Investigation of precipitation in an Al-Zn-Mg alloy after two-step ageing treatment at 100 degrees and 150 degrees C

Fine-scale precipitation of the metastable Zn- and Mg-rich eta' phase and i ts precursors is essential for the mechanical properties of Al-Zn-Mg alloys . However, at present neither the precipitation sequence nor the structure and composition of the intermediate precipitate phases are completely clear . This paper deals with an investigation of precipitation in an industrial Al-Zn-Mg alloy at various stages of a conventional two-step ageing treatmen t at 100 degrees and 150 degrees C. Studies were performed using both trans mission electron microscopy and atom-probe field ion microscopy. Transmissi on electron microscopy (TEM) analysis revealed two parallel precipitation p aths; one involving formation and dissolution of the ordered GP (I) zones, the other involving formation of clusters (type II), having a different ato mic arrangement compared to the Al-matrix, which transform to the eta' phas e. Atom-probe study of the material after short time ageing at 100 degrees C did not show any observable distinction between GP (I) and type II precip itates. In the peak-aged material the best classification of precipitates w as obtained using their morphology (the cigar-like and the plate-like) beca use there was significant overlap in the range of total solute contents of each type of precipitate. Generally the Zn:Mg ratio in all observed types o f precipitates was close to 1:1 and the total solute atom content increased with ageing time. Distribution of alloying elements in the precipitates an d in the surrounding matrix is discussed. (C) 1999 Elsevier Science S.A. Al l rights reserved.