The Influence of Composition on the Clustering and Precipitation Behavior of Al-Mg-Si-Cu Alloys
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IN the continuing quest for automobile weight reduction, the 6xxx series of Al-Mg-Si alloys have been widely used for automotive body panels due to their high strength-to-weight ratio, good formability, and resistance to corrosion.[1–3] The heat-treated 6xxx alloys display a significant increase in hardness due to the formation of a large number of nano-sized semi-coherent metastable precipitates.[4,5] The alloy properties depend highly on the type, size, amount, and distribution of strengthening precipitates. The precipitation sequence of the Al-Mg-Si alloys is as follows[6]: SSSS ! Atomic clusters ! GP zones ! b00 ! b0 ; U1; U2; B0 ! b; Si The needle-like b¢¢ phase is semi-coherent with the matrix and responsible for the peak hardness of these alloys.[7,8] It has a monoclinic structure with a composition of Mg5Al2Si4 as determined by Hating et al.[9] The
ZHIHONG JIA, LINGFEI CAO, ROBERT SANDERS, and QING LIU, Professors, and LIPENG DING, Ph.D. Student, are with the College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. SHICHEN LI, Professor, is with the College of Materials Science and Engineering, Central South University, Changsha 410083, China. Contact e-mail: [email protected] Manuscript submitted May 19, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
rod-like b¢ phase is formed in the over-aged stage with a hexagonal cell.[10] U1, U2, and B¢, also known as Type A, Type B, and Type C, are formed in the Si-rich alloys during the over-aging.[10] Most of these post-b¢¢ phases are semi-coherent rod/lath phases which produce coarse microstructures and low strength. When Cu is added, the precipitation sequence changes to[11] SSSS ! Atomic clusters ! GP zones ! b00 ; L=S=C; QP; QC ! b0 ; Q0 ! Q; Si: This implies that the addition of Cu suppresses the formation of b¢¢ and that other Cu-containing metastable precipitates such as L, S, C, QP, and QC are formed in the peak-aged condition.[12,13] Q¢ (isostructural to the equilibrium Q phase) is mainly formed in the over-aged condition. After solution heat treatment and quenching, these alloys are normally stored for a period at room temperature (RT), where they undergo natural aging to a T4 temper before stamping and final paint-baking by the auto producer. While the strengths of 6xxx alloys increase significantly during NA, their precipitation hardening response during AA is dramatically suppressed. It is understood that clustering of solute atoms during NA retards the precipitation of b¢¢ phase during final aging. Serizawa et al.[14] reported that two types of nanoclusters, i.e., Cluster (1) and Cluster (2), were formed near RT and 373 K (100 C), respectively. Cluster (1) has a wide range of Mg/Si ratios and cannot transform to b¢¢ at higher aging temperatures, and Cluster (2) has a similar Mg/Si ratio to that of b¢¢, which enhances the subsequent
formation of b¢¢. The formation of Cluster (1) retards the precipitation of b¢¢ during AA because quenched-in vacancies and solute elements are consumed. Using the low-temperature
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