Correlation of tensile strength with fracture modes of KAOWOOL- and SAFFIL-reinforced 339 aluminum
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I.
INTRODUCTION
THE reinforcement of aluminum alloys with discontinuous (i.e., short) ceramic fibers has been studied by many investigators.[1–21] The usual fabrication technique is to create a preform of the fibers and to infiltrate it with the molten alloy under pressure, a process known as squeeze casting. This approach produces relatively inexpensive composites, which are suitable for automotive applications.[22,23,24] One of the potential advantages of these composites is the increase of tensile strength compared to the unreinforced alloy. This is generally true at elevated temperatures, but not at ambient temperatures, wherein the literature is replete with apparently conflicting results. Some of the earlier results were rationalized by Baxter,[25] who pointed out that for a composite of this type, which contains randomly oriented fibers, the fibers oriented perpendicularly to the stress direction play a key role, and the composite strength depends upon the strength of the fiber-matrix interface. From this model, two values of composite strength were calculated: a maximum strength smax for the case of perfect bonding, i.e., when the interface is stronger that the matrix, and a lower value of strength smin when the interface is weaker than the matrix so that the transverse fibers delaminate. (The former condition is more easily achieved at elevated temperatures, where fiber reinforcement is beneficial, but at ambient temperatures, the strength of the interface is more critical, resulting in the reported variability.) Thus, this model established the importance of defining two factors: (1) the fracture mode of the composite, an aspect which is commonly neglected; and (2) the strength of the matrix alloy. The latter was assumed to be unchanged by the presence of the fibers, since in general, no evidence WILLIAM J. BAXTER, Principal Research Scientist, and ANIL K. SACHDEV, Senior Staff Research Engineer, are with the General Motors R&D Center, Metallurgy Department, Warren, MI 48090-9055. Manuscript submitted April 21, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
was provided to the contrary. However, it is well known that the fibers can change the microstructure of the alloy[13] and thereby its mechanical properties. For example, Clyne et al.[13] have shown that for several aluminum alloys in the as-cast condition, the addition of SAFFIL*[26] fibers (96 pct *SAFFIL is a trademark of ICI Americas, Inc., Wilmington, DE.
Al2O3 4 pct SiO2) can increase the microhardness by 10 to 50 pct. On the other hand, we have found[27] from nanoindentation of KAOWOOL**[28] (47 pct Al2O3 53 pct SiO2) **KAOWOOL is a trademark of Thermal Ceramics, Inc., Augusta, GA.
reinforced 339 aluminum that the heat treatment is very important: in the T5 condition, the hardness of the alloy is not affected by the fibers, but in the T6 condition, the hardness is decreased substantially by segregation of magnesium from the alloy to the fiber/matrix interface. Thus, to evaluate the tensile strength of this type of composite, it is essenti
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