Intermetallic-matrix composites (IMCs) are intended for applications involving higher temperatures and stresses than conventional metallic matrix composites. Consequently, environmental effects, thermo-mechanical fatigue, interfacial thermodynamic stability, creep resistance, processing and cost are only a few of the obstacles to introduction in real products. The primary goal of current IMC development efforts is to provide ‘revolutionary’ materials for components that are enabling technologies in advanced demonstrator gas turbine engines. As compared to production applications, the hurdles for entry into demonstrator engine programs are not as high due to the limited operating time and the small number of parts to be produced. This paper examines the barriers to the introduction of IMCs in the context of application to production aerospace structures.

Intermetallic compounds are similar to ceramics because they are stoichiometric, with limited compositional ranges and brittle behavior. The limited ductility forces a reliance on powder techniques for shaping and consolidation. The high temperature character of intermetallics is beneficial to high temperature service, but this same attribute contributes to difficulty in processing. This paper reviews the several powder approaches to forming intermetallic structures. Examples are given on powders, consolidation options, and properties. Densification maps are introduced for estimation of consolidation cycles. Unfortunately, many of the composites exhibit little strengthening benefit from incorporation of reinforcing phases.

In this US Bureau of Mines study, a variety of TiAl based composites were produced in situ by reaction synthesis. Mixtures of elemental Ti, Al and B and Ti, Al, and Si powders were reactive hot-pressed to form TiAl reinforced with 10, 20, 25 or 60 vol. pct. TiB2 or Ti5Si3. Microstructural evaluation of the resultant composites confirmed that the reaction products were primarily TiAl and TiB2 or Ti5S3, with a small amount of Ti3Al. The hot-press temperature and pressure had a significant effect on the density of the composites. In general, higher temperatures and initiating the reaction under pressure promoted dense composites. Room temperature biaxial flexure strength tests indicated that the addition of the reinforcing phases can improve the strength of TiAl. Potentiodynamic experiments revealed that TiAl, TiAl+TiB2 and TiAl+Ti5Si3 composites display active-passive corrosion behavior in both acidic and alkaline solutions.

Combustion synthesis of NbAl3-matrix composites with Al2O3 and B additions was studied using the thermal explosion mode. The addition of B to the reaction mixture resulted in the formation of NbB2, small amounts of NbB and unreacted Al. The Al2O3 addition did not affect the NbAl3-matrix reaction completion but the final product density increased with increasing Al2O3 loading. In both NbAl3-matrix composites, the reaction was initiated above the melting point of Al.

Room-temperature intermetallic compound formation occurs when one of the component metals has a very low melting point or when two metals in close contact interdiffuse very rapidly. Compound formation at room temperature at the interface of superposed thin films has been observed in several instances, often in systems relating to electronic materials. The overall amount of compound produced in such configurations, however, is limited, due to the intrinsic limitations involved in the thin layer geometry. Bulk quantities of intermetallic can be produced at ambient temperature in solids by increasing the interface area between the components that interdiffuse rapidly. This condition can be achieved by having small size powder particles of one component coated with a layer of the second component. The very large interface area leads to rapid formation of bulk quantities of compounds even at ambient temperature. By appropriate control of the initial constituents and the coating parameters, it is possible to custom-prepare various intennetallic compounds present in binary systems such as silver-tin, gold-tin and silver-indium in which fast interdiffusion takes place.

A number of discontinuously reinforced, intermetallic matrix composites (i.e., TiAl/TiC, TiAl/TiB2, TiAl/Ti5Si3) were formed in situ through self-propagating, high-temperature synthesis (SHS) between elemental powders. This Bureau of Mines study characterizes the abrasive wear behavior of these composites. Wear behavior is discussed with respect to process history, and type and volume fraction of reinforcement. Generally, higher process temperatures lead to dense composites, resulting in better wear resistance. The wear behavior of the SHS intermetallic composites is compared to other intermetallics, produced by conventional techniques.

The tensile behavior of metal-intermetallic layered composites produced by reacting dissimilar elemental foils was studied by U.S. Bureau of Mines researchers. The layered composites were produced by initiating an SHS reaction between Ni and Al foils or Ti and Al foils. The reaction consumed the aluminum foil, resulting in well bonded metal (Ni or Ti) metalaluminide layered composites. Tensile tests revealed that the tensile behavior of the composites was dependent upon the thickness of the intermetallic layer (which in the present case corresponds directly to volume fraction). Composites, in which the intermetallic layers had cracked extensively, behaved in a ductile or tough manner. Not surprisingly, these composites consisted of a relatively thick metal layer compared to the intermetallic layer. It was found that the cracking of the intermetallic layers occurred prior to the deformation of the metal layer. Those composites that had relatively thick intermetallic layers, compared to the metal layer, behaved in a brittle manner. These composites had few cracks in the intermetallic region.

Vapor phase processings for manufacturing near-net-shaped composites, continuous fiber reinforced composites or lamellar composite will be hopeful methods. The method for synthesis of TiAl by a magnetron sputtering using a multiple target has been successfully established, and the tensile property of the microcomposite with SiC fibers has been studied. The TiAl produced by a vapor phase processing was evaluated by Auger electron spectroscopy and the x-ray analysis. The tensile strength properties of the SiC/TiAl microcompositesof which the interface bonding was controlled with the powers of sputtering were estimated. The properties of SiC/TiAl microcomposites are decreasing with increasing the sputtering power. The irradiation-cured SiC fiber has better compatibility with TiAl than the oxidation-cured SiC fiber. The strength of the microcomposite has been improved with using a thin C-coated irradiation-cured SiC fiber.

Analytical and finite element method (FEM) simulations of creep in idealized ductilephase toughened intermetallic composites are described. For these strong-matrix materials, the two types of analyses predict similar time-independent composite creep rates if each phase individually exhibits only steady-state creep. The composite creep rate increases above that of the monolithic intermetallic with increases in the stress exponent of the intermetallic, the volume fraction of the ductile phase, and the creep rate of the ductile phase. FEM analysis shows that the shape of the ductile phase does not affect the creep rate but may affect the internal stress and strain distributions, and thus damage accumulation rates. If primary creep occurs in one or both of the individual phases, the composite also exhibits primary creep. In this case, there can be significant deviations in the creep curves computed by the analytical and FEM models. The model predictions are compared with data for the Nb5Si3/Nb system.

This paper presents a review of the current status of fibers and fiber coatings for intermetallic matrix composites (IMCs). The properties of commercial and developmental fibers are reviewed and their potential as reinforcements for various intermetallics discussed. The potential fibers for near-term IMC applications are SiC (both monofilament and tow), single crystal Al2O3 and YAG/Al2O3 eutectic monofilaments, and sol-gel derived Al2O3- mullite nanocomposite tow fiber. The chemical compatibility of these fibers with various intermetallic matrices is reviewed. The issues related to the coefficient of thermal expansion (CTE) mismatch between the fiber and the matrix, attaining optimum interfacial bond strength, and fiber damage during composite processing are addressed. The effectiveness and limitations of fiber coatings in accommodating various fiber-matrix incompabilities, both chemical and mechanical, are discussed.

A new sol/gel fiber which exhibits exceptional high temperature properties was recently developed at 3M. This fiber has the composition 85% Al2O3-15% SiO2 (85A-15S). High temperature tensile strength and creep properties were measured in the temperature range 1000°C – 1300°C. The creep rate for the 85A-15S fibers was three orders of magnitude less than single phase polycrystalline alumina fibers such as Nextel 610, and 90% of room tensile strength was retained at 1250°C. These exceptional high temperature properties were attributed to a unique, two-phase microstructure consisting of globular and elongated grains of a-Al2O3 and mullite (3Al2O3-2SiO2). The room temperature single filament strength of the 85% Al2O3-15% SiO2 fibers was 2130 MPa, and the elastic modulus was 260 GPa.

Polycrystalline, stoichiometric β- silicon carbide fiber tow suitable for ceramic or metallic matrix composite use has been prepared using a polymer precursor route to obtain textile grade, weavable fibers. Individual filaments have a diameter of 10 μm, an average tensile strength of up to 3.4 GPa (500 ksi), an elastic modulus up to 430 GPa (62 msi) and a density > 3.1 g/cm3. Crystallites average about 60 nm by x-ray line broadening. The SiC fiber shows improved mechanical and thermal stability properties, especially in an inert atmosphere, as compared to commercial polymer-derived SiC ceramic fibers. Thermochemical and mechanical properties, as well as fractography and microstructure will be presented.

Strength degradation of single crystal Al2O3 fibers due to the effect of fiber/matrix interaction and processing of NiAl and superalloy matrix composites, was investigated. Strength loss was quantified by tensile testing fibers that were exposed to the matrix alloy using two different methods. In one method, the fibers were incorporated into a composite by either the Powder Cloth (P-C) or binderless powder technique. The fibers were then extracted from the composite by chemical dissolution of the matrix and subsequently tensile tested and examined by scanning electron microscopy. In the other method, fibers were sputter-coated with a similar matrix composition and heat-treated to simulate conditions similar to those experienced during composite powder fabrication methods. In the sputter coating method, the contribution of fiber-matrix reaction on fiber strength loss was isolated from the effects of the various mechanical loads which are present during powder fabrication. For all matrices studied, significant strength loss was observed both in fibers extracted from composites and in fibers sputter-coated and annealed. Although surface ridges and pores were observed on the degraded fibers, it remains uncertain whether these features were responsible for the strength loss.

Unidirectionally-reinforced Timetal® 21S composite specimens were subjected to elevated temperature heat treatments. The SiC fibers were then chemically extracted from the matrix, and their tensile strengths were measured at room temperature. A Weibull statistical analysis of fiber strength distribution was performed to compare the Weibull parameters of fibers from the as-consolidated and heat-treated composites. Fractographic analysis of the tested fibers was used to identify the flaws which caused failure in each condition. Surface flaws were found to initiate low strength failures in all conditions, and the number of surface initiated failures increased with an increase in severity of heat-treatment. A relationship between the fiber/matrix chemical reaction and surface flaw development is demonstrated. A fracture mechanics analysis that explains the relationship between surface flaw size, fiber fracture toughness, and the measured tensile strengths is suggested.

The interfacial chemical behavior of two candidate coating systems for use on fine diameter (∼10 μm) Al2O3 in TiAl is presented. The performance of the Nb/Y2O3 system appears to be related to the transport rate of Ti through the Y2O3 layer. Interfacial reaction is then observed at points where the Y2O3 thickness is less than about 1 μm. The main reaction product formed appears to be an oxygen saturated TiAl phase. An alternative system, C/TiN/Y2O3 is then presented as a solution to the Ti transport problem and may offer more desirable interfacial mechanical properties.

Composites consisting of Ni3Al modified with Zr, Fe, and B (IC-50) reinforced with continuous sapphire single crystal fibers have been prepared by vacuum hot pressing foil/fiber multilayers. Previous research detected formation of Zr-rich precipitates and sulfur segregation at fiber/matrix interfaces in these materials subsequent to fabrication. In this work, fiber bonding to the Ni3Al matrix has been characterized using fiber push-out tests. Segregation and precipitation at fiber/matrix interfaces has been examined using scanning Auger electron spectroscopy and scanning electron microscopy. For comparison, composites have also been prepared from Ni, Ni-Al, and Ni-Al-Zr solid solution matrices, to evaluate the effects of matrix strength and reactivity on processing and fiber/matrix reactions.

A significant materials-based activity to develop Ti-aluminide metal matrix composites for high temperature aerospace structural applications is now underway. A review of the approaches, progress, and status of the development of continuously-reinforced Ti-aluminide metal matrix composites with matrices which contain a significant volume fraction of the ordered orthorhombic Ti2AlNb phase will be presented. The principal application considered is a gas turbine compressor rotor ring, and this influences the development approaches and properties goals. Specific development activity that will be presented includes modification and control of the matrix composition and microstructure, fiber coating treatments to control interdiffusion between the fiber and the matrix, and to improve the ability of the interface to support a mechanical load, and efforts to improve the properties of SiC monofilaments used as reinforcements. Critical issues that define the requirements for additional studies will be presented.

Large-scale fiber displacement behavior, usually characterized by a “frictional” sliding stress (τslide), has been studied in two sapphire-reinforced TiAl systems. Experimental results from fiber pushout and reverse push-back tests indicate that the large-scale sliding behavior of debonded fibers leads to an average τslide-value which progressively decreases during fiber displacements. Previous studies of SCS-6 (SiC) fiber-reinforced glass and metal matrix composites have attributed decreases in τslide to the fracture and wear of fiber asperities. However, given a matrix in which fiber asperities do not easily wear (e.g., a TiAl alloy), SEM examination of the fiber/matrix interface indicates that matrix plasticity plays a dominant role in the decrease of τslide with fiber displacement. Experimental evidence suggests that the observed decrease in τslide can be attributed to (1) a decrease in fiber roughness perceived by the matrix due to matrix grooving and (2) a relaxation of radial clamping as a result of material removal from the interface.

The effect of a Ag/Ta coating on the fracture resistance of a notched SCS-6 fiber-reinforced Ti3Al matrix composite under both static and fatigue loading was investigated. The crack propagation patterns and damage mechanisms under various loading conditions were characterized and compared with the uncoated composite counterparts. The results show that the Ag/Ta-coated composites exhibit typical mode-I failure patterns under both static and fatigue loading, while mixed mode-I & H failure patterns were observed in the uncoated SCS-6/Ti-25-10 composite. Localized interfacial debonding, matrix cracking and fiber fracture in the crack wake were the responsible damage mechanisms for the Ag/Ta-coated SCS-6/Ti-25-10 composite. Extensive interfacial debonding, crack splitting and branching, however, were found to be the dominant mechanisms in the uncoated SCS-6/Ti-25-10 composite.

An experimentally-based description of strengthening in a series of TiB2-reinforced near-y titanium aluminides is presented. The analysis provides a means to examine the strengthening contributions which are individually attributable to compositional variants, to indirect sources which are present due to the influence of the particulate on the underlying microstructure, and to those directly evolving from the presence of the reinforcement and its interaction with dislocations. Once the indirect strengthening contributions are isolated and arithmetically removed from the magnitude of the experimentally determined composite strength, the resulting direct strengthening increments are found to obey a linear relationship with the inverse-squareroot of the edge-to-edge interparticle spacing. The description of the dependence on spacing is found to be compatible with other studies where similar relationships have been reported.