A model study of cavity growth in superplasticity using single premachined holes
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I.
INTRODUCTION
THE ability of some fine-grained polycrystalline materials to exhibit very large strains to failure at relatively low flow stresses, termed "superplasticity," offers an attractive and commercially viable method for forming components with complex shapes. Nevertheless, the utility of superplastic metals is often limited because of the development of internal cavitation and the subsequent degrading of the properties of the component when in service.tll It is now well established that most superplastic materials cavitate during deformation.t2-5] Although it may be possible, at least in principle, to reveal the presence of internal cavities using nondestructive techniques such as photoacoustic spectroscopy,t61 these procedures have not been developed to any significant extent so that cavitation remains as an important limitation on the use of superplastic forming operations. Cavity failure occurs as a result of the nucleation, growth, and interlinkage of cavities. Cavities may nucleate in superplastic materials at particles, t7,8,91 at interphase boundaries in microduplex alloys,t~~ or at the interfaces between the reinforcement and the matrix in superplastic composites, t13,141 The subsequent growth of the cavities is generally modeled using the processes of diffusion growthttS] or plasticity-controlled growth[~6.~7]developed for the high-temperature creep of large-grained materials. In practice, however, the cavities may be larger than the very small grains of the superplastic alloys so that several grain boundaries may impinge on each cavity simultaneously, thereby leading to an enhancement in the diffusion growth process.[lS.191
Model experiments, using specimens with premachined holes, were first conducted by Tait and Taplin[20,211and subsequently by other investigators,t2z-251 These experiments
ATUL H. CHOKSHI, Associate Professor, is with the Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India. TERENCE G. LANGDON, Professor, is with the Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453. Manuscript submitted July 25, 1995. 2532--VOLUME 27A, SEPTEMBER 1996
have the advantage of permitting observations on a single hole while avoiding the statistical problems associated with averaging data collected from a wide spectrum of cavity sizes. Accordingly, the present research was initiated specifically to investigate hole growth in a superplastic alloy where it is now well established that failure often occurs as a result of cavitation.tg.26-2s] II.
EXPERIMENTAL MATERIAL AND PROCEDURES
The experiments were conducted using a commercial quasisingle phase superplastic copper alloy, designated Coronze CDA 638, which is known to exhibit very extensive cavitation during superplastic flow.ET,9,22,26,2s-331 The alloy was obtained from the Olin Corporation and contained (in wt pct) 2.8A1, 1.8Si, and 0.4Co, with the balance as Cu. Tensile specimens were machined parallel to the rolling direction with a gage len
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