Differential scanning calorimetry and electron diffraction investigation on low-temperature aging in Al-Zn-Mg alloys
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I. INTRODUCTION
THE 7000 Al-Zn-Mg system has been widely studied due to the excellent mechanical properties developed in the alloys after age hardening.[1,2,3] A large number of investigations have been concerned with the mechanisms of zone and precipitate formation in the alloys, and models for the decomposition process have been proposed.[4–9] However, results reported in the literature are contradictory, especially those concerning the early stages of the decomposition process. It is often reported that, after solution treatment, the AlZn-Mg supersaturated solid solution decomposes in the sequence:[6,10,11]
of metastable and stable precipitate particles,[1,2,3,7,8,9] and Ryum[7] has proposed the following precipitation paths for Al-Zn-Mg alloys: SSS → h
a reaction
SSS → VRC → h8 → h
b reaction
SSS → GP(I) → h8 → h
g reaction
Recently, Stiller et al.[8,9] have carried out an atom probe and transmission electron microscopy (TEM) investigation and proposed the two following precipitation reactions for a two-step aged commercial Al-Zn-Mg-Zr alloy:
SSS → GP zone → h8 → h (MgZn2)
SSS → GP(I) → dissolves during second aging step
Recent observations indicate that the precipitation process in the alloys is more complex than that listed previously.[5,7,8,9] Decomposition of the supersaturated solid solution near room temperature takes place by the formation of GP zones (solute clusters, often termed GP(I)). Simultaneously, vacancy-rich solute clusters (VRC) are also formed, and transform to, or nucleate, a second type of zone, GP(II). Both GP(I) and GP(II) have a marked effect on the formation
SSS → VRC → GP(II) → h8 → h
X.J. JIANG, Researcher, and J. TAFTO, Professor, are with the Centre for Materials Science/Department of Physics, University of Oslo, Gaustadalleen 21, N-0349 Oslo, Norway. B. NOBLE, Reader in Materials Analysis, is with the School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom. B. HOLME, formerly Researcher with the Centre for Materials Science/Department of Physics, University of Oslo, is Research Scientist, SINTEF Materials Technology, P.B.124 Blindern, N0314 Oslo, Norway. G. WATERLOO, Researcher, is with Hydro Raufoss Automotive Research Centre, 2830 Raufoss, Norway. Manuscript submitted April 3, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
The differential scanning calorimetry (DSC) technique has been used in many investigations to characterize the solid state reactions accompanying the dissolution of precipitates, as well as the formation of additional precipitates. This technique in conjunction with TEM has been found to be an effective means of studying the precipitation processes in Al-Zn-Mg alloys.[12–25] However, there is still a great deal of uncertainty about the nature of the zones and clusters that form in Al-Zn-Mg alloys when they are aged at low temperatures. The aim of the present work is to study the nature of the zones and clusters in an Al-Zn-Mg alloy that has been aged
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