Review—History and Future Issues in the Research and Development of Al-Zn-Mg Alloys

This review summarizes the history and future challenges in the research and development of Al-Zn-Mg alloys, with particular emphasis on the contributions made in Japan. The historical development of Extra Super Duralumin (ESD), Zr-containing Al-Zn-Mg alloys with reduced quench sensitivity, and the internationally standardized AA7003 alloy is reviewed together with the metallurgical concepts underlying their development. Previous studies on furnace cooling, quench sensitivity, stress corrosion cracking, exfoliation corrosion, two-step aging, grain boundary fracture, and thermomechanical processing are systematically reviewed. Particular attention is paid to the roles of Cr and Zr in controlling recrystallization, reducing quench sensitivity, and improving resistance to stress corrosion cracking. The exceptionally low quench sensitivity of Al-Zn-Mg alloys, which enables substantial age hardening even after furnace cooling, is discussed together with recent kinetic analyses demonstrating that hardness evolution and electrical conductivity changes are governed by different precipitation reactions. These findings suggest that conventional interpretations based solely on quenched-in vacancies are insufficient to explain the precipitation behavior of Al-Zn-Mg alloys. Future research challenges include clarifying the mechanisms responsible for age hardening after furnace cooling, precipitation kinetics from the viewpoint of chemical bonding, hydrogen-assisted stress corrosion cracking, and the roles of minor alloying elements in controlling grain boundary phenomena. This review provides historical perspectives and identifies key scientific and technological issues that should be addressed in future research and alloy development of Al-Zn-Mg alloys.