Vacancy Behavior and Solute Cluster Growth During Natural Aging of an Al-Mg-Si Alloy
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INTRODUCTION
SOLUTE clustering and precipitation are key processes in the strengthening of aluminum alloys. A detailed understanding of the dependence of these processes on alloy composition and temperature is necessary for rational approaches to the design of agehardenable alloys. While precipitation processes were studied for some time and transmission electron microscopy is a common tool for such analysis, observations of the solute clustering processes that precede precipitation are a little more challenging and higher resolution characterization tools are required. The increasingly widespread availability of the atom probe (AP) technique has led to many recent characterization studies on the growth kinetics,[1–3] composition,[2–5] morphology,[1,4] and number densities[1,6] of solute clusters formed in Al-based alloys. These studies provide atomic resolution information on the distribution of the atoms in the alloy. Nuclear magnetic resonance (NMR) studies of aluminum alloys demonstrated the utility of that technique to quantify the fraction of solute atoms in solid solution and in equilibrium and nonequilibrium configurations including clusters.[7–13] However, neither AP nor NMR is capable of providing information about the behavior of atomic vacancies during clustering, which is unfortunate as these point defects play a key role in the diffusion of solute atoms. Instead, a technique, which is sensitive to vacancy defects, such as M.D.H. LAY, OCE Research Fellow, and A.J. HILL, Division Chief, are with CSIRO Process Science and Engineering, Clayton South VIC 3168, Australia. H.S. ZUROB, Associate Professor, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, L8S 4L7, Canada. C.R. HUTCHINSON, Associate Professor and ARC Future Fellow, is with the Department of Materials Engineering, Monash University, Clayton VIC 3800, Australia. Contact e-mail: [email protected] T.J. BASTOW, Research Scientist, is with CSIRO Materials Science and Engineering, Clayton South VIC 3168, Australia. Manuscript submitted January 12, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
positron annihilation lifetime spectroscopy (PALS), must be used. Previous PALS studies on Al-Mg-Si alloys[14,15] revealed that there are up to five stages in the change of positron lifetime during natural aging after solution heat treatment and quenching. Studies of aged Al-Cu-Mg based alloys have correlated changes in the average positron lifetime, sav, with precipitation and hardness.[15–19] These past studies demonstrate the usefulness of PALS for contributing information about the role of vacancies in the microstructural changes occurring during aging. Solute clustering in Al alloys can often occur at room temperature (natural aging (NA)) and in some commercial alloys, such as the 6xxx series alloys (Al-Mg-Si), can significantly influence the subsequent age hardening response.[20] NA can affect subsequent artificial aging in either a beneficial[21,22] or detrimental manner[23–25] depending on the agin
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