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I.

INTRODUCTION

THE A1-Cu-Mg alloys are capable of developing considerable strength by precipitation. The nature of the precipitating phase is determined by the (Cu/Mg) ratio in the alloy, l~ When the ratio is close to 0.4, the main precipitating phase is A16CuMg4. As the ratio increases to about two, S phase (A12CuMg) begins to precipitate predominantly. When the (Cu/Mg) ratio is more than eight, the main precipitating phase is 0 (A12Cu). The (Cu/Mg) ratio in the A1-1.52 pct Cu-0.75 pct Mg alloy is close to two. Strengthening of this alloy is caused by the precipitation of the metastable precursors of the equilibrium S phase in the sequence. 121 supersaturated solid solution ~ GPB zone ---->complexes --->S' ~ S Addition of silicon to such alloys improves the mechanical properties appreciably, t3'41 However, the influence of silicon additions on structure and kinetic changes have not been well established. The only effect of silicon was thought to be simply a finer distribution of the S phase. 131 However, Suzuki et al. t51 reported the formation of three phases, /3' (Mg=Si), S' (AI2CuMg), and an unknown X phase, in the presence of 0.25 pct Si and phases X, Q(AIsCu2MgsSi6), and 0' (A12Cu) in the presence of 0.5 pct Si. It was recently shown f6/ that 0.23 pct Si remains completely in solid solution in the A1-1.52 pct Cu-0.74 pct Mg alloy in the solution-treated and quenched condition. This alloy shows the same precipitating phases A.K. JENA, Professor, is with the Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur, India. A.K. GUPTA, is with the Research and Development Centre, Alcan International, Kingston, ON, Canada. M.C. CHATURVEDI, Professor, is with the Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada. Manuscript submitted May 20, 1992. METALLURGICAL TRANSACTIONS A

and the same precipitation sequence as those found in the silicon-free alloy, t21 The only change that occurs is found in the rate of precipitation in the first stage. Further increase in the silicon content of this alloy causes the formation of insoluble particles of the Q phase tTl and results in the formation of 0' and a few unidentified phases. 18'91Thus, precipitation behavior in the A1 alloys with C u / M g ratio close to 2:1 appears to change drastically with increase in the silicon content beyond 0.23 pct. Therefore, the kinetics of the first stage of precipitation in A1-1.52 pct Cu-0.75 pct Mg alloys containing various amounts of silicon have been investigated.

II.

EXPERIMENTAL

Cast alloys of high-purity aluminum were supplied by the Aluminum Company of Canada, Kinston, Ontario, Canada. The chemical compositions of the alloys are listed in Table I. Ingots of the alloys were scalped, upset forged, homogenized, and hot- and cold-rolled to the final thickness of 1.2 mm. Homogenization was carried out at 530 ~ for 48 hours. The alloys were hot-rolled at 500 ~ in several passes to obtain a net reduction in thickness of about 70 pct and cold-rolled 70 pct to the f