This work deals with the application of ultrasonic nondestructive evaluation for characterizing fiber-reinforced metal matrix composites. The method involved the use of a recently developed technique in which the fiber reinforcement acts as a reflector to incident ultrasonic shear waves. Single fiber and multifiber, single ply composites consisting of SiC fibers in several titanium alloy matrices were investigated. The ultrasonic images obtained were correlated with the results of metallographic characterization of the composites. The results showed that the ultrasonic response of the metal matrix composites is significantly influenced by the microstructure of the matrix through which the incident wave traverses. The general effects of matrix on ultrasonic wave propagation are reviewed, and the ultrasonic signals obtained from various SiC fiber-reinforced titanium alloy composites are discussed in terms of the scattering effects of matrix microstructure.

The lack of high-temperature environmental resistance is a major issue in the application of orthorhombic-based titanium aluminide alloys (O alloys) and their composites. Improvement in environmental capability can be achieved by applying diffusion barrier coatings to the surface of the orthorhombic matrix alloy. However, since thin coatings are prone to foreign-object damage, an approach based on thicker multilayer materials may be more prudent for fracture-critical applications. In the present study, foils of the orthorhombic alloy were diffusion bonded on either side with an γ alloy, the latter used in an attempt to provide environmental protection. Mechanical tests suggested that the γ alloy was successful in preventing degradation of the O alloy due to oxidation and interstitial embrittlement under thermal cycling conditions. Processing below the β transus of the O alloy provided an improvement in the stress and strain to failure of the joined material compared to materials processed above the transus. However, in either case, the strengths of the joined materials were significantly lower than that of the uncoated O alloy with similar microstructures. Results suggest that the low strength of the joined materials may be due to cracking of the γ alloy, resulting in premature failure of the O alloy. Finite element analysis (FEA) was performed to understand the stress distribution in the joined material and to investigate approaches for reducing the residual stress. Several approaches for improving the stress and strain to failure of the joined material are presented.

Microstructure-property understanding obtained for a nominally Ti-25Al-17Nb (at. pct) monolithic sheet alloy was used to heat treat a unidirectional four-ply SCS-6/Ti-25Al-17Nb metal-matrix composite (MMC) and a fiberless “neat” material of the same alloy for enhancing mechanical properties. The unreinforced alloy and [0] 4 composite recorded significant improvements in ductility and strength, which were related to the microstructural condition. Modeling of the tensile strength based on fiber fracture statistics helped in understanding how improved matrix microstructure provided more efficient utilization of fiber strength. In comparison to the [0] 4 MMC, improvement of the [90] 4 response was negligible, which was related to an α 2 stabilized zone around the fiber. A Nb coating on the fiber was used to modify the local microstructure, and it produced a modest improvement in strength and ductility in the transverse direction. Structure-property relations of the matrix under different heat-treatment conditions are described in terms of deformation and failure mechanisms of the constituent phases; α 2 (ordered hexagonal close-packed), B2 (ordered body-centered cubic), and O (ordered orthorhombic based on Ti 2 AlNb).

Characterization of the disproportionated NdFeCoZrB alloy

The lack of high temperature environmental resistance is a major concern in the application of orthorhombic alloys and their composites. In an attempt to mitigate this issue, in the present study, foils of the orthorhombic alloy were vacuum diffusion bonded on either side with Ti-8Al-2Cr-2Mn foils. The thermal stability of the joint was examined using microscopic analysis and mechanical testing. Microscopic analysis indicates that the reaction product is mainly niobium rich alpha(2) The ultimate tensile strength of the joint was not noticeably affected by thermaI cycling in air. These results suggest that the alpha(2)+gamma-alloy was successful in preventing degradation of the O-alloy due to interstitial embrittlement. However, the strength of the "as-fabricated" material was significantly lower than that of the uncoated O-alloy with a similar microstructure. Therefore, it appears that under the present processing conditions, this coating may not be suitable. However, joining below the beta-transus or incorporating compliant layers may have potential.

This work utilizes a novel procedure for obtaining quantitative information on the mechanical properties of the fiber-matrix interface in composite materials. The method simulates actual experiments in detail, including fiber breakage, matrix yield and/or cracking, and interface failure. In a recent study, the procedure was implemented for the following commonly performed experiments: (a) the fragmentation test for metal matrix composites (MMCs); (b) the pushout and pullout tests for MMCs as well as ceramic matrix composites. In the simulations, the test configuration is discretized into a lattice which delineates the matrix, the fiber, and the interface. Details can be found in [1]. The simulations provide further understanding of the mechanisms involved during the relevant testing. In addition, through back-analysis, quantitative values of the, homogenized, interface properties can be obtained. In this paper, we first describe simulations of the pushout/pullout and fragmentation tests for a titanium matrix, silicon carbide (SiC) fiber composite. Relevant interface properties are evaluated by simulating the former test. Using these values, we study the response of both test configurations, and then compare the numerical results with actual experimental data. Further, we combine recent experimental results with relevant simulations for the so-called transverse test of the same material composition.

An ultrasonic nondestructive methodology for evaluating the consolidation and microstructure of advanced fiber-reinforced composites has been developed to aid in their design and fabrication. The use of this nondestructive evaluation (NDE) technique can enable optimization of the processing parameters to obtain complete densification around the fibers. In addition, the methodology can be used to ensure that the composite panels are devoid of any global problems such as fiber swimming, ply delamination, embedded manufacturing anomalies such as voids, etc. Such a post processing NDE is also essential before any interfacial characterization is performed. The technique described in this paper, being generic, is applicable to both metal matrix and ceramic matrix composites.

This paper presents a novel approach to evaluate the elastic properties and the behavior of the interphase region formed by a chemical reaction between the matrix and the fiber materials in metal matrix and ceramic matrix composites. Contrary to the traditional approach which does not allow any relative displacement at the interface without fracture, this paper considers elastic deformation of the interphase zone between the matrix and the fiber by replacing the zone by an “equivalent elastic interface”. The elastic behavior of the equivalent elastic interface describes the local elastic rigidity and deformation of the interphase zone and can be quantified by a mechanics parameter called “shear stiffness coefficient” which is proportional to the ratio of the shear modulus to the local thickness of the interphase material. This paper also outlines an ultrasonic reflectivity modeling that can be used for the experimental measurement of the interfacial shear stiffness coefficient along the length of an embedded fiber. Further, an experimental method of measurement of the shear stiffness coefficient is presented and experimentally measured values are tabulated. The significance of the quantification of such a parameter is that the elastic property of the interface obtained can be used as a common basis among material scientists designing and developing the composite systems, and groups studying material behavior for life prediction. Also, the parameter can be used by production engineers to assure that the designed properties of the composite are being achieved, and by the end users to ensure that the designed and produced properties are being retained in use.

Non-destructive evaluation techniques using ultrasonic shear wave and longitudinal wave interrogations have been employed to evaluate the consolidation behavior of a Ti-14Al-21Nb/SiC model composite containing a single SiC fiber. Partially and fully consolidated composite samples prepared by diffusion bonding methods were examined by these techniques, and the ultrasonic images obtained correlated with the results of metallographic characterization. A similar evaluation was carried out on a Ti-6Al-4V/SiC single-ply composite to illustrate the applicability of these techniques to composites containing high fiber volume fractions. The results indicate that ultrasonic NDE provides a quick and reliable tool for monitoring the consolidation of metal-matrix composites.

Single-fiber fragmentation tests with continuous silicon-carbide fibers in a Ti-6Al-4V alloy matrix have been conducted with in situ ultrasonic imaging to monitor the fragmentation process. Straining proceeded incrementally on a specially designed load frame with acoustic emission detection (AE) performed during each increment, and shear-wave back reflectivity (SBR) ultrasound images were acquired following each increment. Metallographic examination of the fragmented fiber was performed following the straining sequence by electropolishing and scanning electron microscopy. Good agreement was found between the fiber breaks imaged by ultrasound, the number of breaks detected by acoustic emissions, and the breaks observed by metallography.

This study explores a novel procedure for obtaining quantitative information on the mechanical properties of the fiber-matrix interface in composite materials. The method, based on lattice discretization of a medium, simulates actual experiments in detail, including fiber breakage, matrix yield and/or cracking, and interface failure. The paper concentrates on two experiments performed commonly, the so-called fragmentation test for metal matrix, and the pushout/pullout test for metal as well as ceramic matrix composites. Based on the documented capability of the method to simulate actual experimental data, reliable values of (homogenized) interface properties can be obtained. In addition, the simulations provide further understanding of the mechanisms involved during the relevant testing. Although this study presents results from basic problems, the method is general enough to include effects of residual stress, of high temperature environment, and of dynamic crack propagation, as well as three-dimensional details of the interface failure process. The potential exists for simulating nondestructive wave-based techniques aimed at evaluating interface properties.

The results of the application of supervised backpropagation neural networks to the classification of ultrasonic signals obtained from a model metal matrix composite are presented. This composite is made of a single fiber embedded in a Ti-24Al-llNb matrix and is used for the characterization of the fiber-matrix interface. The neural network is implemented in the frequency domain with two hidden layers and shows excellent discrimination capability when the network is trained with a judicious choice for the training set of ultrasonic signals. The sensitivity of the performance of the network to the number of examples used for training and the robustness of the algorithm to the change in the training set are discussed

Titanium-aluminum alloy metal matrix composites (MMC) and Ti-Al intermetallic matrix composites (IMC), reinforced with continuous SCS6 SiC fibers are leading candidates for high temperature aerospace applications such as the National Aerospace Plane (NASP). The nature of deformation at fiber / matrix interfaces is characterized in this ongoing research. One major concern is the mismatch in coefficient of thermal expansion (CTE) between the Ti-based matrix and the SiC fiber. This can lead to thermal stresses upon cooling down from the temperature incurred during hot isostatic pressing (HIP), which are sufficient to cause yielding in the matrix, and/or lead to fatigue from the thermal cycling that will be incurred during application, A second concern is the load transfer, from fiber to matrix, that is required if/when fiber fracture occurs. In both cases the stresses in the matrix are most severe at the interlace.

TiAl alloys with Cr, V, and Nb additions show promise as high temperature materials due to their high temperature strength and modulus. Dynamic recrystallization has been shown to be important for the processing and superplastic forming of these materials. Earlier studies have indicated that dynamic recrystallization may also occur during tensile straining at or near the proposed use temperature of these alloys. A systematic study has been conducted to determine the effects of various parameters - temperature, strain, strain rate, and microstructure, on the occurrence of dynamic recrystallization near expected use conditions. The conditions near the ductile to brittle transition temperature which bound the onset of dynamic recrystallization in tension and compression were investigated. The findings will be presented and correlations will be drawn between the contributions of dynamic recrystallization and high temperature to the observed ductility.