The effect of strontium on the Mg 2 Si precipitation process in 6201 aluminum alloy
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I.
INTRODUCTION
Alloy 6201, a member of the heat treatable 6000 series Al alloys, is an electrical conductor alloy used widely in the transmission and distribution of electricity due to its good mechanical properties coupled with a high electrical conductivity. The strengthening of 6000 series alloys is obtained by precipitation of the Mg2Si phase during elevated temperature aging. The microstructural changes that occur during artificial aging of 6000 series alloys have been well studied.[1–6] The process starts with the formation of Guiner–Preston (GP) zones of Si and Mg2Si phases from the supersaturated solid solution. These later transform to coherent needlelike GP zones of monoclinic Mg2Si (b"). Further aging treatment yields the formation of a semicoherent, hexagonal Mg2Si rod-shape (b') phase. Finally, noncoherent equilibrium cubic Mg2Si plates (b) form from the b' particles. Additions of Sr to 6000 series alloys including 6201 have been shown to modify the AlFeSi intermetallic phases, which form during ingot solidification. In these alloys, Sr promotes the formation of more desirable Chinese-Script a-AlFeSi (Al8Fe2Si) instead of platelike b-AlFeSi (Al[7] In addition to this modifying effect, there is some 5FeSi). evidence to indicate that Sr influences the Mg2Si precipitation kinetics in 6000 series Al alloys[8] as well as in AlSi-Mg foundry alloys.[9] In this investigation, the effects of Sr on the Mg2Si precipitation process in 6000 series Al alloys have been studied using commercial 6201 aluminum wires. The transmission electron microscopy (TEM) work was complemented by electrical resistivity measurements and tensile testing of the wires at different stages of the aging treatment.
M.H. MULAZIMOGLU, Senior Metallurgical Engineer, is with the Aluminum Division, American Racing Equipment Inc., Rancho Dominguez, CA 90221. A. ZALUSKA, Electron Microscopist, Department of Physics, and F. PARAY, Research Associate, and J.E. GRUZLESKI, Gerald Hatch Professor, Department of Mining and Metallurgical Engineering, are with McGill University, Montreal, PQ, Canada H3A 2A7. Manuscript submitted November 5, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
The chemical composition of the commercial alloy is listed in Table I. The alloy was cast in a commercial Properzi continuous casting machine as a pentagonal bar with a cross-sectional area of about 30-cm square. The cast bars were hot-rolled (500 7C) down to a 10-mm-diameter rod. Homogenization of the rods was carried out at 540 7C for 4 hours, followed by water quenching. The quenched rods were then cold drawn to 2.8 mm in diameter without intermediate annealing and naturally aged for several months. Finally, the wires were artificially aged at 175 7C for up to 48 hours. Microstructural examination of the alloy was performed by using TEM. Thin foils from wires were prepared by jet polishing using an electrolyte consisting of 20 pct perchloric acid and 80 pct methanol at 225 7C and a current of 1 A at 10 V. These thin foils were examine
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