Effect of hot deformation on microstructure and quenching-induced precipitation behavior of Al-Zn-Mg-Cu alloy

The effect of hot deformation on microstructure and quenching-induced precipitation behavior of an Al-Zn-Mg-Cu alloy 7055 was investigated by electron backscattered diffraction (EBSD), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM) and scanning transmission electron microscope (STEM). In the solution heat treated and slowly-quenched alloy without hot deformation, there are equiaxed grains with low dislocation density, and there is major quenching-induced η (MgZn2) phase located at grain boundaries and on some Al3Zr dispersoids in the interior of grains and minor T(Al2Zn3Mg3) phase on Al3Zr dispersoids inside grains. While in the solution heat treated and slowly-quenched alloy after hot rolling, there are recrystallized grains and subgrains with higher dislocation density, and there is a larger amount of quenching-induced η and T phase on incoherent Al3Zr dispersoids inside recrystallized grains and at (sub) grain boundaries; moreover, quenching-induced S (Al2CuMg) phase and Zn-Cu rich Y phase are observed in the interior of subgrains. The precipitation behavior of these quenching-induced phases has been discussed primarily based on their chemical compositions, nucleation sites and different microstructure features in the solution heat treated alloy with and without hot deformation.