Fiber and interface fracture in singlecrystal aluminum/SiC fiber composites
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I.
INTRODUCTION
METAL-matrix composites possess a unique combination of stiffness, ductility, and elevated-temperature strength that makes them attractive for high-performance applications, such as in aerospace components. The mechanical properties, particularly fracture resistance, depend strongly on the relative strengths of the reinforcement and interfacial zone, [1-41 Metallurgical control of these relative strengths is thus the basis for fracture-resistant design, which offers unique opportunities in metal-matrix composites, since their interfacial microstructures can be both complex and manipulable. There are a number of options available. Reactions between the matrix and reinforcement can create additional phases and compositional gradients. We will refer throughout this paper to the additional phase in the interfacial zone as the interphase (Figure 1). The fiber surface may be chemically altered for purposes of minimizing handling damage, improving fiber wettability, and/or controlling matrix reactivity. The matrix may contain alloying elements introduced to react with the interfacial phases or affect their stability. Postfabrication heat treatments may also be used to alter the strength of the matrix or interface. In addition to these metallurgical effects, the generally greater coefficient of thermal expansion of the metal matrix tends to produce residual stresses. Interfacial failure micromechanisms are illustrated in Figure 1. Interfacial strength can affect the transverse fracture toughness of fiber composites in several ways. An impinging transverse crack is preceded by a stress field which contains both tensile and shear components. [5.61Their magnitudes, relative to the shear and tensile strengths of the interphase and fiber, determine the initial fracture mode. Shear failure at the interface between the fiber and interphase (or if weaker, between the interphase and the matrix) can increase fracture ROGER B. CLOUGH, FRANCIS S. BIANCANIELLO, and URSULA R. KATYNER, Metallurgists, are with the National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899. HAYDN N.G. WADLEY, Professor, is with the Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22903. Manuscript submitted September 19, 1989. METALLURGICAL TRANSACTIONS A
toughness through crack blunting and frictional energy absorption during fiber pullout. [~1 In addition, as investigated by Ochiai and Murakami, t2] tensile failure of a finite thickness interphase can serve as a notch to decrease the effective fiber strength, accelerating the fracture process. Application of such models requires experimental data. Unambiguous measurement of fiber and interfacial strength requires micromechanical test methods, to make strength measurements of individual failure events, combined with metallographic observations. We therefore prepared tensile samples using a simple cylindrical geometry with a single fiber centrally aligned along the sample axis, using a Bridgman growth technique. To obtain
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