Unraveling precipitation evolution and strengthening function of the Al-Zn-Mg-Cu alloys with various Zn contents: Multiple experiments and integrated internal-state-variable modeling

Abstract

• The functions of Zn content in precipitation evolution and strengthening of Al-Zn-Mg-Cu alloys were systematically investigated. • The increased Zn content would promote the development of precipitates and enhance aging hardening. • The diffusion flux of soluble Zn and the coordination of Mg atom controlled the crystallographic microstructures. • An improved internal-state-variable physical model was developed by integrating precipitation development with electrical resistivity and hardness evolutions. • The unified model considered the intrinsic characteristic of precipitates and was adaptive to heat-treatment variables and chemical compositions. Zn content is one of the most concerned factors in the development of next generation ultra-strength Al-Zn-Mg-Cu alloys owing to its essential role in precipitation strengthening. In the present work, the underlying functions of Zn content in precipitation evolution and strengthening function of Al-Zn-Mg-Cu alloys were systematically investigated by combining multiple experiments and an integrated internal-state-variable model. The experimental results indicated that the increased Zn content in Al-Zn-Mg-Cu alloys would promote the development of precipitates and enhance aging hardening. The diffusion flux of soluble Zn and the coordination of Mg atom controlled the crystallographic microstructures evolution during precipitates nucleation, growth and transition processes. By integrating precipitation development with electrical resistivity and hardness evolutions, an improved internal-state-variable physical model was then developed for the aging responses of Al-Zn-Mg-Cu alloys. The unified model considered the intrinsic characteristics of precipitates such as crystallographic orientation, morphology, component, and distribution. The specific improvements were to balance the combined functions of Zn element and Mg element and consider the plate-like morphology and directed growth as indicated by experiments. This model was also adaptive to heat-treatment variables and chemical compositions, and owned the notable advantages to simultaneously rationalize the observed microstructural characteristics, mechanical and electrical properties following artificial aging of Al-Zn-Mg-Cu alloys. In addition, a preliminary model framework between electrical resistivity and hardness for Al-Zn-Mg-Cu alloys was established.