Cavity growth rate in superplastic 5083 Al and AZ31 Mg alloys
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Mamoru Mabuchib) Department of Energy Science and Technology, Kyoto University, Kyoto 606-8501, Japan (Received 17 April 2004; accepted 10 August 2004)
The plasticity-controlled growth rate of cavities during superplastic deformation was statistically investigated for 5083 Al alloy and AZ31 Mg alloy. When the cavity growth rate was evaluated on the basis of macroscopic strain calculated using the displacement of the specimen, the growth rate for the Al alloy was larger than that for the Mg alloy. However, the growth rate of the Al alloy was in agreement with that of the Mg alloy when the cavity growth rate was evaluated on the basis of the microscopic strain due to grain boundary sliding. The results obtained lead to two conclusions: (i) the rate of cavity growth is not affected by the kind of materials, that is, the nature of the grain boundary, and (ii) the microscopic strain due to grain boundary sliding should be used to evaluate exactly the rate of cavity growth for superplastic deformation.
I. INTRODUCTION
Cavitation often occurs during deformation at elevated temperatures. In particular, significant cavitation is caused during superplastic deformation.1,2 The dominant deformation process of superplasticity is grain boundary sliding.3–5 Hence, the origin of cavitation during superplastic deformation is related to grain boundary sliding. In general, cavitation is divided into three stages; cavity nucleation, growth, and coalescence. A cavity grows by a stress-directed vacancy diffusion6,7 or by plastic deformation of the surrounding matrix.8,9 Namely, there are two cavity growth mechanisms: diffusion-controlled growth and plasticity-controlled growth. Usually, cavity growth is diffusion-controlled when the cavity size is small and is plasticity-controlled when the cavity size is large.10 It was reported that cavity growth is mostly plasticity-controlled for superplastic metals.1,11 The current paper describes the results of a detailed investigation of the cavity growth rate in superplastic 5083 Al and AZ31 Mg alloys. The specific objectives of the current research are twofold: to examine exactly the cavity growth rate from the microscopic strain due to grain boundary sliding and to compare the cavity growth rate of the Al alloy with that of the Mg alloy. The latter
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2004.0431 3382
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J. Mater. Res., Vol. 19, No. 11, Nov 2004 Downloaded: 16 Mar 2015
objective is important to understand the nature of the cavity growth. Recently, it has been reported that the grain size strengthening parameter of Mg is larger than that of Al,12,13 resulting from the larger potential of the grain boundary for suppression of dislocation movement in Mg. Besides, grain boundary sliding occurs significantly in Mg,14 due to its large grain-boundary diffusion coefficient.15 These indicate that the nature of the grain boundary of Mg is different from that of Al. It is interes
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