Atomic-scale investigation of the heterogeneous precipitation in the E (Al18Mg3Cr2) dispersoid of 7075 aluminum alloy

Abstract

The heterogeneous precipitation of the η (MgZn2) phase on the E (Al18Mg3Cr2) dispersoids of the 7075 aluminum alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectrometry (EDX). It is found that coarse η particles are heterogeneously precipitated at the E particle interface after water quenching and isothermal aging at 120 °C. The incoherent E/Al interface is responsible for the high tendency of heterogeneous precipitation of the η phase. Two different orientation relationships (ORs) between the η, E and Al matrix are identified: OR1 [21¯1¯0]η//[011]E//[1¯12]Al, (011¯0)η//(133¯)E//(201)Al, OR2 [1¯12]E//[0001]η//[011]Al, (011¯0)η//(220)E//(344¯)Al. The η phase is preferential to nucleate along the {111}E or the {220}E planes, depending on its OR. The heterogeneous nucleation of η phase on the E particle could stabilize the E/Al interface by introducing a coherent E/η interface, which increases the drive force of heterogeneous precipitation. The reorientation of η phase and mutual diffusion of solute atoms could assist the coherency of the E/η interface. The present results suggest that increasing the coherency of the E/Al interface is a promising method to suppress the heterogeneous precipitation of the η phase.