Abstract
Two types of nanoclusters, Cluster (1) and Cluster (2), formed at around room temperature and 100 °C, respectively, affect the age-hardening behavior in Al-Mg-Si alloys. Formation of Cluster (1) during natural aging (NA) is more accelerated in the high-Mg (9M10S) alloy than in the low-Mg (3M10S) alloy. Hardness at the early stage of two-step aging at 170 °C is not increased for the natural aging samples. On the other hand, hardness is directly increased for the pre-aged (PA) specimens. Furthermore, the formation of Cluster (1) during natural aging is suppressed by the formation of Cluster (2) during pre-aging at 100 °C. To understand the effects of heat treatment histories and Mg contents on the microstructure, transmission electron microscopy (TEM) was utilized. All the images were obtained at (001) plane, and peak aged samples with different heat treatments were used. Lower number density of precipitates is confirmed for the natural aging samples compared with the single-aged and pre-aged specimens. A higher number density of precipitates is confirmed for 9M10S in comparison to 3M10S. Hardness results correspond well to the TEM images.
Highlights
Al-Mg-Si alloys have been used for the body panels of automobiles due to their good age-hardening response, formability and corrosion resistance
Pre-aging is conducted at 100 ◦ C for 3600 s using an oil bath in order to understand the effects of the pre-aging on the multi-step aging behavior
The exothermal peaks are considered to be the formation of nanoclusters, with β”, β0 and β phases being indicated by peaks I, III, IV and V, respectively
Summary
Al-Mg-Si alloys have been used for the body panels of automobiles due to their good age-hardening response, formability and corrosion resistance. The precipitation sequence in Al-Mg-Si alloys is SSS → GP Zone → β” → β0 , U1, U2, B0 → β (1). Where SSS and β represent the super-saturated solid solution and an equilibrium phase, respectively [1,2]. SSS can be formed through solid solution treatment. The energetically stable β phase, compared with metastable phases such as GP Zone, β” and β0 , can be obtained with a prolonged aging treatment. Information on the chemical composition and the size distribution of nanoclusters has been reported using a three-dimensional atom probe (3DAP) in Al-Mg-Si alloys [3,4,5,6]. 1. 1.Differential curves over over the the full full range rangeofofthe theas-quenched as-quenched
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