On the decomposition behaviour of Al-Zn(4.5 at.%)-Mg(2;3 at.%) alloys during continuous cooling from the temperature range of homogeneity

The decomposition processes taking place in the Al-Zn (4.5)-Mg(2;3) alloys were studied by means of XSAS and resistivity measurements and by TEM investigations after continuous slow cooling to certain temperatures T_i and during an isothermal ageing at T_i following the continuous cooling procedure. During the cooling between 380 °C and 320 °C mostly particles of the η-phase are growing. The formation of particles homogeneously distributed in the matrix starts at 165 °C in the alloy with 2 at.% Mg and at 175 °C in the one with 3 at.% Mg. Below these temperatures new subcritical GP zones are formed and particles already present in the matrix are growing. The dominating process depends on T_i.