NiAl and NiAl/TiB2 composites were tested in air at 800, 1000, and 1200°C. The oxidation resistance of the composites depends on the fabrication route, and subsequently on the reinforcement phase morphology and distribution. The oxidation resistance of NiAl reinforced with large TiB2 particles was found to decrease with increasing TiB2 content. NiAl reinforced with large TiB2 particles was completely oxidized at 1200°C after a 100 hour exposure.

Selected eutectic compositions in Ni-Al-Cr ternary systems were processed by directional solidification (DS) with various growth rates. Fracture toughness tests were performed at room temperature and 400 °C; fracture surfaces of broken specimens were examined using SEM to investigate fracture behavior of each alloy. The alignment of eutectic phases was found to play an important role in composite toughening for the intermetallic matrix. Binary eutectic composites consisting of bcc α-Cr and B2 β-NiAl phases with a directional, well-aligned structure showed improved fracture properties over NiAl single crystals. Ternary eutectics, which contain an fcc γ-Ni phase, offered an excellent fracture resistance at room temperature.

Mechanical tests were performed on directionally solidified (DS) NiAl in-situ eutectic composites in order to evaluate the effect of ductile reinforcements on the fracture resistance of the B2 ordered intermetallic compound NiAl. Reinforcements consisted of i) Mo fibers, ii) Cr fibers, and iii) Cr(Mo) solid solution plates. Near stoichiometric NiAl ingots were prepared by induction melting as reference material to compare with the eutectic composites. Resistance curves were obtained for the NiAl/Mo fibrous eutectic alloy as well as for the NiAl/Cr(Mo) layered material. The initiation fracture toughness of the DS NiAl/Mo and NiAl/Cr(Mo) eutectic composites is larger than that of the stoichiometric NiAl, with the layered material. producing the better properties. The mechanisms for such increase in fracture toughness are discussed.

Ductile phase reinforcement is an attractive approach for improving room temperature ductility and toughness of intermetallics. Two alloys of nominal composition (at.%) Ni70Al30 and Ni50Fe30Al20 were directionally solidified to produce quasi-lamellar microstructures. Both alloys exhibit ∼10% tensile ductility at 300 K when the ductile phase is continuous, while the Ni70Al30 alloy has a tensile ductility of ∼4% when the γ′ phase is discontinuous. Observations of slip traces and dislocation substructures indicate that a substantial portion of the ductility enhancement is a result of slip transfer from the ductile phase to the brittle matrix. The details of slip transfer in the two model materials and the effect of the volume fraction and morphology of the ductile phase on the ductility enhancement in the composite are discussed.

Ordered Ni3A1 alloys and their composites are attractive materials for elevated-temperature structural applications due to their many favorable properties. The addition of alloying elements can significantly lower the Ni3Al order-disorder transition temperature and also result in the formation of a Ni solid solution. As the percentage of Ni solid solution increases, the composite's room-temperature flexural strength increases. The effect of rocessing parameters on the material's microstructure is discussed. The complex matrix microstructure also has a significant effect on the composite's creep properties. Normal power-law creep was exhibited by the composite material when tested in compression.

The mechanical behavior of a continuous Al2O3 fiber-reinforced NiAl matrix composite was investigated. The interfacial mechanical properties were measured at room temperature using a fiber pushout test. Tensile test was conducted at room and elevated temperatures. Four-point bending creep test was also conducted at 700 °C. The key microstructural parameters controlling the mechanical behavior and fracture processes were identified. The effect of fiber surface coating on the interfacial properties and mechanical behavior of the composite was also studied.

MoSi2 based composites represent an important new class of “high temperature structural silicides”, with significant potential for elevated temperature structural applications in the range of 1200–1600 °C in oxidizing and aggressive environments. The properties of MoSi2 which make it an attractive matrix for high temperature composites are described and the developmental history of these materials traced. Latest results on elevated temperature creep resistance, low temperature fracture toughness, and composite oxidation behavior are summarized. Important avenues for future MoSi2 based composite development are suggested.

The addition of Mo5Si3 as a reinforcing second phase in a MoSi2 matrix has been investigated for possible high temperature strengthening effects. MoSi2 with up to 45 vol % Mo5Si3 was fabricated using powder metallurgy (PM) and arc-casting (AC) techniques. Effects of processing routes, which result in different microstructures, on their mechanical properties are given. PM composites, which have an equiaxed microstructure, exhibit a limited increase in hardness. Higher hardnesses are observed in script-structured AC eutectics and Er-modifiedeutectics throughout the temperatures studied (25–1300°C). Crack propagation paths induced by indentation show long transphase cracks in the AC materials vs short intergranular and interphase cracks in the PM composites at high temperatures.

Transmission electron microscopy discloses that the interface in the AC composites has a low-index orientation relationship between the two phases and shows regularly faceted interfacial structures, while planar interfaces are found in the PM composites. These observations suggest the interface is stronger and lower in energy in the AC composites, which is consistent with the higher hardness values and long transphase cracks observed.

Dislocation analysis shows the presence of ordinary dislocations (<100>, <110> and 1/2<111>) in MoSi2 in the as-fabricated composites. These types of dislocation are also responsible for the high temperature plastic deformation in compression in both the monolithic MoSi2 and the composites. <331> types of dislocation are only found in MoSi2 either near the interface of the AC composites or in materials deformed below 1000°C.

Niobium was added to MoSi2 in the form of particles, random short fibers and continuous aligned fibers. It was found that the morphology of Nb played a role in the toughening that occurred (as measured by the area under load displacement curves from room temperature three point bend tests and the examination of fracture surfaces). The Nb particles did not toughen MoSi2. The random short fibers appeared to toughen MoSi2 via crack deflection along the fiber matrix interface. Aligned fibers imparted the greatest toughness improvements, as toughening resulted from fiber deformation. However, larger diameter fibers displayed a greater ability to toughen MoSi2 than smaller diameter fibers. This was attributed to the constraint resulting from the interfacial layer between the MoSi2 matrix and the Nb fiber. Maximum toughness occurs when the fiber is able to separate from the matrix and freely deform.

Monolithic MoSi2 and MoSi2-TiC particulate composites with 10 vol % and 15 vol % TiC were tested in compression between 950°C and 1200°C. The MoSi2-TiC composites can be deformed plastically at lower temperatures than MoSi2 can before brittle fracture occurs. The composites exhibit much lower strain hardening rates and attain zero strain hardening rate at much lower strains than monolithic MoSi2. The differences between the composites and monolithic MoSi2 in plasticity and in strain hardening behavior is attributed to efficient dislocation generation into the matrix from sources at the MoSi2-TiC interfaces.

In an effort to enhance the toughness and creep strength of MoSi2, the role of various metallic and ceramic reinforcements is being examined. In this work, the effects of an oxide, a carbide and a nitride reinforcement on the compression creep behavior of MoSi2 are explored. Variations in the deformability of reinforcements and their relative strength and flaw population appear to influence the creep strength of the composites. Refinements in grain size also improve crack tolerance of the composite during deformation at 1200°C.

The compressive creep behavior of MoSi2 reinforced with 30v/o SiC fabricated by in situ XDTM process was investigated at 1050°C–1300°C in anaerobic and aerobic test ambients. Creep experiments performed with the composite in dry nitrogen and in air showed power-law type constitutive behavior and a stress exponent of ∼3.5. Creep deformation occurred by dislocation glide accompanied by cavitation, and the apparent activation energy for creep at 1100°C–1300°C was bi-vaiued with a threshold temperature of ∼1170°C. Microstrucural observations by TEM indicated that the rate-controlling process changed from dislocation glide to dislocation climb at higher temperature, corresponding to the change in activation energy. Creep damage occurred by cavitation at SiC-matrix interfaces and at grain boundaries within polycrystalline SiC particles. This process was apparently facilitated by the accumulation of glassy phase at these sites during creep. Creep experiments in air showed there was no appreciable atmospheric effect on the response of the composite, while an increased strain rate was observed in the base alloy due to an increase in glass phase resulting from thermal oxidation.

Films of Mo, MoSix and Mo/MoSix (1.22<x<1.35) multilayers were formed by physical vapor deposition (PVD) and ion beam assisted deposition (IBAD) onto (100) Si, glass and graphite substrates. Ion to atom arrival rate (R) ratios for IBAD varied from 0.01 to 0.1 and film thicknesses varied from 200 to 1100 nm. The Si/Mo ratio decreased with increasing R ratio. The oxygen content of Mo films was greater than silicide films, but both decreased substantially with increasing R ratio. Ar incorporation increased with increasing R ratio to a maximum of 1 at% in Mo and 5 at% in MoSi1.22. Mo films exhibit a strong (110) fiber texture at low R ratios. At the highest R ratio, a tilting of the (110) fiber texture by 15° occurs, along with the development of a distinct azimuthal texture indicative of planar channeling of the ion beam along (110) planes. The microstructure of the multilayer consists of small Mo grains and an amorphous silicide. Average film stress in Mo films increases from tension to a maximum value of 0.63 GPa and becomes compressive with increasing normalized energy. The stress in the MoSix films decreases with increasing normalized energy and saturates at a compressive stress of -0.24 GPa at 25 eV/atom. Indentation fracture experiments using a Vickers indenter with a 300 g load show a fracture behavior that is consistent with a residual stress effect for the IBAD monolithic MoSix and microlaminate, but which is influenced by additional factors in the PVD microlaminate.

A high-strength in situ composite of MoSi2/SiC was synthesized using a solid state displacement reaction between Mo2C and Si by blending Mo2C and Si powders and vacuum hotpressing the powders at 1350°C for 2 h followed by 1 h at 1700°C. The resulting microstructure consisted of 1-μm diameter β-SiC particles (30 vol%) uniformly dispersed in a fine grained MoSi2 matrix. Transmission electron microscopy was used to study the fine-scale morphology and phase distribution of the composite. Evidence for a small amount of grain boundary glass phase was observed using diffuse dark field imaging. The β-SiC particles were distributed mainly on grain boundaries and triple points within the MoSi2 matrix. These findings were used to rationalize the observed mechanical property behavior.

Erbium additions to MO5Si3-MoSi2 eutectic composites have been investigated in an effort to reduce oxygen impurity levels found in the MoSi2 starting powders. Additions of 0.35 atomic percent Er were found to refine the script lamellar microstructure and increase the hardness of the composite as compared with the unmodified material. This increase in hardness was maintained over the temperature range 25°C to 1300°C. Oxygen rich Er particles have been observed in the Er-modified material suggesting that Er has acted as a gettering agent for oxygen. An intermetallic phase Er2Mo3Si4was also observed in the Er-treated material. The character of this phase was determined by convergent beam electron diffraction (CBED), wavelength dispersive spectroscopy (WDS) and X-ray dispersive spectroscopy (XEDS). The particles of Er2Mo3Si4 were found to be heavily faulted which caused a reduction in the experimental diffraction symmetries. Thus, ambiguities arose during CBED analysis. Therefore, a standard sample of Er2Mo3Si4 was prepared and used to aid in the identification of the faulted particles.

The growth of SiC during solidification of Mo-Si-C melts was investigated to explore the potential for developing in-situ refractory reinforcements for MoSi2 matrices. Volume fractions of up to 20% were readily incorporated by arc melting. Primary β-SiC grows as equiaxed particles, plates and hopper crystals, whereas secondary SiC grows with ribbon, thin flake or whisker morphologies. The dominant facets of both primary and secondary SiC are of the (111) and {002) type, which are also characteristic of the equilibrium crystal shape. There is a clear orientation relationship between the phases wherein the close packed planes and directions of the C11b and B3 structures are parallel.

Interfacial failure along the fiber-matrix interface during fiber push-out tests is examined for conditions where (1) a “thin-slice” specimen geometry is used and (2) matrix plasticity is necessary for large scale fiber displacement. The influence of both test geometry and the combination of thermally and mechanically induced stresses on the potential failure process is discussed. Experimental results are presented which illustrate a range of interfacial failure processes as a consequence of different combinations of specimen geometries, thermally induced residual stresses, and mechanically applied stresses. It is concluded that mode I crack opening and growth induced by specimen bending can be a major contributor to the interfacial failure process. Accurate quantitative measurements of interfacial shear properties must therefore rely on test configurations in which specimen bending is eliminated.

The mechanical properties of new ceramic reinforcing fibers need to be well characterized before their incorporation into composite materials. Critical fiber properties include strength and Weibull modulus, both off the spool and after matrix extraction, bundle strength, modulus and creep resistance. Important composite properties include thermochemical stability, interface debond energy and interfacial sliding resistance. Tailoring these interfacial properties invariably involves the use of a fiber coating that can, in turn, influence fiber properties. Methods of measuring strength related properties are addressed and the results of a computer simulation to assess the quality of measured data using statistical methods are presented. The simulation was developed to determine the errors associated with a strength/Weibull modulus determination based on a limited number of samples. Finally, an assessment of the effect of mixing of high and low quality fiber on bundle strength and composite properties is made.

A newly developed apparatus for performing fiber push-out testing at elevated temperatures has been applied towards testing fiber-reinforced intermetallic and metal matrix composites. This new capability shows the effects of the relief of residual stresses and increased matrix ductility with increasing temperature on fiber debonding and sliding behavior.

The central problem in the study of composite materials is the adhesive strength of the electronic bond between reinforcement and matrix. We have introduced a unique method of measuring the interface bond strength of a wide variety of engineering interfaces (e.g. metal/ceramic, semiconductor/metal, metal/polymer). Specimens are composed of a relatively thin overlayer on a thick substrate. The specimens are shocked using a magnetic hammer which accelerates a thin metal flyer onto the substrate. The shock, upon reflection at the free surface, is incident on the bonded interface as a tensile wave, spalling the overlayer. The method is unique in using free surface velocity measurements to determine the interface stress at the instant of separation. The debonding process is sufficiently rapid (on the order of 1.0 ns) that debonding occurs by the simultaneous breaking of atomic bonds, rather than by propagation of cracks nucleating at stress concentrations near existing flaws.