Single fiber push-out and push-back tests combined with acoustic response monitoring were used to examine the interfacial behavior in two titanium alloy-SiC fiber composites. Distinctly different behaviors were observed in the two systems. The differences were attributed to the formation of a substantial interfacial reaction layer in one of the composites which changed the interfacial chemistry and the resulting debond topography. The reaction layer caused an increase in the interfacial bond strength and in the roughness of the debonded interface. The latter resulted in substantially increased sliding friction. Although both composite interfaces exhibited some roughness, only one showed a seating drop during fiber push-back. This is related to the fact that the reaction layer which formed in one of the composites was severely degraded during fiber pushout. Although this interface was still rough, the roughness correspondence between fiber and matrix was destroyed during sliding, such that seating was no longer possible.

Fiber push-out tests were used to evaluate several double coating systems for sapphire fibers in γ -TiAl. The double coatings consisted of an inner debond coating, followed by an outer coating of alumina that serves as a diffusion barrier. The inner coatings were of three types: carbon black, colloidal graphite and porous alumina. By comparing estimates of interface toughness computed from the push-out tests, each double coating is found to permit debonding and sliding of the sapphire fibers. Consequently, each of the double coatings would afford improved fracture toughness for sapphire reinforced γ -TiAl.

Processing routes for fabrication of continuous fiber, intermetallic-matrix composites are reviewed. These methods include conventional and isostatic hot pressing of layups of matrix material (e.g. foil or powder cloth) and fiber mats; consolidation of monotapes made by techniques such as arc, plasma, vapor, or electron beam deposition or tape casting; and liquid metal infiltration-base methods. The advantages and disadvantages of the various methods are discussed. Particular attention is focussed on HIP consolidation via foil-fiber-foil techniques. Process modeling techniques to assess the effects of pressure, temperature, and time on consolidation behavior are described. By this means, maps to delineate the interaction of process variables in such methods can be developed and applied for process optimization.

Consolidation of continuous fiber-reinforced titanium aluminide matrix composites (TMC) by the foil/fiber/foil method has traditionally taken an empirical approach utilizing processing cycles derived by simple trial and error. In an effort to reduce the empirical nature of producing TMC, a simple but effective analytical approach is employed. This approach analyzes the effect of fiber and foil geometries on consolidation parameters by combining a physical constitutive creep model with computational methods of interpreting raw materials characterization data. Examples of SCS-6/super a2(Ti-25Al-10Nb-3Mo-lV) and Saphikon/γ-TiAl composites consolidation are discussed by comparing the model predictions with equivalent validation specimen microstructures.

A novel way of producing particulate intermetallic matrix composites based on Nb-Al in one step using pulsed laser deposition (PLD) has been investigated. One unique characteristic, inherent to laser ablation, is the generation of particulates. These particulates condense on the substrate and become part of the film. In some cases, such as in high Tc superconducting or optical films applications, it is believed that the presence of particulates diminishes the quality of the films. In this work, we demonstrate that it can be advantageous in some applications to incorporate these particulates into the films.

Nb-Al films were prepared by laser ablation from a Nb3Al target using a 248nm KrF excimer laser at various fluences and substrate temperatures. Transmission (TEM) and scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and Auger electron spectroscopy (AES) were used to characterize the samples. The resultant films consist of a homogeneous matrix with uniformly distributed inclusions. The size of the particles generated by PLD is in the range of a few tens of nanometers to a few microns. EDX shows that the matrix has a composition of Nb7A13 and the particulates contain barely detectable Al. The mechanism of the depletion of Al in the particulates will be discussed. The merit of having a ductile Nb phase embedded in the intermetallic matrix is evident as the cracks generated during TEM sample thinning process propagate in the brittle matrix and finally are arrested at the ductile inclusions.

In pursuing the development of intermetallics as high temperature structural materials, a composite approach is considered necessary for achieving room temperature toughness. If not further qualified, the term “composite” generally implies the application of mechanical processes by which a strong reinforcing phase is dispersed in the matrix, often as aligned continuous fibers. While this approach appears simple and promising in principle, in practice it is limited by the availability of compatible fibers, controlled processing, and microstructural homogeneity and reproducibility. Alternatively, the composite microstructures may be created in-situ either synthetically or naturally. In synthetically derived composites, the desired phases may be deposited layer by layer using such techniques as chemical vapor deposition (CVD), and potentially a variety of lithographic techniques may be employed to control the microstructure. However, such techniques are currently rate limited and not well developed for the large dimensions required for structural composites. In contrast, the in-situ composites, which rely on phase separation by either eutectic solidification or solid state precipitation, are economical and especially well suited for generating naturally compatible ductile phase toughened composites with uniform fine scale microstructures. This paper attempts to classify these approaches in perspective, discuss the benefit of in-situ composites relying on the natural phase separation mechanisms, and review the current activities with emphasis on the concept of ductile phase toughening.

The possibility of incorporating SiC or TiC particulate reinforcements in a Ni3Al matrix through the melt processing route was examined. Interface bonding and thermal stability of SiC and surface treated SiC in Ni3Al has been investigated through sessile drop and DTA experiments. Feasibility of various processing techniques such as the mechanical mixing method and the vacuum infiltration method was also explored.

Microlanminates of Nb3Al and Nb were synthesized in-situ by high rate magnetron sputtering. Three composites were fabricated. They had thicknesses ranging from 19 to 146 μm (5.5 mils) with 11 to 91 layers. The volume fraction of the Nb lamellae was approximately 0.4 for two and 0.5 for the third. Room temperature fracture of the composites revealed that some cleavage cracking in the Nb3Al intermetallic layers was arrested by the ductile Nb layer. The Nb layers failed by chisel point fracture and shear.

New materials with improved high temperature properties are required for higher efficiency gas turbines. One group of alloys which show promise in this area are the eutectic systems Cr-Cr3Si, Nb-Nb3Si, and V-V3Si. Crystals of these refractory metalsilicide eutectics were directionally solidified using Czochralski crystal growth in conjunction with cold crucible levitation melting. The crystals were examined using scanning electron microscopy and transmission electron microscopy. In the present paper, these systems are examined with respect to the directionally solidified eutectic microstructures, phase equilibria, crystallography and mechanical properties.

Bend strength, compression strength, and fracture toughness of niobium beryllide intermetallic compounds have been assessed at temperatures from ambient to 1200°C. Hot-isostatically-pressed (HIP) Be12Nb showed significantly improved lowand high-temperature mechanical properties over vacuum-hot-pressed (VHP) material. Strengths at 20°C were 250 MPa in bending and 2750 MPa in compression with a fracture toughness of ∼4 Mpa√m, much higher than previously measured for this compound. High-temperature (≥ 1000°C) mechanical properties were also improved with bend strengths of 250 MPa at 1200°C as compared to only 70 to 100 MPa for the VHP material. However, severe pest embrittlement was detected in the HIP material at temperatures between 650 and 1000°C.

Model high-melting point Nb3Al + Nb intermetallic composites have been fabricated in situ by vacuum hot pressing and reaction sintering elemental powders mixed in the ratio Nb + 7wt. % Al. In both cases, microstructures feature islands of ductile Nb solid solution (∼20 vol. %) in a brittle Nb3Al intermetallic matrix. Thermal treatment for 24 h at 1800°C results in a lamellar microstructure containing a uniform and fine distribution of filamentary Nb in a Nb3Al matrix following the massive peritectic transformation. In this paper, the fatigue and fracture resistance of these two microstructures are examined and compared to pure Nb3Al and Nb. Preliminary results suggest that the Nb phase can provide significant toughening to Nb3Al via crack bridging, plastic stretching and interfacial debonding mechanisms. Measured plane-strain fracture toughness values for the as hot-pressed and fully-aged microstructures are ∼6–8 Mpa√m compared to √fm for pure Nb3Al. However, under cyclic loading, the composites tend to show a strong dependence on applied stress-intensity level; fatigue thresholds range between 2–3 Mpa√m.