Strength and Fracture Behavior of Aluminide Matrix Composites with Ceramic Fibers
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e microstructure of the composites was observed by scanning electron microscopy (SEM) and electron dispersive X-ray analysis (EDX). The constitutional phases were identified by X-ray diffraction (XRD). The strength of the composites was measured by three point bending test with a span length of 30 mm at 573-1273 K in air. The bending test was performed using 3 mm x 4 mm x 40 mm testing bars at a crosshead speed of 8.3 x 10,3 mm secl. RESULTS AND DISCUSSION Microstructure of composites
Because FeAl alloys have perfect chemical compatibility with A120 3 [5], the deterioration of fibers by the interfacial reaction is negligible in the Fe-40at%A1 composites with A120 3 fibers. A120 3 fibers are also sufficient as reinforcement materials for FeAl matrix composites from the viewpoint of strength at elevated temperatures because the composites are expected to be used up to -1000 K. The thermal stress originating from mismatch of coefficient of thermal expansion (CTE) (Fe-40at%A1 : 21.8 x 10-6 K-1 ;A120 3 : 9.4 x 10-6 K-') [6] between the fibers and matrix can be the most serious problem for deterioration of the composites. However, neither cracking nor debonding of fibers was observed in the composites fabricated by reactive hot-pressing [3]. The thermal stress generated on cooling is considered to be relaxed by the plastic deformation of the matrix. In the case of Ni3A1 matrix composites, A120 3 is chemically compatible with the matrix [7]. However, SiC fibers have an advantage over A120 3 fibers in Ni3A1 composite, which are expected to be used at higher temperatures than FeAl composites, because SiC fibers exhibit excellent mechanical properties even at 1273-1473 K [8]. Although SiC fibers have superior mechanical properties, the deterioration of fibers by the interfacial reaction is unavoidable in Ni3A1 matrix composites. In fact, the reinforcement completely disappeared when JP-SiC whiskers were added into the Ni3AI matrix [9]. In the present work, Si-Ti-C-O fibers [8] were selected as the reinforcement. Figure 1 shows a typical cross-sectional microstructure of Ni-24at%A1-O.lat%B matrix composites with Si-Ti-C-O fibers. The reaction product formed around the fibers is identified to be graphite by EDX and XRD. The EDX analysis also indicates the dissolution of 1-2 at% Si into the matrix.
Figure 1 Cross-sectional SEM micrograph of Ni-24at%AI-0.1 B/Si-Ti-C-O fiber composites.
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Mechanicalbehavior of composites A120 3 fiber reinforced FeAl composites
The strength of the Fe-40at%Al matrix composites reinforced with A120 3 fibers at ambient temperatures have been described in a previous paper [3]. The ultimate strain of Fe-40at%A1 matrix is much larger than that of A120 3 fibers. In this case, the fibers are predominately broken into short fragments before matrix cracking (multiple-fracture) [10] during the bending test because the load carried by the fibers can reach to their fracture strength. Fiber fragments longer than the critical length (1c) can provide significant strengthening by their load beari
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