Precipitation and dissolution processes in age-hardenable Al Alloys — A comparison of positron annihilation and X-ray small-angle scattering investigations. II. AlZn(4.5 at%)–Mg(x) (x = 0, 0.2, 1, and 2.5 at%)

The precipitation and dissolution processes going on during continuous heating (heating rate of 1 K/min) in four AlZn(4.5)–Mg(x) alloys are studied by means of XSAS, TEM, and PAS investigations. The XSAS and TEM investigations give strong evidence that only GP zones as well as η′-particles of overcritical size are transformed directly into the more stable phases during the heating procedure. GP zones and η′-particles of subcritical size are dissolved attaining their dissolution temperature and on the expense of this material the more stable phases are formed. The PAS results are very strongly influenced by the Mg additions which can be understood by the formation of VZnMg complexes. Most probably in case of the presence of zones the positrons are mainly trapped by monovacancies bound to Mg atoms inside the Zn-rich GP zones. Due to this special trapping effect the PAS is not in a direct and simple way related to the various precipitation processes going on in the AlZnMg alloy as known from the AlZn alloys. The formation of partially coherent and incoherent precipitates causes an increase of the h-parameter which can be understood by localization of positrons at parts of the incoherent particle-matrix interfaces of high-angle boundary type.