The effect of Cu additions on the precipitation kinetics in an Al-Mg-Si alloy with excess Si
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THE 6000-series wrought Al-Mg-Si(-Cu) alloys are widely used as medium-strength structural alloys. In the continuing drive for automobile weight reduction, Al-MgSi(-Cu) alloys are considered to be the most promising candidates for heat-treatable body sheet materials. Numerous investigations on the precipitation process,[1,2,3] the process of two-step aging,[4] and the effect of alloy composition on the age-hardening response[5,6] have been carried out. Since the alloys for body sheet applications must be age hardened during the paint-baking process, substantial age hardening must occur during artificial aging for less than 30 minutes at about 175 ⬚C. It has been reported that the Al-Mg-Si alloys containing excess amounts of Si, with respect to the Al-Mg2Si quasibinary composition, exhibit a faster agehardening response.[5] A recent atom-probe study by the present authors[7] revealed that the chemical compositions of the Guinier–Preston (GP) zones and the  ⬙ precipitate change with the alloy composition, and that the number density of the GP zones in the paint-bake condition increases as the number of Si atoms available to form Mg-Si coclusters increases. There are several reports on the effect of the quaternaryelement addition on age hardening of Al-Mg-Si alloys.[1,8–10] Cu additions generally increase the kinetics of precipitation during artificial aging, as well as reduce the deterioration of the age-hardening response arising from natural aging of the Al-Mg-Si alloy. Collins[8] reported that the presence of Cu causes a small reduction in the solubility of Mg2Si, which increases the supersaturation of the Mg2Si phase for a given alloy composition. Chaterjee and Entwistle[9] reported that Cu reduces the rate of migration of Mg and Si atoms, which M. MURAYAMA, Researcher, and K. HONO, Head of 3rd Laboratory, are with the Materials Physics Division, National Research Institute for Metals, Tsukuba 305-0047, Japan. W.F. MIAO, formerly Postdoctoral Fellow with the Department of Materials Science and Engineering, Carnegie Mellon University, is with Nanomat, Inc., North Huntingdon, PA 15642. D.E. LAUGHLIN, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890. Manuscript submitted September 8, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
retards the formation of Mg-Si clusters at room temperature. They proposed that the smaller clusters are reverted at the temperature for artificial aging. This, in turn, increases the supersaturation of Mg2Si, resulting in a larger driving force for precipitation at the temperature of artificial aging. Suzuki et al.[11] also reported that Cu additions increase the initial rate of hardening and the peak hardness. This was attributed to the increase in the number density of the needle-shaped precipitates with increasing Cu content. More recently, Laughlin et al.[10] reported that the Cu level has a large effect on the age-hardening kinetics in the underaged regime and a smaller but noticeable effect on the value
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