Tensile deformation and fracture toughness of 2014 + 15 vol pct SiC particulate composite
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I.
INTRODUCTION
INVESTIGATIONS of the fracture characteristics of particulate SiC-reinforced aluminum alloys have shown that particle addition usually lowers the fracture toughness. Fracture toughness values for unreinforced aluminum alloys are in the range of 25 to 75 MPaVrn,[I] while the composites have plane strain values of 7 to 25 MPaVrn. [I] This drop in toughness has been attributed to (1) alterations in flow stress, (2) fracture of SiC particles, [2.3] (3) poor dispersion of SiC,[4] and (4) decreases in tensile ductility. [4] Volume fraction of SiC particles, matrix alloy chemistry, and processing are all believed to be contributing factorsYA.5] The research reported here was conducted to investigate fracture using the tools of micromechanics. Both the growth of large fatigue cracks and fracture toughness have been investigated, but this paper reports only the work done directly on fracture toughness; the results of fatigue crack growth have been published separately. [6] The goals of this research have been to understand the origins of fracture toughness as quantitatively as possible and to determine if there is anything which might be done to elevate the fracture toughness of these composites to levels closer to those of the matrix aluminum alloys.
II.
MATERIAL
The material investigated was fabricated in 1985 by Science Applications International of San Diego, CA, now called the Dural Aluminum Composites Company, a subsidiary of Alcan. Particulate SiC at a concentration of 15 pct by volume was dispersed in 2014 aluminum alloy (4.4Cu, 0.8Si, 0.8Mn, O.4Mg, in wt pct) using a proprietary method. After casting, the material was extruded at 450 °C with a 8.2: 1 ratio to produce bars of 16 x 38 mm cross section. The material was solutionized at 500 °C for 2 hours and precipitation aged to peak DAVID L. DAVIDSON, Institute Scientist, is with Southwest Research Institute, P.O. Box 28510, San Antonio, TX 78228-0510. Manuscript submitted May 17. 1990. METALLURGICAL TRANSACTIONS A
hardness (T6) at 160 °C for 16 hours, according to the recommendations of the manufacturer (not the standard age hardening schedule for unreinforced 2014 alloy). This material was designated 2014-PA + 15 vol pct SiC for comparison to other composites tested. 17 ,8] This material was chosen for a detailed micromechanics investigation because of the 13 variations in manufacturing technique, matrix alloy, SiC content, and heat treatment tested; this combination gave the highest value of fracture toughness measured. A detailed account of the microstructure of this material has already been given,[6] so the description given here is only a summary. Characterization was carried out by use of optical microscopy and both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Dispersion uniformity may be judged from Figure 1, a low-magnification optical micrograph. No statistical description of the uniformity of dispersion was devised, although the need for such a characterization is necessary. Higher magnification inv
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