Effect of lithium on the mechanical properties and microstructure of sic whisker reinforced aluminum alloys
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I.
INTRODUCTION
A L T H O U G H whisker reinforced metal matrix composites are not a recent introduction as evidenced by the early work of Sutton and Chorne ~ and Divecha e t al, 2 there has been a revival of interest in the last five years due to the introduction of silicon carbide whiskers derived from rice hulls. These whiskers are potentially inexpensive and are currently being produced by Exxon Enterprises Materials Division (formerly, Silag Inc.) in quantities suitable for many aerospace applications. Many early whisker materials used in aluminum alloys produced an increase in elastic modulus and an increase in the strength of pure aluminum and dilute aluminum alloys but little or no strength increase compared to commercially available high strength aluminum alloys. Some reasons for this are: 1. An apparently weak bond at the whisker-matrix interface which allows the whiskers to pull out of the matrix before their full strength can be utilized) 2. The aspect ratios of the whiskers are markedly reduced during extrusion. Data from Exxon Enterprises Materials Division 4 indicates that a four-fold reduction in aspect ratio occurs during extrusion. 3. Early SiC reinforcement from Exxon contained a substantial proportion of particulate material which on theoretical grounds would be expected to produce a lower strength composite. Recent (after 1979) improvements in whisker quality and composite fabrication techniques have resulted in composites which show increased strengths compared to commercial aluminum alloys such as A1 2024 and A1 7075. 4 The present program was designed to improve further the mechanical and physical properties of A1-SiC Whisker composites by the use of an aluminum-lithium alloy matrix. The anticipated advantages of this approach were a reduction in composite density and an improvement in the bond between the whiskers and the matrix due to the reactivity of the lithium. Lithium serves similar purposes in Du Pont's A1-Li-A1203 (FP) composites where the reinforcement is in the form of continuous fibers. An increase in the matrix-whisker bond strength would reduce the amount of whisker pullout and might be expected DONALD WEBSTER is Staff Engineer with Lockheed Missiles and Space Company, P. O. Box 504, Sunnyvale, CA 94088-3504. Manuscript submitted July 27, 1981. METALLURGICALTRANSACTIONSA
to allow the stress in the whisker to reach a higher level and hence increase the strength of the composite. II.
MATERIALS
Four A1-Li-SiC composites were made by powder metallurgical techniques by Exxon Enterprises Materials Division. The composites had nominally 25 wt pct SiC Whiskers and were made by blending the whiskers with four aluminum-lithium alloys made by Homogeneous Metals Inc. After blending the composites were liquid phase hot pressed to full density as 7.6 cm billets and extruded to 1.6 cm rods. Since the dissemination of information regarding metal matrix composite manufacturing technology is specifically restricted by the provisions of the United States Munitions Control List (1979), no fur
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