High-strain-rate Superplastic Flow in 6061 Al Composite Enhanced by Liquid Phase
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S.H. Hong Dept. of Material Science and Engineering, Korea Advanced Institute of Science and Technology, Kusung-dong, Yusung-ku, Taejon, 305-701, Korea
H.G. Jeong Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
S.H. Min Dept. of Metallurgy, Kangnung National University, Kangwon-do, Kangnung, 210-702, Korea (Received 2 May 2001; accepted 15 October 2001)
High-strain-rate superplastic behavior of powder-metallurgy processed 0%, 10%, 20%, and 30% SiC particulate reinforced 6061 Al composites was studied over a range of temperatures from 430 to 610 °C. The strength of the 6061 Al composites was lower than that of the 6061 Al matrix alloy in the temperature range where grain boundary sliding is believed to control the plastic flow. The difference in their strength was also observed to be temperature dependent, increasing with increase in temperature. Abnormally high activation energy for superplastic flow was another important feature of the 6061 Al composites. These behaviors in particle weakening and activation energy have strong resemblance to those noted in the high-strain-rate superplastic 2124 Al composites studied previously. The observed particle weakening was attributed to liquid-enhanced superplastic flow and discussed by adopting the concept of effective diffusivity considering mass flow through liquid phase formed at the solute-segregated region near SiC/Al interfaces.
I. INTRODUCTION 1
Kim and Sherby have recently reported “particleweakening” phenomenon in a superplastic 2124 Al composite when grain boundary sliding (GBS) controls the plastic flow. The 2124 Al matrix alloy and 2124 Al composite were processed by a powder-metallurgy (PM) route at elevated temperatures, and the strength of the 2124 Al composite was lower than that of 2124 Al matrix alloy even after effects of grain size and threshold stress were properly considered. This unusual behavior was dubbed as particle weakening in comparison with particle strengthening2,3 often noted in metal matrix composites at relatively low temperatures, where dislocation climb creep is the rate-controlling process. The strength discrepancy between the 2124 Al matrix alloy and 2124 Al composite was seen to decrease with decrease in temperature and virtually vanish at 460 °C. It was also observed that the activation energies for superplastic flow
a)
II. EXPERIMENTAL PROCEDURES
Materials were fabricated using a PM route at elevated temperatures. The chemical composition of 6061 Al powders was 0.35% Cu–0.88% Mg–0.01% Mn–0.57%
e-mail: [email protected] J. Mater. Res., Vol. 17, No. 1, Jan 2002
http://journals.cambridge.org
of the 2124 Al composite increased with increase in volume fraction of reinforcement particle and decreased with increase in particle size. The authors claimed that the liquid phase formed at interface between reinforcement particles and Al matrix is responsible for the particle weakening.1 In this study, deformation behavior of 6061 Al alloy without and with SiC reinforcement has been examined to investig
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