Modeling the Age-Hardening Behavior of SiC/Al Metal Matrix Composites
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INTRODUCTION
THE age-hardening behavior of aluminum alloys has been widely studied and many models have been developed to describe this phenomenon.[1–10] However, these models only describe either the thermodynamics and kinetics for the precipitation process or the characteristics, sizes, and shapes of the precipitates on strengthening. Nothing had been taken into account to connect these two aspects until Shercliff and Ashby[11,12] made a first attempt to make a relationship between the process parameters (such as composition, aging temperature, and time) and the yield strength or hardness. The model successfully described the yield strength variations of 2000 and 6000 series aluminum alloy due to age hardening. Deschamps and Brechet[13,14] subsequently incorporated the nucleation, growth, and coarsening of the precipitates into the model. The predicted values of the model were in good agreement with the experimental results. The incorporation of SiC platelets or whiskers in aluminum matrix results in an increase in the elastic modulus and yield strength. Two explanations have been used to account for the strengthening effect of the SiC on the Al matrix. One is the shear lag model and modified shear lag model[15–19] based on the Al matrix transferring the applied stress to SiC, and the other is the quench strengthening model[20,21,22] based on the increase in dislocation density of the Al matrix due to MIN SONG, Associate Professor of Engineering, XIA LI, Master Student, and KANG H. CHEN, Professor of Engineering, are with the State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted May 31, 2006. Article published online March 27, 2007. 638—VOLUME 38A, MARCH 2007
the difference in the coefficient of thermal expansion between SiC and the Al matrix. These two models, in some way, have been successfully applied to predict the modulus, yield strength, and other physical properties, respectively. However, no model exists to describe the age-hardening behavior of SiC/Al metal matrix composites, although much work[27–32] has been performed to study the age-hardening behavior of SiC/Al composites experimentally. As a first step, this article describes an attempt to assemble a process model for the age-hardening behavior of SiC/Al composites. II.
MODEL
The model developed here includes two groups of submodels (independent of aging time and dependent of aging time) to describe the yield strength variations due to the SiC reinforcement. The first group includes a series of mechanisms enhancing the yield strength (Orowan strengthening, quench strengthening, grain strengthening, and geometric-necessary dislocations strengthening) caused by the incorporation of the SiC particles, while the second group includes a series of submodels (solution strengthening and age strengthening) describing the yield strength variations during aging. A. Strengthening Submodels Independent of Time The components of the strengt
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