Transmission electron microscopy and differential scanning calorimetry studies on the precipitation sequence in an Al-Mg
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sso Weiland Alcoa Technical Center, Alcoa Center, Pennsylvania 15069 (Received 19 January 2005; accepted 19 June 2005)
The precipitate sequence in a 6022 aluminum alloy was investigated by means of differential scanning calorimetry (DSC), transmission electron microscopy, and Vickers hardness measurements. The solution-treated samples were quenched and then immediately subjected to DSC and isothermal aging experiments. It was observed that in the early stages of aging there are some unknown small precipitates that form prior to the formation of ⬙ precipitates. Studies on isothermally aged and DSC heated samples suggest that some of the ⬙ needles transform during growth to lath-shaped precipitates. An alternative precipitation sequence for AA6022 is proposed.
I. INTRODUCTION
AA6022 is an important member in the family of Al– Mg–Si alloys and was developed in 1995 for use as automotive sheet panel. In body panel sheet materials, two important properties need to be optimized, the formability and the strength. It has been found that the paint bake process plays a key role in optimizing these properties. In this process, the solution-treated alloy is baked at a temperature of 175 °C, and thus the precipitation sequence needs to be understood better to optimize the process. Recently Miao and Laughlin1,2 studied the precipitation sequence in 6022 aluminum alloy, and they proposed the following reaction solid solution ␣ → GP zones → ⬘→ ⬘ + lath-likeprecipitates →  + Si . Studies assisted by high-resolution transmission electron microscopy (HRTEM) and atomic probe field ion microscopy (APFIM) show that GP zones in Al–Mg–Si alloys are spherical clusters and are fully coherent with the matrix.1,3–5 The ⬘ precipitates are fine needleshaped precipitates along 〈100〉Al with a C-centered monoclinic structure.3–6 The ⬘ precipitates are reported to be rodlike precipitates along 〈100〉Al and have a hexagonal crystal structure with a ⳱ 0.705 nm and c ⳱ 0.405 nm.6 The structure and composition of the  phase a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0330 J. Mater. Res., Vol. 20, No. 10, Oct 2005
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has been established to be of the fluorite structure with a composition of Mg2Si.7 The lath-shaped precipitates are believed to be one of Q phase precursors and have been denoted as Q⬘.1,2 Both the Q and Q⬘ phases have hexagonal crystal structure with lattice parameters of a ⳱ 1.04 nm and c ⳱ 0.405 nm,8 and several chemical compositions of the phase have been reported (e.g., Al4Cu2Mg8Si79). The only difference between Q and Q⬘ appears to be the degree of coherency of the phase with the matrix.8 During body panel processing, AA6022 is isothermally aged at 175 °C for 30 min prior to subsequent deformation process. However, the lack of formability of this alloy is one of the major issues during the body panel manufacturing, especially the bending process. It is well accepted that in hardening aluminum alloys, the formability high
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