In-Situ Dissolution Behavior of Secondary Alloy Phases in Al–Zn–Mg–Cu-Base AA7055 Aluminum Alloy

The present study reports the dissolution behavior of the secondary alloy phases of an Al–8.3Zn–2Mg–2.4Cu–0.15Zr alloy (AA7055) during in-situ heating within a scanning electron microscope operating under highly evacuated condition. It has been shown that the dissolution of the major secondary alloy phases (i.e., η (MgZn2)-base and T (Al2Mg3Zn3)-base phases present in the inter-dendritic channels) is initiated at a substantially low temperature (~300 °C) under SEM operating condition. It is to be contrasted with the fact that the typical homogenization temperature of 7xxx series Al alloys is always greater than 450 °C under atmospheric pressure. Characterization of morphological changes during the in-situ heating to different predetermined temperatures reveals that the dissolution process occurs by the thinning, discontinuation,, and spheroidization (TDS) mechanism which encompasses spheroidization and thinning processes. In addition, the dissolution kinetics of the fine eutectic phases have been found to be significantly accelerated compared to that under atmospheric pressure. The results are further augmented by conventional ex-situ homogenization experiments. The compositional variations of secondary phases have been investigated as a function of time and temperature. The result shows that elemental compositions of Fe-bearing phases vary marginally, whereas that of Zn, Cu-rich phases move from non-equilibrium to equilibrium values.