Superplastic constitutive equation and rate-controlling process in aluminum matrix composites with discontinuous fiber a
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Superplastic constitutive equation and rate-controlling process in aluminum matrix composites with discontinuous fiber and particle reinforcements Mamoru Mabuchi National Industrial Research Institute of Nagoya, Hirate-cho, Kita-ku, Nagoya 462, Japan
Kenji Higashi College of Engineering, Department of Mechanical Systems Engineering, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 593, Japan (Received 18 March 1996; accepted 9 April 1997)
Superplastic behavior of aluminum matrix composites with discontinuous reinforcements has been investigated in a temperature range below the melting temperature measured by differential scanning calorimetry. The experimental results of the mechanical properties revealed that the rate-controlling process of superplastic flow was associated with dislocation movement controlled by lattice self-diffusion. The strengthening due to the presence of reinforcements was retained. It is suggested that the strongest strengthening process of the dislocation-pileup mechanism and the diffusional relaxation-limitation or dislocation bypass mechanism affects the rate-controlling process.
I. INTRODUCTION
It has been demonstrated that aluminum matrix composites with discontinuous reinforcements show superplastic behavior.1–7 In particular, some composites showed superplasticity at high strain rates (.1022 s21 ).8 High-strain-rate superplasticity is very attractive for commercial applications because one of the current drawbacks in superplastic forming technology is a slow forming rate which is typically ,1024 s21 . The superplastic deformation mechanisms in metal matrix composites are now under serious discussion. Mishra and Mukherjee9 showed that the activation energy for superplastic flow was 313 kJymol for a highstrain-rate superplastically deformable SiCwyAl–Cu– Mg composite. This value is higher than the activation energy for lattice self-diffusion of aluminum. On the other hand, it was reported that the deformation characteristics of superplastic aluminum matrix composites were divided into two regions from the viewpoint of the activation energy: region I (in a temperature range below the partial melting temperature) showing the activation energy for lattice self-diffusion in the metal matrix and region II (in a temperature range above the partial melting temperature) showing the activation energy higher than that for lattice self-diffusion.10 This fact suggests that the deformation mechanisms of the composites are similar to those of metals in a temperature range below the partial melting temperature, though the superplastic deformation mechanisms are drastically changed by the presence of a liquid phase. A purpose of the present study is to investigate superplastic behavior in a temperature range below the 640
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 3, Mar 1998
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melting temperature of aluminum matrix composites from the viewpoint of microstructural effects of the matrix grain size and the rein
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