The effect of thermal exposure on the microstructure an fiber/matrix interface of an AI 2 O 3 /AI composite
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I.
INTRODUCTION
THE fiber/matrix
interface is probably the most critical region in a metal matrix composite (MMC) since the mechanical properties are largely limited by the efficiency of the fiber/matrix bond. Treatments, such as exposure to high temperature, which may affect the interfacial reaction layer, can drastically alter the mechanical properties. The effects of thermal exposure and thermal cycling on the structure and properties of a-A1203/AI-Li composites, produced by the DuPont Company under the designation FP/AI, have been the subject of several investigations. 1-5 For example, a degradation of longitudinal and transverse strength was reported by Olsen I after exposure to a temperature of 457 ~ for 500 hours. Similarly, Kim e t a l . 2 found a general decrease in transverse tensile strength with increasing time of exposure at 500 ~ although a plateau occurred between 27 and 100 hours. In contrast to these reported degradations in properties, Skinner e t a l . found that 5.2 days at 500 ~ had no significant effect upon the work of fracture of a-A1203/ A1-Li.3 Furthermore, it was of no consequence for this property whether the sample was exposed for a given period of time at constant temperature or whether it was cycled between high and low temperature. However, although there have been several investigations of the effects of thermal exposure on the mechanical properties, there has been little direct metallographic examination of the interface structure which is produced as a result of these treatments and, if such studies are carried out at all, they are frequently limited to optical or scanning electron microscope (SEM) observations. Indeed, even the identity of the interfacial reaction products in as-fabricated a-AI203/A1-Li is still open to debate despite the fact that the system has now been under investigation for well over a decade. For example, Champion e t a l . 4 identified small amounts of a-LiAIO2 by X-ray diffraction (XRD) of extracted fibers whereas Olsen ~ found only A1203 and proposed I.W. HALL is Assistant Professor, Center for Composite Materials, Spencer Laboratory, University of Delaware, Newark, DE 19716. V. BARRAILLER, formerly with the Center for Composite Materials, University of Delaware, is Research Engineer, Renault Company, Pads, France. Manuscript submitted April 26, 1985. METALLURGICALTRANSACTIONSA
the presence of a nonstoichiometric form of this compound. Kim e t a l . 2 identified the spinel of Li20 9 5A1203 (=LiAI5Os) by XRD, and Prewo 5 reported the same phase detected by electron diffraction. Thus, three separate phases have been proposed, but it still remains unclear why one technique should reveal a particular phase while a different technique fails to reveal it. The purpose of the present work was, therefore, to characterize the interface and to investigate how the interface responded to thermal exposure treatments. Initial studies were performed using the conventional techniques of SEM and XRD in order to permit correlation with other work. Thereafter, transmiss
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