As the temperature demands increase in advanced aerospace systems, engineering materials that can withstand 1200° to 1600°C exposure in air must be developed. MoSi2 is an intermetallic compound that has many, though not all, of the necessary physical and mechanical properties for use in this temperature regime. At ambient and moderate temperatures MoSi2 is brittle, but once the ductile-to-brittle transition temperature (DBTT) is exceeded, it rapidly loses strength. Recent progress in, and current status of, the research efforts in MoSi2 matrix composites will be reviewed, with an emphasis on the critical issues that currently impede progress to application.

The objective of this investigation was to identify a pest resistant MoSi2-base matrix composition having properties suitable for SiC reinforcement. A 30 vol.% addition of fine Si3N4 particulates to MoSi2 significantly improved the low temperature accelerated oxidation resistance and thereby eliminated pest failure. Addition of Si3N4 also improved the high temperature oxidation resistance, strength and more importantly lowered the CTE of MoSi2 such that cracking was eliminated in a hybrid composite consisting of 30 vol.% Si3N4 and 30 vol. % SCS-6 fibers.

The effects of ductile reinforcement morphology on the mechanical behavior of MoSi2 reinforced with 20 vol.% Nb are presented. While all the composites exhibit improved fracture toughness relative to the monolithic MoSi2, the most significant improvement is obtained in the composite with laminate reinforcement, followed by coarse fiber, fine fiber and particulate reinforcements. Bend strength measurements indicate a ductile-brittle-transition-temperature of ∼1250°C, and highlight the attractive properties of laminate reinforced composite. Preliminary studies of room temperature fatigue crack growth in particulate reinforced composites show a stable fatigue crack growth, which is not achieved in monolithic MoSi2. Possible ways of achieving a better combination of mechanical properties are also discussed.

Induction heating of disk shaped specimens was used to compare and contrast the thermal fatigue behavior of MoSi2 and MoSi2-based composites. Specimens were subjected to 5 s heating and cooling cycles between temperature limits of 700°C and 1200°C. The monolithic material and a MoSi2- 10 vol% TiC composite exhibited poor thermal shock resistance and could not be thermally cycled according to this temperature-time profile. A 30 vol% TiC composite exhibited much better thermal shock and thermal fatigue resistance as compared to the monolithic material, but exhibited undesirable oxidation. MoSi2-10 and 30 vol% SiC particulate composites exhibited excellent thermal shock and thermal fatigue resistance compared to that of the monolithic material. A MoSi2-10 vol% SiC whisker composite did not show improved thermal fatigue resistance due to the initial processing defects present in the material. The monolithic material and the 10 vol% TiC composite were also subjected to 30 s heating and cooling cycles between temperature limits of 700°C and 1200°C. Both of these materials exhibited better thermal fatigue resistance at this temperature-time profile, but the 10 vol% TiC composite also exhibited undesirable oxidation. The fatigue results are discussed with reference to the initial microstructure of the specimens and the stress-strain history of the specimens which was obtained by a thermoelastic finite element analysis.

The U.S. Bureau of Mines is examining the wear behavior of a variety of advanced materials, including the intermetallic compound MoSi2. The high stiffness and hardness of MoSi2 make it attractive for use in applications requiring wear resistance. This research reports on the results of pin abrasion wear tests for a variety of powder processed MoSi2 and MoSi2 matrix composites. The effect of the addition of ductile (Nb) and brittle (SiC) reinforcements, as well as the influence of reinforcement type and orientation geometry, on abrasive wear is discussed. Comparisons of the wear behavior of MoSi2 and MoSi2-based composites with other materials, such as refractory metals, aluminides, and ceramics, are made.

NiAl-based composites are a new class of engineering materials being developed for high temperature structural applications in oxidizing and aggressive environments. This paper discusses some of the recent advances in developing continuous fiber-reinforced polycrystalline NiAl-based composites. Several key issues including matrix alloy development, fiber development, fabrication development and mechanical behavior will be addressed.

The microstructure and mechanical behavior of Al2O3 fiber-reinforced NiAlFe composites fabricated using the pressure casting technique were investigated. The mechanical response and fracture behavior of these unidirectional composites under tensile and three-point flexural loading were characterized. The results were compared to those measured for Al2O,/NiAl-based composites fabricated by powder metallurgy techniques. Preliminary results indicate that pressure casting appears to be a viable processing route for fabricating continuous fiber-reinforced NiAl-based composites.

This work reports on the synthesis, characterization and preliminary mechanical properties of nanocrystalline NiAl/Al2O3 composite materials. Nano-scale crystallites of NiAl were formed by high energy milling of pre-alloyed powder with and without Al2O3 under an Ar atmosphere. Consolidation of the nano-scale powders was done by sinter-forging at < 0.4 Tm in air. Characterization of the powders and consolidated microstructures and hardness data are reported.

The results of preliminary investigations of the room-temperature fatigue and fracture mechanisms in model NiAl composites are presented. The composite systems studied include: NiAl reinforced with ductile second phase particles (Mo); ductile fibers (Mo); brittle second phase particles (zirconia partially stabilized with yttria) and brittle fibers+particles (Al2O3 + zirconia). Mechanisms of fatigue crack growth in heat treated specimens of the fiber-reinforced composites are also elucidated. The investigations indicate that both ductile and brittle reinforcements can enhance the toughness of NiAl significantly, and the ductile phase particulate reinforcement may even contribute to room-temperature ductility in the composite.

Cryomilling of prealloyed NiAl powders, followed by extrusion, has been used to produce a particulate strengthened NiAl-13vol.% AlN material. At 1300 K, the compressive strain rate-flow stress diagram has two distinct deformation regimes, with the transition occurring near 150 MPa. The low and the high stress regimes have power law creep exponents of ∼ 6.1 and 14.2, respectively. Microstructural characterization of the as-extruded and tested samples has been performed to develop an understanding of the superior mechanical properties of the material. The microstructure of the as-extruded material was inhomogeneous and consisted of mantle regions containing a mixture of small NiAl grains (diameter ∼ 50–150 nm) and fine AlN particles (size ∼ 5–50 nm) that surround larger NiAl grains (diameter ∼ 0.3–8.0 μm) which were mostly particle free. In the low-stress regime, samples tested to steady state exhibited a structure composed of subgrain boundaries in the particle-free NiAl grains. In addition, some of the subgrains had developed a well defined dislocation network. AlN patricles occasionally found within large NiAl grains acted as pinning centers for dislocations. Small NiAl grains and the AlN particles constituting the mantle coarsened during these tests. In the high-stress regime, samples tested to steady state exhibited a high density of dislocations in most of the particle-free NiAl grains. Subgrain boundaries were found occasionally but dislocation networks were rare. The AlN particles had not significantly coarsened due to the shorter times at temperature.

An increase in the athermal component of the yield stress at low temperatures is the caused of the high temperature strengthening of discontinuously reinforced NiAl matrix composites. The reinforcements stabilize the microstructure. The strengthening in the temperature range of 300–1273K, is believed to be related to the grain size refinement and effective pinning of grain boundaries. This conclusion was obtained from analysis of data from AlN, TiB2 and Al2O3/NiAl particulate composites.

The deformation and fracture behaviors of two directionally solidified multi-phase Ni-Fe-Al ordered alloys were investigated. One alloy consisted of continuous β+γ lamellae with fine γ precipitates within the γ phase. The NiAl-based β phase of this alloy exhibited <100> slip even when deformed parallel to the [001] growth direction. This material exhibited an initiation fracture toughness of ∼ 30 MPa √m and tensile ductility of 10%. The second alloy consisted of aligned but discontinuous γ lamellae within a continuous β phase. Again, the γ phase contained γ precipitates, but unlike the previous alloy, the β phase also contained a fine dispersion of bcc precipitates due to spinodal decomposition. The β phase of this alloy deformed by <111> slip. This four-phase alloy exhibited a fracture toughness of ∼ 21 MPa √m and tensile ductility of 2%. Observations of the plastic zone in both alloys indicated significant plasticity in the β phase due to easy slip transfer from the ductile second phase. The enhanced fracture resistance of these multiphase materials compared to single phase β alloys is attributed in large part to intrinsic toughening of the β phase by an increased mobile dislocation density due to efficient dislocation generation from the β/γ interfaces.

Fracture toughness as a function of temperature was evaluated for two Cr2Hf+Cr intermetallic composites, each in two different microstructural conditions. The proeutectic microstructures based on Cr-6.5Hf (at%) showed a significant increase in fracture toughness with an increase from room temperature to 600°C. The coarse microstructure obtained by heat treatment at 1500°C showed evidence of ductile behavior of Cr at a lower test temperature (200°C) relative to that of one heat treated at 1250°C (400'C). In the eutectic microstructures based on Cr-13Hf, only a small increase in fracture toughness at 600°C was seen. The results are analyzed in the light of fracture micromechanisms.

Several compositions of the intermetallic alloy Ni3Al were examined with respect to (a) their wetting of Al2O3 and (b) their energy absorption capability in Al2O3/Ni3Al laminates. Wetting is enhanced by small additions of carbon (e.g., 0.1 at. %), and bonding by additions of zirconium (e.g., 1 at. %). Chevron-notched laminate specimens were tested in three-point bending to assess the energy absorption due to the presence of a thin layer of Ni3Al. A nickel aluminide with the composition Ni-22Al-lZr-0.1C-0.1B was found to have a reasonable combination of wetting, adhesion and energy absorption. Interfacial bonding was found to be weak in all laminates.

The creep behavior of Nb-26Ti-48Al alloy and its composite were investigated. After creep testing, precipitates were observed in the composite and mobile dislocations were found to be pinned by these precipitates. The combined effects of the reinforcements and the pinned dislocations were responsible for a reduced creep rate of the composite. The possible creep mechanisms were discussed based on the stress exponent and the activation energy of creep.

We examine the conditions under which differences in thermal expansion between a particle and the matrix leads to crack growth within the matrix. Using linear elastic fracture mechanics, we obtain closed-form, analytical results for the case of a penny shaped crack present in the matrix interacting with a spherical inclusion which is misfitting with respect to the matrix. A simple and direct relationship is established between the strain energy release rate, the crack size, the crack orientation with respect to the inclusion, the crack/inclusion separation, the degree of thermal expansion mismatch and the elastic properties of the medium. We also analyze the size to which these cracks can grow and find that for a given misfit strain and material properties, crack growth is inhibited beyond a certain critical crack size. Finally, the preferred orientation of these cracks as a function of misfit strain is predicted. The implication of these results for thermal cycling are analyzed.

Isothermal compression tests were conducted on a particulate reinforced Ti-44Al–3V+7.5v/o TiB2 composite produced by the XD® process over wide ranges of temperatures and strain rates, 1050 to 1300 C and 0.001 to 1.0 s-1, respectively. Flow localization parameter analysis was performed to analyze the flow behavior of the composite under various processing conditions, and the results were compared to those by dynamic material modeling. Microstructural characterization was also conducted to correlate the flow behavior with post-deformation microstructures. With combined information on flow behavior and microstructures, the processing condition for relatively stable flow and uniform microstructure was determined to be 1300 C and 0.01 to 0.1 s-1.

A new class of intermetallic matrix composites (IMC's) based on orthorhombic titanium aluminides offer attractive properties for high-temperature structural components at temperatures up to 760°C. Results from an ongoing study on the microstructural stability and mechanical properties of the orthorhombic-based alloy Ti-22Al–23Nb (at%), in both monolithic and composite forms, are discussed. Oxygen acquired during processing or as a result of high-temperature exposure in air or vacuum has a pronounced influence on the microstructure of the monolithic and composite materials. Two-phase lath microstructures of ordered beta (βo) + orthorhombic (O) phases produced by processing low oxygen material above the beta transus are morphologically stable at 760°C. Conversely, in higher-oxygen three-phase microstructures containing O+βo+ α2(Ti3Al), lath coarsening and additional precipitation of α2 in oxygen-enriched sheet surface regions is observed. At 760°C/69MPa the two-phase lath microstructure has a higher creep resistance and lower tensile strength compared to the three-phase α2- containing microstructures of the higher oxygen material.

Mechanical alloying(MA) and Reactive Sintering(RS) techniques were successfully used to produce Nb5Si3. Model laminates of mechanically alloyed Nb5Si3 and commercially pure niobium were prepared via vacuum hot pressing. Tensile properties for the as-received, vacuum heat treated and niobium with a solid solution of silicon were obtained at 298K and 77K. 298K bend tests were conducted in an in situ deformation stage in a SEM. 77K bend tests were also conducted. Toughness values are correlated to the ligament properties and effect of constraint.

Two different interatomic potentials of the embedded atom type were used to study the relationships between dislocation core structure and mobility. Core structures were computed for a variety of dislocations in B2 NiAl. Several non-planar cores were studied as they reacted to applied stress and moved. The results show that in some cases, the dislocation core transforms to a planar structure before the dislocation glides, whereas in some other cases the core retains the non-planar structure at stresses sufficient to sustain glide. The effects of stoichiometry deviations on the core structure and motion were also studied.