An experimental study on the alignment of cavities in a superplastic commercial copper alloy
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I.
INTRODUCTION
SUPERPLASTICITY is the ability of a class of alloys to deform extensively prior to failure. Currently, the largest elongation obtained in a superplastic material is 5500 pet. 1,2 This phenomenon is now being utilized to form complex shapes with considerable savings in c o s t . TM However, the occurrence of internal cavitation can significantly limit the use of superplastically formed structural components in the aerospace industry. Recent studies have demonstrated that even a small volume fraction of cavities can lead to a serious deterioration in the mechanical properties of superplastically formed components. 5'6'7Thus, there is a growing need to understand and control the development of cavitation during superplastic deformation. It is now recognized that cavitation during superplasticity may be considerably different from cavitation during creep deformation, and new models have been developed with specific applications to cavitation in fine-grained superplastic alloys. 8-H A striking feature of cavitation in superplastic materials is the frequent observation of cavities that are aligned in stringers parallel to the tensile axis. Cavity alignment in stringers was reported in a commercial copper alloy, ~2-~7Cu-Zn-Ni alloys, ~8't9 stainless steels, 2~ A1 based alloys, 23'24 and Zn-A1 alloys. 25-3~ A detailed study of cavitation in a commercial copper alloy by Shei and Langdon ~2revealed that, at high strain rates, small cavities formed in stringers parallel to the tensile axis whereas at low strain rates, large cavities developed with an essentially random distribution. The alignment of cavities in stringers and the influence of strain rate on cavity alignment has not yet been adequately explained. The present investigation was undertaken to study cavitation in detail at a fixed strain rate with the specific objective of understanding the development of aligned cavity stringers and the influence of large strain on the cavity stringers. ATUL H. CHOKSHI, formedy with the Department of Materials Science, University of Southern California, Los Angeles, CA, is with the Division of Materials Science and Engineering, Department of Mechanical Engineering, University of California, Davis, CA 95616. Manuscript submitted February 17, 1986.
METALLURGICALTRANSACTIONS A
II.
EXPERIMENTAL
The material chosen for this study was a commercial copper alloy, Cu-2.8 pct Al-l.8 pct Si-0.4 pct Co, where the constituents are given in weight percent. Measurements from scanning electron micrographs revealed that the spatial grain_size d was 2.8 _+ 0.3/xm, where d is equal to 1.74 L and L is the average linear intercept grain size. An examination of an overetched polished section led to the observation of Co-rich particles that were aligned in stringers along the rolling direction.16 In addition, scanning electron microscopy did not reveal the presence of any preexisting cavities. Tensile specimens with a gage length of 6.3 mm were machined with their tensile axes parallel to the rolling direction. The tensile tests we
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