Superplasticity in a high strength powder aluminum
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I.
INTRODUCTION
A N ongoing demand is to increase structural performance and concurrently to decrease the cost of aircraft and spacecraft components. This has led to increased innovative studies in the areas of both materials and processes. In the area of materials, methods are being developed to produce superior aluminum alloy products for aerospace structures through the use of rapidly solidified powders and improved consolidation techniques. ~,2Two important developments in this area have received attention. They are (1) high strength, rapidly solidified powder A1 alloys, with strength levels greater than 7075 A1, and (2) high modulus A1 matrix composites, discontinuously reinforced with silicon carbide particulates or whiskers. Even though SiC has a density (3.2 gm/cc) slightly greater than that of A1 (2.78 gm/cc), the efficiency of modulus improvement is so significant that the A1/SiC materials appear more attractive in terms of specific stiffness than low density alloys3 such as the A1-Li alloys, which are currently under development. However, one of the main drawbacks of AI/SiC composite materials is the poor formability, and therefore bending and shaping from sheet or plate has been virtually impossible. Therefore, considering alternative fabrication techniques to overcome such problems is highly desirable. Superplastic forming has been shown to be a viable technique for the fabrication of complex shapes or difficult materials such as titanium, 4 INCONEL* 718, 5 and titanium *INCONEL is a trademark of the INCO family of companies.
aluminides. 6 Recently, using different approaches, superplasticity has also been demonstrated in aluminum alloys reinforced with SiC. 7'8'9 In the work of Wu and Sherby, 7 the procedure involved long-time thermal cycling and resulted in an optimum elongation-to-failure of - 3 0 0 pct at a relatively slow strain rate of - 4 × 10-5 s -~. In the work of
MURRAY W. MAHONEY, Member, Technical Staff, and AMIT K. GHOSH, Manager of Metals Processing, are with Rockwell International Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360. Manuscript submitted June 25, 1986. METALLURGICALTRANSACTIONS A
Nieh et al., 8 it was shown that a commercial SIC/2124 A1 alloy, following appropriate thermomechanical treatments, could exhibit superplastic-like behavior (300 pct) during isothermal deformation at high strain rates of - 3 × 10-1 s -l. However, in this work it was reported that tests were performed in a solid-liquid region, negating any additional benefits attributable to use of an RST powder alloy as the matrix material. Mahoney and Ghosh 9 used a more conventional approach, fabricating SiC/A1 composites by distributing SiC whiskers between fine-grained, superplastic, 7475 A1 foils with subsequent consolidation to achieve sheet thicknesses of - 1 . 5 mm. This material was formable (350 pct elongation) and had an improved modulus, but did not benefit in terms of superior strength from rapid solidification processes. The availability of a high strength, high modulus and yet formab
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