Precipitation processes in Al-Cu-Mg alloys microalloyed with Si
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I. INTRODUCTION
ALLOYS of the Al-Cu-Mg system form an important part of the 2xxx series alloys and are widely used in structural aerospace applications. Moreover, this system provides the basis for the development of many other important Al alloys. An important example includes Si-modified alloys having compositions that lie in the (a 1 S) region of the experimentally derived Al-Cu-Mg phase diagram proposed by Brook,[1] which have application in skin sections and rivet components of supersonic jet aircraft. Alloy 2618 is based on Al-3Cu3Mg-0.5Si (wt pct) and was originally developed for the Concorde superstructure. Although additions of Si are associated with improved tensile and creep strengths, details of the precise mechanism by which Si enhances hardening and refines microstructure remain unclear. Renewed interest in this system was stimulated by studies of the rapid hardening that occurs in base Al-Cu-Mg alloys[2–6] and the observation that this effect is enhanced in Si-bearing alloys.[2,7] The precipitation sequence observed during elevated temperature aging of Al-Cu-Mg alloys in the (a 1 S) region depends on the Cu/Mg ratio. For a Cu/Mg weight ratio of 2.2, Bagaryatsky[8,9] reported the following precipitation sequence. Supersaturated solid solution → GPB zones → S9 → S8 → S. The Guinier-Preston (GP) zone structures in these alloys were designated as Guinier-Preston-Bagaratsky (GPB) zones
C.R. HUTCHINSON, formerly student at Monash University, is postgraduate student, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903, USA. S.P. RINGER, formerly Associate Professor of the Department of Materials Engineering at Monash University, is now Director of the Electron Microscope Unit at the University of Sydney, NSW, 2006, Australia. Manuscript submitted January 17, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
by Silcock,[10] who showed that they possess a rod-shaped morphology extended along the cube directions of the a matrix. Recent three-dimensional atom probe (3DAP) work by Reich et al.[6] indicates that they are composed of Cu and Mg atoms and that their formation is preceded by the formation of more diffuse Cu-Mg coclusters. Despite reports of the S9 phase by Bagaryatsky,[8,9] Cuisat et al.,[11] Zahra et al.,[12,13] and Ratchev et al.,[5] it is noteworthy that Silcock,[10] Wilson and Partridge,[7] Jena et al.,[14] and Ringer et al.[3,4] were unable to confirm the presence of the S9 phase. The S9 and S8 phases are proposed to be precursors to the equilibrium S phase (Al2CuMg, Cmcm, a 5 0.400 nm, b 5 0.923 nm, and c 5 0.714 nm).[15] There is now thought to be little difference between the S8 and S phases,[3,6,16] which occur as lath-shaped precipitates on {210}a planes elongated along ^100&a. High resolution transmission electron microscopy (HRTEM)[17] and microbeam electron diffraction (MBED)[18] have revealed that the habit plane of the lathshaped S-phase precipitates is (001)s, oriented such that (100)s//{100}a and [010]s//^012&a. It is noteworthy that sli
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