This study examines the longitudinal creep behavior of a unidirectionally reinforced SiC fiber/Ti-1100 composite at 540°C in a vacuum. Primary creep behavior is predicted by the McLean creep model for elastic fibers in a creeping matrix provided the initial damage in the composite is taken into account and no damage accumulates during creep. At higher stresses, where the composite accumulates damage and eventually fails, both the strain and time to rupture can be predicted using the Bundle model, a modification of the McLean/Curtin creep model. In predicting rupture, the strength distribution of the weakest fibers in the composite is the controlling factor.

The Wright Laboratory Materials Directorate at Wright-Patterson AFB has been spearheading the development and evaluation of a new class of metal matrix composites based upon continuous SiC fiber reinforcement of orthorhombic phase containing titanium aluminide matrices. These composites (O TMCs) will be subjected to thermal exposures during primary and secondary component processing, and possibly also during heat treatments to optimize matrix-dominated mechanical performance. Such thermal excursions must not degrade the SiC fiber reinforcement, hence compromising resulting composite properties. Therefore, the effects of heat treatment on the room temperature tensile strength of continuous SiC fibers were studied. The fibers examined included: Trimarc 1®, SCS-6, Ultra SCS and an experimental large diameter version of Ultra SCS. The fibers were heat treated below and above the beta solvus temperature of the orthorhombic matrix alloy utilized for this study, Ti-22Al-23Nb (at%). The fibers were evaluated for ambient temperature tensile strength in the following conditions: (1) as-received: (2) heat treated in vacuum; and (3) consolidated into Ti-22Al-23Nb, heat treated in vacuum, and chemically extracted. Fiber microstructure and fracture analysis was accomplished via secondary scanning electron microscopy (SEM). Chemical reactions between fiber core and the SiC, and between the SiC fiber and the Ti-22Al-23Nb matrix, were also studied by SEM.

It is shown that tensile testing of continuous fibre reinforced composite specimens with varying gauge lengths above the matrix melting temperature provides a method for the measurement of fibre damage within the composite. This method is applied to a composite wire of alumina fibres reinforcing a pure Al or an Al – 2%Cu matrix. It is found that the wire contains a finite initial level of fibre damage. Fibre damage accumulation caused by composite tensile straining is also measured by applying the method to previously stressed wire samples. The rate of damage accumulation for a single phase matrix composite is well described by the Weibull parameters of the virgin fibre, indicating that fibre strength is unaffected by composite processing. The presence of brittle second phases in the matrix increases, however, the rate of damage accumulation as compared to the rate expected from single fibre Weibull statistics.

Unidirectionally reinforced titanium matrix composites (TMC's) have significant potential to replace more dense materials in future aerospace applications; however, their current low transverse strength motivates the development of new interface systems. To guide the selection of an optimum interface, a comprehensive approach to the characterization of the effect of the fiber-matrix interface on the composite properties must be developed, In the present work, simplified test techniques using microcomposite specimens to investigate the interface influence on the fiber strength and the transverse tensile, longitudinal tensile, and fatigue crack growth behavior of the composite are discussed. These techniques have been successfully employed to understand the role of the interface on the composite properties of interest and have significant promise for future interface development studies.

Silicon carbide monofilaments manufactured by chemical vapor deposition are the primary reinforcement in titanium-alloy matrix composites. In this study, W-core/SiC fibers were consolidated in a Ti-22Al-23Nb (atomic percent) matrix. Samples of this composite were then subjected to heat-treatments in an inert environment which ranged in temperature from 800 to 1000 degrees C and duration from 25 to 225 hours. The fibers were chemically extracted from the matrix and tensile tested to measure their strength. The rate of the W-core/SiC reaction was measured and compared to that reported in previous research, and the effect of the reaction products on the fiber strength distribution was characterized to determine the limits of exposure to which such fibers may be subjected.

In any composite development effort it is important to demonstrate that modifications in constituents or in the fabrication process do not adversely affect the other constituents or the resulting composite properties. Previous work has demonstrated that the extracted fiber strength distribution in as-consolidated composites is different than typical virgin fiber strength distributions and that composite heat-treatments in an inert environment may further degrade fiber strength. The degree to which a composite can withstand elevated temperature exposure depends on the kinetics of the fiber-matrix interaction which is related to the matrix chemistry and microstructure. Smith et al. have characterized the reaction kinetics of several Ti-Al-Nb alloys with SiC fiber, but the effect of the reaction on fiber strength had not been measured. The goal of this work was to establish the effect of composite consolidation in a Ti-22Al-23 Nb (atomic percent) matrix on the fiber strength distribution. The consequences of post-consolidation heat treatments, as well as prolonged high temperature exposure, on fiber strength were also evaluated.

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.