹110›, or ‹210› texture when the growth rate was above 13 mm/hr, and showed a large tensile elongation of more than 60% at room temperature. Crystallographic slip and transgranular fracture were observed on the fracture surface. Intergranular fracture was completely suppressed. When the growth rate was below than 13 mm/hr, the alloys precipitated martensitic second phases in the columnar grained Ni₃Al matrix and showed a smaller tensile elongation than that of the alloys grown at the growth rate of 24 mm/hr. These results indicate that the FZ-UDS is a promising method to improve the room temperature ductility of polycrystalline Ni₃Al.

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The interest in titanium aluminide alloys includes elevated temperature applications, for which creep resistance is a primary property. Tests have been made between 650 and 870°C on a variety of microstructures of Ti-24Al-11Nb and Ti-25Al-10Nb-3Mo-1V (at%) alloys. It has been found that microstructure plays an important role in creep of these materials, so that thermal and mechanical history is important. Stress exponents for power-law creep, and apparent creep activation energies, have been determined for these alloys. As is usually found in structural alloys microstructural characteristics which increase ductility and toughness at low temperature tend to accelerate creep considerably, particularly the presence of β phase, and most notably when arranged as locally-continuous β films between plates of the α₂ phase. Solution treatment in the β phase provided optimum creep resistance, but cooling rate effects were different in the two alloys considered. Comparison to near-α titanium alloys developed for creep resistance, shows that the aluminide alloys have better performance, especially above 700°C.

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Fundamental study of the high temperature deformation of titanium aluminide, TiAl, has been conducted in order to develop the manufacturing process using isothermal forging. The flow stress was exactly expressed using Zenner-Hollomon paramter as well as common metals. The recrystrallization occurred during high temperature deformation, and structure was transformed from the lamellar structure to the fine recrystallized grain structure. And there were some deformation conditions where TiAl could be deformed without any defects up to 80% degree of reduction from as cast state.
On the basis of above information, manufacturing of pancakes was carried out by isothermal forging. It was confirmed that it was possible to work plastically even from cast ingots samples and that mechanical properties could be improved by isothermal forging.
These results suggest that the isothermal forging is a extremely effective process for manufacturing processes of TiAl.

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The effect of Mn addition on the mechanical properties of TiAl base alloys was examined by compression test for Ti-35mass%Al, Ti-36mass%Al and Ti-34.5mass%Al-1.45mass%Mn alloys in the temperature range of 296 to 1 073 K at strain rates of 10^-1 to 10^-4 sec^-1. Mn addition to the TiAl alloy increased the proof stress and improved the deformability at both room temperature and high temperatures. The proof stress of the TiAl alloy containing Mn had the positive temperature dependence for both the as-cast and the heat-treated alloys. The reason for the positive temperature dependence of the proof stress in the as-cast alloy is different from that in the heat-treated alloys, i.e. in the former it is due to a large amount of Ti₃Al phase and in the latter alloy it is due to the decrease of the Al content in TiAl phase. The as-cast alloy containing Mn deformed soundly at 1073 K at a strain rate of 10^-1 sec^-1 and had the compressive strain of more than 50%.

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The effect of carbon and nitrogen on mechanical properties of single and dual phase γ TiAl alloys was studied in tensile tests at room temperature as functions of content of interstitial elements such as carbon and nitrogen, titanium/aluminum compositional ratio and grain size. The fracture strain in stoichiometric and aluminum-rich TiAl alloys annealed at 1 423 K was improved from nearly zero plastic strain to 0.6-0.8% plastic strain by the addition of 0.3-0.6 at% carbon. In titanium-rich TiAl alloys annealed at 1 423 K and all of titanium-rich, stoichiometric and aluminum-rich TiAl alloys annealed at 1 573 K, tensile fracture strain was decreased by the addition of carbon and nitrogen. Volume of a unit cell in TiAl phase decreased by the addition of a small amount of carbon and nitrogen. The analysis of TiAl-1.0at%C alloys by an X-ray diffractometer showed the presence of Ti₂AlC phase.

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The effect of cholrine content on the tensile properties of titanium aluminide (TiAl) have been studied on the specimens which are made by self-propagating high-temperature synthesis (SHS) using three kinds of titanium raw powder. Chlorine as an impurity was found to enhance the formation of porosities, resulting in degrading the mechanical properties, especially the tensile strength.

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The use of NiAl as a structural material has been hindered by its lack of tensile ductility or toughness at room temperature. The operative flow and fracture mechanisms in monolithic NiAl leading to these poor low temperature properties are analyzed, demonstrating the need for ductile phase toughening. Progress in ductile phase reinforced intermetallics and NiAl-based materials are reviewed and the primary mechanisms involved in the flow and fracture of ductile phase reinforced alloys are clarified by recent investigations of directionally solidified NiAl-based materials. The mechanical behavior of these model alloys (Ni-39Al and Ni-30Fe-20Al (at%)) are discussed. The prospects for developing a ductile phase toughened NiAl-based alloy and the shortcoming presently inherent in these systems are analyzed.

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From measurements on more than two hundred alloys four binary intermetallic compounds have been identified that melt above 1900°C and are tough at ambient temperature. Effects of alloying on elastic properties and room-temperature toughness are given for two of these compounds, RuTa (an L1₀(tP4) structure) and AIRu (a B2(cP2) structure). For selected alloys other mechanical data are presented. The high-temperature hardness of AIRu alloys are improved by 4%Sc, 0.5%B doubles the compressional strain to fracture, and Cr aids the oxidation resistance.

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In this paper we review experimental and theoretical findings related to the recently discovered mechanism of moisture-induced environmental embrittlement in FeAl-based alloys. We show that when low aluminum content FeAl alloys (35 and 36.5% Al) are tested in air, the aluminum in the alloys reacts with moisture in the air, producing atomic hydrogen. This atomic hydrogen enters the metal in the vicinity of the crack tips and embrittles the FeAl aluminides. As a result, when the alloys are tensile tested in air, it is commonly found that they fracture with limited ductility by transgranular cleavage. When this embrittlement mechanism is suppressed (e.g., by testing in dry oxygen), ductility is found to increase dramatically (to as much as 17-18%), and the fracture mode changes to intergranular. The intrinsic resistance to fracture is therefore quite high in these alloys. First-principles calculations confirm that the intrinsic cleavage strength and energy of FeAl are indeed quite high (comparable to or slightly higher than that of a ductile alloy like Ni₃Al). The calculations also show that absorbed hydrogen can significantly reduce the cleavage strength and energy of FeAl (by as much as 20-70%, depending on the hydrogen concentration), consistent with the proposed embrittlement mechanism. In higher Al content FeAl alloys (40 and 43% Al), there is an additional cause of brittle fracture, namely intrinsically weak grain boundaries. In these alloys, the grain boundaries have to be first strengthened (by the addition of boron) before the moisture-induced environmental embrittlement mechanism becomes evident. Thus, the ductility of Fe-40Al alloys is approximately the same in air and dry oxygen, whereas the ductility of B-doped Fe-40Al alloys is significantly improved when tested in dry oxygen instead of air (from 4.3 to 16.8%). With increasing Al concentration, the grain boundaries in FeAl become increasingly more brittle and, in Fe-50Al, boron is unable to suppress intergranular fracture.

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The improvement of oxidation resistance for the intermetallic compound TiAl and TiAl containing 1.5 mass% Mn was investigated using surface treatments: heat treatment under a low partial pressure oxygen atmosphere, Al diffusion coating and combined treatments utilizing both these processes. The effect of the surface treatments was evaluated by cyclic oxidation tests carried out at 900 and 950°C in static air. Heat treatment under a low partial pressure oxygen atmosphere was very effective in improving resistance for the cyclic oxidation of TiAl at 900°C, but not sufficient for TiAl at 950°C or for TiAl-1.5%Mn alloy at either 900 or 950°C. Al diffusion coating was effective for both TiAl and TiAl-1.5%Mn at 900 and 950°C, but large cracks were found at the edges of all treated specimens. Specimens subjected to the combined surface treatment showed superior oxidation resistance to the Ni-base superalloy Inconel 713C. Specimens heat treated under the low partial pressure oxygen atmosphere followed by Al diffusion coating were found to have a tightly diffused layer with smaller cracks than those treated by diffusion coating alone.

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TiAl P/M intermetallics bearing Si ≤ 3.1%, B ≤ 0.12% and Nb ≤ 13.3% were surveyed in respect of mechanical and oxidation behavior in the range: 600-1 200°C. The oxidation resistance of TiAl alloy is appreciably improved by Si and/or Nb addition, probably due to the protective aluminum-rich oxide on surface. Siliconizing instead of Si alloying will be recommended for good mechanical properties. In general, the tensile strength has a peak value of more than 350 MPa at 800°C or so nearly corresponding to brittle-ductile transition. The creep-rupture strength of TiAl alloy is more or less improved by Nb addition. Consequently, TiAl alloy bearing 1.5% Si and 3-6% Nb will be preferable for use in hot air up to 900°C. A further survey on superplasticity is required for alloy design and microstructural control.

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Possible pathways to mill production of gamma titanium aluminide based alloys are presented with the author's recent results on processing of the alloys. The focus is directed towards the fabrication into sheets by near-net-shaping and the thermomechanical processing for microstructure control. Developments in the alloying, processing, and microstructure controlling technology of titanium based and alpha-2 based alloys are surveyed to assess the opportunities for transfer of the current technology to alloy and process design of gamma based alloys, which may require more severe processing conditions to operate at elevated temperatures in a protective atmosphere and to work at low strain rates under application of high loads. Direct twin-roll casting has been demonstrated to be applicable to the experimental production of gamma alloy sheets. By optimum combinations with thermomechanical processing, macroalloying of beta stabilizing elements has been shown to produce a gamma plus beta microduplex structure that renders superplasticity. These laboratory tests suggest feasibility of the mill processing of gamma based alloys. However, tremendous gaps in both processability and performance are anticipated between laboratory products and mill products, making the transition of laboratory to commercial production more difficult than it has been for conventional alloys in the past. Highly extensive scientific and experimental bases should be established before gamma based alloys can be considered for the practical application in severe aerospace environments with satisfactory reliability and durability.

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Selected intermetallic materials have undergone an evolutionary process whereby some of them could provide major payoffs in aerospace systems. The maturation of intermetallic alloys based on Ti₃Al has provided significant hope for making still greater advances in turbine performance through further developments in other intermetallic materials. The development results obtained to date have highlighted the fact that much of the fundamental basis from which these materials may be understood has simply not been built and has suggested that widespread implementation of these materials lies in the distant future. This paper briefly reviews the recent research results on selected intermetallic alloys being pursued as high temperature structural materials. Specifically, advances and findings from studies performed during the last three years on alloys of the titanium aluminides, nickel aluminides and other intermetallics for service at temperatures over 1 300 K are reviewed. Technical challenges and pacing unknowns are highlighted throughout.

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The production, characteristics, and commercialization of monolithic and composite titanium aluminides are presented with amphasis on use in the demanding aerospace industry. The elevated temperature properties combined with a low density are attractive, but inherently low "forgiveness", and environmental concerns, must be overcome before wide-spread use will occur.

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Intermetallic matrix composites have recently received considerable attention as potential candidates for high-temperature applications. A variety of matrices and reinforcements have been examined to date, and reinforcement type, volume fraction, size, shape, and distribution have been shown to affect microstructure and mechanical properties. Several innovative approaches have been devised to produce discontinuously reinforced composites, ranging from such conventional techniques as blending, mechanical alloying, and rapid solidification processing to more exotic techniques, such as reactive consolidation and XD^TM technology, which depend on the exothermic heat of formation of compounds. Composite mechanical properties of interest typically include high-temperature strength, the strain-rate dependence of strength, and ambient-temperature toughness and/or ductility. Thermodynamic stability of the reinforcement, elastic modulus mismatch with the matrix, and differences in thermal expansion coefficient between the matrix and reinforcement influence these properties. This paper addresses recent advances in these area.

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This study was carried out in order to develop a satisfactory technique for joining TiAl intermetallic compounds. Ti-38mass%Al binary cast alloys were bonded in a vacuum of 26 mPa using the solid state diffusion bonding method by varying the bonding conditions, viz. the bonding temperature varied from 1273 o 1 473 K, the bonding pressure varied from 10 to 30 MPa and the bonding time varied from 0.96 to 3.84 ks.
From metallurgical point of view, the diffusion bonding diagrams with bonding conditions were produced in order to obtain a sound joint without microvoids and oxides such as TiO₂ or Al₂TiO₅ at the bonding interface. According to these diagrams, the joints for mechanical testing were produced at the temperature of 1473 K for 3.84 ks with 15 MPa.
The joint tensile strength at room temperature was about 225 MPa and the joints factured in the base metal zone. However, the joints at the testing temperatures of 1 073 and 1 273 K fractured at the bonding interface and the joint tensile strength was about 40 MPa lower than that of the base metal, because minimal bonding interface migration occurred.
The joints recrystallized at the bonded zone were produced in order to promote the migration of the bonding interface and sequently improved the tensile property at 1 273 K for the joint with the previous bonding conditions and with the post-bonding heat-treatments. The joint tensile strength at 1 273 K with recrystallized grain size of around 130 μm was about 210 MPa and the joints fractured in the base metal zone departed from the bonded zone even at 1 273 K.

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Recent interest in fabricating high modulus, high strength titanium matrix composites has created a need for good quality titanium alloy foils.To fabricate continuous fiber reinforced metal matrix composites, an alternating lay-up of titanium alloy foil and fiber mats is prepared, and consolidated by hot pressing or HIPing. Due to various problems encountered in cold rolling titanium alloy sheets to this gauges (<0.010 in), such foils have often been produced by chemical milling. However, chemically milled foil, often has poor surface finish, excessive gauge variation and may contain holes due to uncontrollable chemical attack. Chemically milled foil can also suffer from hydrogen embrittlement and may pick up other chemical impurities. In the present work, a method of producing rolled foils of titanium alloys and titanium aluminide intermetallics has been developed. The rolled foils exhibit excellent surface finish, good mechanical properties, uniform gauge and are free from any extraneous chemical contamination. Mechanical working coupled with heat treatments allows the flexibility of tailoring the microstructure and mechanical properties of the rolled foil for specific applications. The characteristics of cold rolled and annealed titanium alloy and titanium aluminide foils are described.

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The potential of TiAl as an elevated temperature material is well investigated. However, negligible low-temperature ductility and toughness and low high-temperature strength have limited its practical applications. To improve these properties, this compound was made as a composite material containing a second phase such as Ti₃AlC.
In the present study, using elemental powders, combustion reaction was carried out to form intermetallic/ceramic composites in the Ti-Al-C system. As a result, the compact of Ti and Al powders with C powder reacted exothermically to form TiAl and Ti₂AlC with a small amount of Ti₂Al, and the mixture products had a fine distribution of the Ti₂AlC particles in the mastrix TiAl, and the volume fraction of the Ti₂AlC particles depended on the amount of C present. Subsequently, using these reacted products, button ingots were arc-melted to a full density. The resulting Ti₂AlC particles were ellipsoidal or columnar in shape with sizes from 5-15 μm and appeared to be homogeneously distributed. It was found that the composite materials produced have a good toughness and strength at both room and elevated temperatures. Consequently, present investigations have considerable interest as a new combustion reaction process to make a composite material.

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