Panel composites made of alpha-2 titanium aluminide (Ti-25A1-9Nb-1.6Ta-1.6Cr-lMo) foils and SCS-6 (SiC with carbon core and carbon double coatings) fibers were investigated to determine the influence of postprocessing heat treatments on the thermal fatigue behavior in air. The 4-ply composite panels with 22 vol% fiber showed no edge cracks after 1000 thermal cycles between 150°C and 816°C in alpha+beta or beta-solution plus isothermal-aging treatments. The thermal cycling, however, embrittled the fiber carbon core and coatings in the composites given post-processing heat treatments, with the degree of embrittlement increasing progressively with heat treatment temperature/exposure. The detrimental effects of heat treatments on the matrix were also observed in that beta treatments reduced both the strength and tensile ductility of the matrix alloy when compared with alpha+beta treatments.

NiAl is being evaluated as a potential matrix material as part of an overall program to develop and understand high-temperature structural composites. Currently, continuous fiber composites have been fabricated by the powder cloth technique incorporating either W(218) or single crystal Al2O3 fibers as reinforcements in both binary NiAl and a solute strengthened NiAl(.05 a% Zr) matrix. Initial evaluation of these composite systems have included: fiber push-out testing to measure matrix/fiber bond strengths, bend testing to determine strength as a function of temperature and composite structure (e.g. fiber volume fraction and number of fiber plies), and thermal cycling to establish the effect of matrix and fiber properties on composite life. The effect of matrix/fiber bond strength and matrix strength on several composite properties will be discussed.

The feasibility of developing high-temperature intermetallic composites for use as gas turbine engine components is assessed for a wide range of high temperature intermetallic matrices including aluminides, silicides, Laves and Sigma phases along with Al2O3, SiC, TiC, Si3N4 and Y2O3 as well as ductile refractory metals as either reinforcing phases or coatings. Preliminary evaluations of fabricability and observations of matrix/reinforcing phase compatibility are presented and discussed in terms of various factors, including interstitial impurities, equilibrium phase relationships, kinetics, and physical and mechanical properties of both matrix and reinforcing phases.

Arc-melted alloys of stoichiometric β-NiAl with additions of up to 1.5 atomic percent Re were examined to determine the nature of the quasi-binary phase diagram. Analysis of the Re segregation during solidification confirmed the eutectic nature. Solubility of Re in the β-NiAl structure was limited to approximately 0.5 atomic percent and precipitation of α-Re was observed in the hypoeutectic alloys. An orientation relationship with (101)β // (0001)α and [111]α // [1210]βwas determined for these rod-like precipitates.

Among the more serious problems arising from the use of SiC as a reinforcement for titanium aluminides are chemical instability and thermal expansion mismatch. In this work, metallic layers (Pd + W or Pt + W) have been included in the interface between SCS6 type SiC reinforcing fibers and Ti-48A1-1V (gamma TiA1) matrix to reduce interdiffusion and to retard crack propagation. Pd reacted with the SCS layer on the SiC fibers as well as with the titanium aluminide matrix by diffusion through the W layer. Cracks were found in the resulting Pd-Ti-Al reaction product. Pt did not react with the SCS layer, and reacted with the titanium matrix only in those regions where the W layer was fractured. No cracks were present where the W layer successfully protected the matrix from the Pt. Ti and Pt reacted extensively with the underlying SiC fiber in those areas where the SCS layer was fractured.

Based upon recent mechanical property tests a NiAl-AlN composite produced by cryomilling has very attractive high temperature strength. This paper focuses on the oxidation resistance of the NiAl-AlN composite at 1473 and 1573 K as compared to that of Ni-47Al-0.15Zr, one of the most oxidation resistant intermetallics. The results of cyclic oxidation tests show that the NiAl-AlN composite has excellent properties although not quite as good as those of Ni-47Al-0.15Zr. The onset of failure of the NiAl-AlN was unique in that it was not accompanied by a change in scale composition from alumina to less protective oxides. Failure in the composite appears to be related to the entrapment of AlN particles within the alumina scale.

As part of an effort to develop oxidation-resistant metal matrix composites, the influence of reinforcement materials on the oxidation of Ni3Al was studied. Thermogravimetric measurements and microstructural analyses were used to determine the 800°C oxidation behavior of Ni3Al and composites based on this material. The presence of fiber (Al2O3)- or particle (TiN or TiC)- reinforcement material in the Ni3Al increased susceptibility to oxidation as measured by the amount of reacted material, rate constants, and/or extent of internal attack. The orientation of fibers with respect to the free surfaces was found to affect the oxidation behavior by providing, in certain cases, reactive paths along the fiber-matrix interface that led to significant internal penetration. The results suggest that the choice of reinforcement material and the method of materials processing will be important considerations in the design of oxidation-resistant Ni3Al composites. However, ways to minimize the detrimental influence of the reinforcement material on oxidation resistance often conflict with requirements for obtaining attractive mechanical and physical properties of the composite.

Reinforcing phases used to strengthen a metal matrix cannot be appreciably chemically reactive for the times and temperatures expected during the service life of the composite. Where accurate phase diagrams exist that describe the interactions of the metal matrix and the reinforcing phase, the evaluation process can be completed with these diagrams and, if reactivity is encountered, with appropriate kinetic parameters. These complete phase diagrams are rarely available for complex materials systems. Therefore, we have adopted several ways of using available thermochemical data to estimate the compatibility of various metal matrix composite systems. Usually evaluations involve comparing matrix component activities, using reinforcing phase component thermochemistry to decide whether or not a reaction is expected by examining product chemical activities. Other methods use integral thermochemical properties of reactants and possible products in estimations. In either case a single thermodynamically favored reaction will disqualify a reinforcing phase on an energetic basis if not a kinetic basis. Where no disqualifying reactions are found, the reinforcing phase-metal matrix system is considered a candidate to be proved or disproved in experimental studies which may reveal reaction products not included in the extensive but still limited thermochemical literature. This approach has been advantageously applied to several systems. The study of Ti-Al metal matrices with selected reinforcing phases is discussed as an example.

The compatibility of candidate ceramic reinforcement materials with intermetallic matrices for high temperature composite systems has been evaluated. Powders of FeAl and NiAl were mixed with ceramic powders and consolidated by hot isostatic pressing and subsequent heat treatment. The microstructures of these composites and the nature of the ceramic/matrix interfaces were assessed using a wide variety of electron-beam techniques. The system FeAl/TiB2 was found to be particularly promising. The matrix appears to be bonded to the ceramic particles, which may be the result of diffusion of Fe into the ceramic. The particles stabilized in a previously unreported monoclinic crystal structure, rather than the equilibrium hexagonal form exhibited by the binary compound.

The chemical compatibility between five high temperature intermetallic compounds and six potential reinforcements are investigated. Diffusion couples are prepared by vacuum hot pressing. The most promising reinforcements or coating for each intermetallic compound are identified.

An alloy with the nominal composition 42Nb-28Ti-30Al (at.%) was heat treated in the sigma + beta phase region. The evolution of σ phase from the metastable β phase and the stability of the two-phase microstructure at various aging temperatures were evaluated using TEM techniques. The results indicate that the β phase in equilibrium with the σ phase at high temperatures decomposes to the orthorhombic phase at temperatures below 1200°C.

Chemical compatibility between oxide coated Nb filament reinforcements and MoSi2 was investigated. It was determined that ZrO2, Al2O3, and mullite coatings were chemically compatible with both Nb and MoSi2. Comparison between coated and uncoated filaments indicated that the coatings reduced the thickness of the interaction zone. The fracture toughness of the Nb filament reinforced composites showed an increase, while W filament reinforced composite showed a decrease, in the toughness compared to that of the matrix. The results are discussed in terms of the mismatches in the coefficients of thermal expansion and the bonding characteristics of the reinforcement/matrix interface.

In the NiAl-Cu3 Al pseudobinary section of the Ni-Al-Cu ternary system, the structural similarity of the binary phases yields a miscibility gap between NiAl (B2 structure with ao≃0.2880 nm) and Cu3 Al (bcc structure above 567° C with ao≃0.2946 nm). Due to the extensive mutual solubility, alloys can be solution-treated as single-phase NiAl and then aged at temperatures within the miscibility gap to investigate possible decomposition reactions. Several NiAl-Cu alloys (containing up to 30 at% Cu) were solution-treated at 1000° C, water-quenched, and then aged at either 475° C or 525° C. The decomposition of the supersaturated NiAl was characterized using XRD and SEM. A modulated microstructure was observed for an initial aging of 24 hours, and is consistent with the presence of a conditional spinodal between the NiAl and the Cu3Al. Long term aging resulted in the formation of equilibrium fcc Cu-rich precipitates. In comparison with the brittle nature of the as-quenched samples, the initial aging treatment yields samples which exhibit extensive slip development during hardness testing. Analysis of the deformation/fracture zone produced with a diamond cone indenter suggests that the ductile phase produced by the aging treatment, yielding an in-situ composite, enhances the resistance of NiAl to both crack initiation and propagation.

Reaction zone phases and kinetics over a temperature range of 650-1200°C have been characterized for Ti-14Al-21Nb/SiC and Ti-6Al-2Sn-4Zr-2Mo/SiC fibrous composites. The matrix of the Ti-1421/SiC materials was α2 with varying amounts of beta phase and transformed beta phase, and the Ti-6242 matrix consisted of alpha plus beta. The reaction zone in the as-HIP Ti-1421/SiC contained fine-grained TiC, a region of coarser-grained TiC, a carbide layer containing Ti, Al and Nb, and a region of(Ti, Nb)5(Si, Al)3. The matrix adjacent to the reaction zone was α2-Ti3Al with no beta phase. In the as-HIP Ti-6242/SiC composites, the reaction zone contained TiC near the fiber, and a layer of (Ti,Zr)5Si3 adjacent to the matrix. In heat-treated samples, the reaction zone thicknesses varied in a linear fashion with the square-root of aging time, indicating diffusion-controlled growth. Incubation periods for reaction zone growth were observed in Ti-1421/SiC samples aged at 760-1000°C. The overall kinetics for the two matrix alloys were approximately the same, and the activation energy was measured as 63-73 kcal/mole for Ti-1421/SiC and 73kcal/mole for Ti-6242/SiC.

Niobium aluminide based composites fabricated by powder metallurgy techniques were tested for oxidation resistance between 1000 and 1400°C. The oxidation rate of these materials was improved over conventionally cast or pack aluminized NbAl3. The inclusion of oxide dispersoids decreased the rate constant by an order of magnitude at 1000°C compared to monolithic NbAl3 and to a value of the same order of magnitude as NiAl at 1170°C. The most heavily reinforced material studied had a rate constant an order of magnitude greater than NiAl at 1400°C.The oxide scale was generally adherent and compact but contained oxide nodules.

Candidate diffusion barrier coatings were applied to aluminum oxide and silicon carbide fibers, and consolidated in a gamma titanium aluminide alloy. Interfacial coatings of titanium carbide and titanium diboride were applied to the fibers using chemical vapor deposition, while calcium oxide coatings were applied from a solution. Sections of these specimens were vacuum encapsulated in quartz tubing and heat treated for 50 hours at 1100°C. The thermochemical stability of the fibers was evaluated using energy dispersive x-ray spectroscopy to compare the fiber-matrix interface of the specimens before and after heat treatment. Both the as-manufactured and the coated aluminum oxide fibers showed no discernable interaction with the titanium aluminide matrix. Titanium diboride effectively prevented the silicon carbide fiber from reaction with the matrix. The silicon carbide fiber which was coated with titanium carbide showed significant reaction between the fiber and the coating after heat treatment.