Superplastic behavior and cavitation in high-strain-rate superplastic Si 3 N 4p /Al-Mg-Si composites
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I.
INTRODUCTION
IT has been demonstrated[1–9] that aluminum matrix composites with discontinuous reinforcement materials exhibit high-strain-rate superplasticity. High-strain-rate superplasticity is very attractive for commercial applications, because one of the major problems in the current superplastic forming technique is very slow forming rates of typically 1025 to 1023 s21. The mechanisms of high-strain-rate superplasticity for the composites are under debate. The in situ transmission electron micrograph (TEM) observation[10] revealed that partial melting occurs at the matrix/reinforcement interfaces and at grain boundaries at elevated temperatures. In addition, experimental results[6,11,12] showed that a maximum elongation is attained at a temperature close to the onset temperature for melting. This suggests that a liquid phase plays an important role in high-strain-rate superplasticity for the composites. It was shown in the previous articles[13,14] that cavities are formed during superplastic deformation for the high-strain-rate superplastic composites as well as for superplastic metals; however, the rate of increase in the cavity volume fraction for the composites is much lower than that for the superplastic 7475 Al alloy. Recently, Wada et al.[15] showed that cavity formation is limited by the presence of a liquid phase, because a liquid phase serves to relax the stress concentrations at the matrix/reinforcement interfaces.[16] In general, a high volume fraction of reinforcements has a deleterious influence on elongations for the composites, because cavity or crack formation is enhanced due to the stress concentrations around the reinforcements. However, HAJIME IWASAKI, Associate Professor, and TAKASUKE MORI, Professor, are with the Department of Materials Science and Engineering, College of Engineering, Himeji Institute of Technology, Hyogo 671-2201, Japan. MAMORU MABUCHI, Researcher, is with the National Industrial Research Institute of Nagoya, Nagoya 462-0057, Japan. KENJI HIGASHI, Professor, is with the Department of Metallurgy and Materials Science, College of Engineering, Osaka Prefecture University, Osaka 599-8531, Japan. Manuscript submitted June 27, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
if the stress concentrations are sufficiently relaxed by a liquid phase, a high volume fraction of reinforcements may have no deleterious influence on elongation. In the present article, superplastic behavior and cavitation are investigated for two high-strain-rate superplastic Si3N4p /Al-Mg-Si (6061) composites with a Vf 5 20 and 30 pct, respectively, where Vf is the volume fraction of reinforcements. The results in the present investigation showed that, in a superplastic region, a maximum elongation for the 30 vol pct Si3N4p /Al-Mg-Si composite is larger than that for the 20 vol pct Si3N4p /Al-Mg-Si composite. This is investigated from the viewpoint of cavitation. II.
EXPERIMENTAL
A fine-grained 20 vol pct Si3N4p /Al-Mg-Si (6061) composite and a 30 vol pct Si3N4p /Al-Mg-Si (6061) composite were p
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