Chemical stability of zirconia-stabilized alumina fibers during pressure infiltration by aluminum
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I.
INTRODUCTION
O N E of the principal difficulties encountered in the fabrication of metal matrix composites is the occurrence of chemical reactions at the interface between reinforcement and matrix. E.I. Du Pont de Nemours Inc., Wilmington, DE, has recently developed a new continuous ceramic fiber which is designed to increase both high-temperature stability and strain to failure, compared to an analogous fiber entirely composed of alumina. [I] This fiber, designated PRD-166,* contains small grains *PRD-166 is a trademark of E.I. Du Pont de Nemours, Wilmington, DE.
of ZrOz (of 0.1 /zm average diameter) to prevent grain growth of the A1203 grains (of 0.5/xm average diameter) at higher temperatures. This fiber has been used for the reinforcement of Fe-28AI-2Cr-lTi, Ni-lTi, and Ti-48.4A11Mn. t2,3,41These composites, fabricated by pressure casting, showed varying degrees of fiber/matrix interaction, which depended both on matrix chemistry and processing conditions. Although grain growth in alumina fibers is of no concern with an aluminum matrix (because of the relatively low melting temperature of aluminum and its alloys), the higher strain to failure of these fibers renders them technologically interesting for the reinforcement of aluminum and its alloys. The resulting composite would have high specific modulus and specific strength along the fiber direction, with reasonably high transverse strength.
J.A. ISAACS, Research Scientist, is with Aluminium Ranshofen, A-5282 Ranshofen, Austria. F. TARICCO, Research Engineer, is with Centro Ricerche Fiat, 10043 Orbassano, Italy, V.J. MICHAUD, Postdoctoral Associate, and A. MORTENSEN, Alcoa Associate Professor, are with the Deparanent of Materials Science, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted March 11, 1991. METALLURGICAL TRANSACTIONS A
In solidification processing of fiber-reinforced aluminum, the fibers come into contact with the molten matrix. To retain the properties of the fiber in the resulting composite material, chemical reactions between fiber and matrix must be minimized. This requirement prompted the present investigation, in which the chemical stability of PRD-166 fibers in pure aluminum during fabrication of composites by pressure infiltration is examined.
II,
EXPERIMENTAL PROCEDURES
A. Sample Preparation Three samples, designated A, B, and C, were cast using different procedures. In all composites, the reinforcement consisted of PRD-166 zirconia-stabilized alumina fibers, which were donated by E.I. Du Pont de Nemours, Wilmington, DE. In the processing of samples A and B, the as-received fibers were aligned by winding the fiber tow onto a second bobbin along its circumference. Aligned fiber bundles were then cut off the second bobbin, weighed for a desired volume fraction in the composite, wet with distilled water, and compressed into the desired shape using a small press. The water between the fibers was then frozen with liquid nitrogen. This produced a solid ice matrix composite, which could be handled during
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