Nanoprecipitates and Their Strengthening Behavior in Al-Mg-Si Alloy During the Aging Process
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THE precipitation strengthening 6xxx series Al-Mg-Si alloys are generally used as candidate materials for automobile weight reduction, due to their high strength-to-weight ratios. Solute atom clustering and nanophase precipitation are the key processes in the strengthening of the 6xxx series aluminum alloys.[1–17] A detailed understanding of the dependence of these processes on aging treatment and their influences on strengthening is necessary. By the increasing use of atom probe tomography (APT) in many recent characterization studies, the growth kinetics,[3,4,10] compositions,[3,4,6,11–13] morphology,[3,4,6,10,13] and number densities[3,4,10,13] of the various precipitates formed in aluminum alloys have been widely investigated at the atomic scale. The precipitation sequence of different precipitates during the aging process is generally accepted as supersaturate solid solution (SSSS), solute atom clusters, Guinier–Preston (GP) zones, metastable b¢¢ precipitates, metastable b¢ precipitates, and then stable b precipitates.[5,7,13] The different types of precipitates have different strengthening effects. Moreover, the precipitation sequence evolution during artificial aging (AA) with and without pre-natural aging (NA) is different. Solute atom clusters and GP zones generally precipitate after NA treatment or at the early stage of AA treatment and have significant strengthening effects on the alloys. It is hard to observe them by transmission HUI LI and WENQING LIU are with the Key Laboratory for Microstructure, Shanghai University, Shanghai, 200444, P.R. China. Contact e-mail: [email protected] Manuscript submitted March 27, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
electron microscopy (TEM), due to their coherent orientation relationship with the matrix and the weak scattering contrast between Mg, Si, and the Al matrix.[2,5–7] The recent emergence of powerful APT has made it possible to visualize the nanoscale precipitates in aluminum alloys.[3–5,9–13] The sizes of solute atom clusters are about 1 to 2 nm, while the sizes of GP zones are somewhat larger than those of the solute atom clusters. The Mg/Si ratios of solute atom clusters and GP zones vary between 1 and 2 in different samples by different characterization methods.[3–7,9–13] Furthermore, the solute atom clusters and GP zones generally happen at room temperature, which is called NA. Natural aging cannot be avoided during practical processing and may have an adverse effect on the age hardening of the subsequent AA treatment.[4,8,9] Solute atom clusters and GP zones may dissolve into matrix or transform into metastable b¢¢ precipitates during prolonged aging at proper aging temperature. The b¢¢ precipitates with needle shape and 20 to 30 nm in length can obviously be observed in TEM. Thus, the crystal structure of b¢¢ precipitates is well studied in the literature, e.g., References 5,7, and 14 through 17. The composition of b¢¢ precipitates is determined as about Mg5Si6 during TEM examination.[5,7,14–17] However, APT results in different reports[5,7,13–17] give
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