High-temperature, light-weight materials represent enabling technology in the continued evolution of high-performance aerospace vehicles and propulsion systems being pursued by the U.S. Air Force. In this regard, titanium aluminide matrix composites appear to offer unique advantages in terms of a variety of weightspecific properties at high temperatures. However, a key requirement for eventual structural use of these materials is a balance of mechanical properties that can be suitably exploited by aircraft and engine designers without compromising reliability. An overview of the current capability of titanium aluminide composites is presented, with an effort to assess the balance of properties offered by this class of materials. Emphasis is given to life-limiting cyclic and monotonic properties and the roles of high-temperature, time-dependent deformation and environmental effects. An attempt is made to assess the limitations of currently available titanium aluminide composites with respect to application needs and to suggest avenues for improvements in key properties.

This paper presents examples of the use of transmission electron microscopy to characterize matrix/reinforcement interaction in titanium aluminide matrix composites reinforced with continuous SCS-6 type SiC. As a result of the high temperature required for consolidating this type composite, reaction products form in the interface. Using diffraction and x-ray energy dispersive spectroscopy techniques, reaction products in Ti3Al and Ti2AINb alloy matrix composites have been identified. TiC1 –x and Ti5 Si3 compounds are common in these composites, with AlTi3C also present depending on consolidation temperature and matrix composition. Residual stress calculations indicate that these reaction products may be subject to cracking during cooling from consolidationtemperatures.

The production of titanium aluminide intermetallic compound foil represents a significant manufacturing challenge. Cold rolling, which imparts excellent thickness uniformity and surface finish characteristics that are of benefit in composite fabrication, is especially difficult with these alloys. However, recent modifications in Ti aluminide alloy compositions and advances in thermomechanical processing have made it possible to produce foil of thickness less than 100 μm, having the microstructure and mechanical property characteristics required for composite fabrication and improved performance. This paper describes the properties of a new Ti aluminide alloy, of nominal composition Ti-22AI-23Nb (at.%), comprising a three phase microstructure of α2 (Ti3Al), an ordered orthorhombic phase (Ti2AINb) and an ordered beta phase. The discussion emphasizes the processing of this alloy through cold rolling to foil, and the associated microstructures and mechanical property characteristics that are relevant to the use of this foil to form a composite matrix.

study was undertaken to examine the attributes of utilizing a high niobiumcontaining titanium aluminide (orthorhombic) composition, specifically Ti-22AI-23Nb (a%), for use as a matrix for a continuously reinforced metal matrix composite. Both unreinforced “neat” panels and 35v% 4-ply unidirectional ([0]4) SiC (SCS-6) panels were fabricated by HIP'ing using a foil / fiber / foil approach. The microstructure of these panels were examined via optical, scanning electron and transmission electron microscopy. Reaction zone kinetics including both the primary reaction product and any beta-depleted zone growth were determined at 982°C. Analytical electron microscopy was employed to identify fiber / matrix interfacial compounds as well as local phases and their associated chemistries. Preliminary mechanical properties were obtained which included: longitudinal and transverse tensile, matrix thermal stability, thermal fatigue, thermal mechanical fatigue and transverse composite creep. The results were compared with panels fabricated from the baseline matrix composition, Ti-24AI-11 Nb.

A SiC/Ti-24AI-11 Nb (at. %) composite (30–35 vol. %) was thermally cycled in air and an inert environment between 150 °C and 815 °C for various cycle counts. Various hold times at maximum temperature were employed to determine timedependent effects on composite integrity. Laminate orientations investigated included: [0]4, [0]8, [90]4, [90]8 and [0/90]2S. Acoustic emission produced during thermal fatigue of selected specimens was employed to monitor damage progression. Post-cycling room temperature tension tests as well as optical and scanning electron microscopy were used to document damage, which was particularly acute when hold times at temperature were employed on tests performed in air. The roles of the environment, composite thickness, and off-axis fibers during thermal fatigue on the composite strength and integrity are discussed.

We have studied interdiffusion and phase relations in two titanium aluminide/Nb/titanium aluminide composite structures at 1100 °C using solid state diffusion approach. The composite structures were prepared with two practical intermetallic alloys, γ (Ti-48Al-2Nb-0.3Ta, in at.%) alloy and super-α2 (Ti-25Al-10Nb-3V-1Mo, in at.%) alloy, and pure Nb sheet by diffusion bonding technique. The interfacial microstructures evolved during thermal diffusion annealing were characterized using optical microscopy, SEM, TEM and X-ray diffraction. The effect of Nb on the phase stabilities of two intermetallic alloys and the phase relations among different phases formed in the diffusion zone due to interdiffusion were analyzed in terms of rules of diffusion paths on the basis of the observed diffusion structures and experimental diffusion paths.

The mechanisms of fatigue crack advance are examined for a lamellar α2 + γ alloy. Crack growth rates are highly dependent on the orientation of the loading axis to the lamellae direction. Thus, the material has some of the characteristics of a composite. For crack growth perpendicular to the lamellae, the mechanisms of crack advance are similar to those of other titanium alloys, while crack growth parallel to lamellae has other characteristics.

In the in situ lamellar γ-TiAl based composites, very large elastic stresses (≈ 1GPa) would be generated at coherent interfaces between close packed planes for three reasons: the tetragonality of TiAl, the larger atomic spacing in α2-Ti3Al than in γ and the differences between thermal expansion coefficients in α2 and γ. These stresses appear to be partially relaxed by the creation of van der Merwe dislocations, diffusion across α2/γ interfaces and cracking along interfaces. Measurements of lattice parameters by Convergent Beam Electron Diffraction (CBED) reveal stresses of the order of 100MPa. The presence of these stresses and the very specific form of the resulting stress tensor are used to discuss the hard/soft mode deformation of TiAl composites.

To evaluate various in-situ reinforcement schemes, a computer controlled containerless directional solidification system has been used to produce NiAl-based polyphase composites containing up to two intermetallic phases and at least one ductile phase. Systems evaluated include Ni-Al-Cr, Ni-Al-Mo, Ni-Al-V ternary systems that form NiAl/α-refractory metal eutectics and a three phase eutectic in the Ni-Al-Cr-Nb system. Initial screening of these in-situ composites has included morphological characterization, four point bend testing, temperature dependent yield strength evaluation and compressive creep testing. Occasional growth defects termed “banding” currently interrupt the continuity of these composite structures and limit the attainment of optimum properties. However, both the creep strength and toughness of NiAl were improved by in-situ reinforcement.

Alumina (A12O3) fibers were incorporated into gamma titanium aluminide(TiAl) based powders by hot isostatic pressing (HIP'ing). The microstructure of as-HIP'd and heat treated composite specimens were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM studies reveal the presence of an amorphous reaction zone at the fiber/matrix interface. Numerous dislocations, dipoles and loops as well as twins are observed in A12O3 fibers. In addition, it is determined that the fiber/matrix interface stability is significantly affected by the matrix microstructure.

Diffusion couple experiments were carried out for Mo/γ-TiAl at 900, 1000 and 1100°C for periods of time ranging from 121 to 553 hrs. Using the Boltzmann-Matano analysis, the two diagonal interdiffusion coefficients for the two phases formed in these couples were obtained assuming the cross diagonal terms to be negligible. These two phases are δ-(Mo, Ti)3Al and β-(Mo,Al)Ti. The three intrinsic diffusion coefficients were also obtained using the Darken-type relationships between the interdiffusion and intrinsic diffusion coefficients. The calculated interdiffusion coefficients from the three intrinsic diffusion coefficients are in reasonable accord with those obtained directly from the Boltzmann-Matano analysis.

High temperature compression tests have been performed on a Ti-44a/o Al-3a/o V-7.5v/o TiB2XD composite over the temperature range 1000 to 1300°C and the strain rate range 10-3 to 10s-1. The workability of this material for metalforming processes was determined using both dynamic material modeling and workability testing approaches to cover both internal and surface cracking. At higher temperatures and strain rates internal fracture occurred in the material, while at lower temperatures and higher strain rates surface cracks occurred. Use of lower temperatures and strain rates inhibited internal instabilities and surface cracking in the composite. A finite element model (FEM) was developed to describe the stress and strain states during the deformation process. Mechanical flow behavior obtained from the compression tests was used as input to the model. A fracture criterion developed from Kuhn's surface crack equation was coupled with the FEM model to predict bulk formability in a forging process.

Finite element as well as indentation fracture mechanics modeling have been used to analyze the evolution of fiber damage that was observed at the ends of fibers intersecting a free surface in sapphire-reinforced TiAl matrix composites. Experimental observations indicate that, under certain conditions, surface cracks introduced during cutting will propagate along the fiber axis due to thermally-induced residual stresses. Finite element computations predict that significant thermally-induced residual tensile stresses exist near the ends of sapphire fibers which are embedded within TiAl-based matrices and are oriented normal to a free surface. Crack growth behavior induced by microhardness indentations is used to experimentally verify the FEM predictions. The results indicate that a biaxial tensile residual stress state exists near the fiber ends due to a thermal expansion mismatch. The magnitude of the residual stresses are a sensitive function of interfacial bond strength and elastic/plastic properties of the interfacial region and may be sufficient to propagate pre-existing cracks.

Cyclic crack-propagation behavior is examined in a series of γ-TiAl intermetallic alloys reinforced with pancake-shaped, ductile β-TiNb particles as a function of microstructure and specimen orientation. In contrast to results under monotonic loading, TiNb reinforcements are found to be far less effective in impeding crack extension under cyclic loading due to their susceptibility to premature fatigue failure, and consequently to the diminished role of shielding from crack-bridging mechanisms. Modest improvements in fatigue-crack growth resistance are observed in TiAl/TiNb composites compared to monolithic γ-TiAl, provided the particle faces are oriented perpendicular to the crack plane; however, properties are compromised in orientations where the particle edges are stacked normal to the crack plane. Microstructural effects on cyclic crack growth are less prominent in the composites, with crack-growth rates exhibiting a strong dependence on the applied ΔK level; measured exponents for the da/dN-ΔK relationship range between 10 and 20, and are found to decrease with increasing ductile phase content, yet are independent of particle thickness.

Fractographic analysis of SCS-6/Ti-24Al-11 Nb(a/o) (Ti-24-11 hereafter) and SCS-6/Ti-1 5V- 3Cr-3Al-3Sn(w/o) (Ti-15-3 hereafter) composites subjected to fatigue crack growth conditions indicates that the interface is prone to wear damage as a result of fiber/matrix sliding. In this study, the effect of fatigue loading on the integrity of the Interface was studied by using fiber pushout testing to compare the interfacial shear strength of composite specimens in the asreceived condition with specimens that were previously subjected to fatigue loading. Fatigue loading was also simulated by pushing fibers back and forth (multiple reverse pushouts). It was concluded that interfacial sliding during fatigue loading results in interfacial damage and degradation of the interfacial shear strength. Tensile testing of extracted fibers exposed to fatigue-induced interfacial damage was also performed to determine the effect of interface damage on the fiber strength. Interfacial damage also resulted in decreased fiber strength of the SCS-6 fiber. Fracture and wear of the outer carbon coatings on the SCS-6 fiber is the main contributing factor in the deterioration of these interfaces.

As part of a study to assess NiAl-based composites as potential high-temperature structural materials, the mechanical properties of polycrystalline NiAl reinforced with 30 vol.% continuous single crystal Al2O3 fibers were investigated. Composites were fabricated with either a strong or weak bond between the NiAl matrix and Al2O3 fibers. The effect of interfacial bond strength on bending and tensile properties, thermal cycling response, and cyclic oxidation resistance was examined. Weakly-bonded fibers increased room-temperature toughness of the composite over that of the matrix material but provided no strengthening at high temperatures. With effective load transfer, either by the presence of a strong interfacial bond or by remotely applied clamping loads, Al2O3 fibers increased the high-temperature strength of NiAl but reduced the strain to failure of the composite compared to the monolithic material. Thermal cycling of the weakly-bonded material had no adverse effect on the mechanical properties of the composite. Conversely, because of the thermal expansion mismatch between the matrix and fibers, the presence of a strong interfacial bond generated residual stresses in the composite that lead to matrix cracking. Although undesirable under thermal cycling conditions, a strong interfacial bond was a requirement for achieving good cyclic oxidation resistance in the composite. In addition to the interfacial characterization, compression creep and room temperature fatigue tests were conducted on weakly-bonded NiAl/Al2O3 composites to further evaluate the potential of this system. These results demonstrated that the use of A12O3 fibers was successful in improving both creep and fatigue resistance.

NiAl is well known for its low density, superb oxidation resistance and low ductile to brittle transition temperature. It is equally renowned for its low room temperature fracture strength and poor high temperature creep strength. A compositing approach has been used to introduce chopped and aligned FP Al2O3 into a matrix of fine grain NiAl via a powder metallurgical approach. This resulted in composites with approximately 40 vol. per cent undamaged alumina reinforcement. Fiber orientation effects on strength have also been characterized. Room temperature tests resulted in yield strength increases of 425% for chopped FP reinforcements and 800% for aligned composites. Elevated temperature tests conducted at 1200°C were even more dramatic in their strength increment with 200% and 3600% increases respectively. Fractographic results show matrix ductility, fiber-matrix decohesion and minimal fiber pull-out.

Extrusion textures in monolithic NiAl and an NiAl/AlN particle composite are compared. The NiAl has a generally increasing grain size with extrusion temperature, and a corresponding transition from <110> to <111> fiber texture. The results suggests that <110> is more closely related to the deformation texture, while the <111> is a recrystallization orientation formed by preferential growth. The composite exhibits <311> fiber texture, and this is consistent with slip in <100> directions. The effect of the AlN particles is to prevent the orientation changes observed in the monolithic NiAl during recrystallization.

The present research was carried out to study the cryogenic synthesis NiAl from elemental powders with the view of forming particle dispersed NiAl. It was found that nanocrystalline NiAl with a crystallite size in the range 5nm to 10nm is produced during cryomilling. Additionally, the nanocrystalline NiAl maintains the fine crystallite size when annealed at high temperatures. It is thought that the resistance of the microstructure to coarsening is due to the presence of nano-scale particles of AIN formed by reaction between milling medium, liquid nitrogen, and aluminum. The transformation of Ni and Al to ordered NiAl was studied using x-ray diffraction. This showed that cryomilling not only produces a solid solution but also can induce in-situ ordering of stoichiometric NiAl. The B2 NiAl structure forms during milling after the elemental powder blend is cryomilled for about 40 hours. It also forms when the “mechanical” solid solution is annealed at room temperature for about twelve hours.

Fatigue tests were performed on a novel, extruded, stainless steel/NiAl composite having good impact and tensile properties. A high fatigue limit was observed to occur at approximately 67% of the σUTS. The fracture surface showed a distinct change in morphology between the fatigued and fast fracture areas and the formation and growth of microcracks was postulated as the initial fatigue mechanism. The microcrack development was monitored by intermittent measurement of the elastic modulus and associated hysteresis. Microstructural characterization by means of SEM, TEM and EDS revealed the existence of approximately 100nm diameter Al2O3 particles decorating the interface between the NiAl and the stainless steel tubes.