Influence of zirconium and copper on the early stages of aging in Al-Zn-Mg alloys
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x series Al-Zn-Mg alloys have been widely studied, due to their excellent mechanical properties developed by age hardening[1] and their extensive use in the aircraft and automotive industries. It has been reported that two types of Guinier-Preston (GP) zones, i.e., solute-rich clusters (often termed GP(I)) and vacancy-rich solute clusters (a second type of zone, termed GP(II)) form during the decomposition of the supersaturated solid solution of AlZn-Mg alloys.[2,3,4] Both GP(I) and GP(II) have a marked effect on the formation of metastable (⬘) and stable (, MgZn2) precipitates in Al-Zn-Mg alloys.[1,2] Commercial high-strength aluminum alloys based on the Al-Zn-Mg system frequently contain copper to give enhanced strength, and zirconium may be added to control grain size and the degree of recrystallization. Both zirconium and copper are known to influence GP zone formation in Al-Zn-Mg alloys and can, therefore, have a large effect on the formation of the ⬘ transition phase and, hence, influence the mechanical properties. It is, therefore, important that the effect of zirconium and copper additions on GP zone formation in Al-Zn-Mg alloys be fully defined. Some work has already been carried out on the effect of zirconium additions,[5,6] in which it has been established that X.J. JIANG, formerly Researcher, Centre for Materials Science/Department of Physics, University of Oslo, is Researcher, R&D Materials Technology, Hydro Aluminium a.s, N-4265 Ha˚vik, Norway. B. NOBLE, Reader in Materials Analysis, is with the School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, Nottingham NG7 2RD, United Kingdom. V. HANSEN, formerly Researcher, Centre for Materials Science/Department of Physics, University of Oslo, is Professor, Department of Mechanical Engineering and Materials Science, Stavanger College, N-4004 Stavanger, Norway. J. TAFTO, Professor, is with the Centre for Materials Science/Department of Physics, University of Oslo, N-0349 Oslo, Norway. Manuscript submitted January 26, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
the presence of zirconium suppresses GP zone formation and retards ⬘ nucleation. Work has also been reported on the effect of copper on the precipitation processes in Al-ZnMg alloys.[7–16] However, the precise role played by zirconium and copper is still not fully understood. In particular, the way in which these elements affect the two different types of GP zones (GP(I) and GP(II)) has not been defined. The present work examines an Al-5.5 wt pct Zn-1.2 wt pct Mg alloy with controlled additions of zirconium and copper. A detailed differential scanning calorimetry (DSC) investigation has been carried out, supported by transmission electron microscopy (TEM) analysis, in an attempt to get a better understanding of the role of zirconium and copper on GP zone formation in Al-Zn-Mg alloys.
II. EXPERIMENTAL PROCEDURE The compositions of the alloys used in the present study are listed in Table I. Solution treatment was undertaken in an air furnace by holdin
