Unraveling the leverage effect in phases stability and mechanical properties of Al-Zn-Mg-Cu alloy

Abstract

In order to further improve the comprehensive performance of advanced aluminum (Al) alloys, especially 7xxx Al-Zn-Mg-Cu alloys, tuning the relative ratios between major alloying elements (Zn, Mg, Cu) while maintaining the total alloying amount has been widely adopted. However, the specific influence of these relative element ratios on various properties has yet to be well understood. In this work, the leverage effect of those alloying element pairs (Al:Cu, Mg:Zn, Cu:Mg, and Cu:Zn) on the phase stability and mechanical properties of Al-Zn-Mg-Cu alloy is systematically investigated by means of first-principles calculations, cluster expansion, and quasi-harmonic Debye model. The tradeoffs in formation energy, elastic strength, and anisotropy are unraveled when the element pairs deviate from the equal ratio (1:1), indicating a strong leverage effect in element pairs. Among those element pairs, Al:Cu and Cu:Mg are the major lever in phase stability, in which the stable phase reversed on the two sides of 1:1; while the Mg:Zn lever dominates the elastic strength and anisotropy. Cu:Zn, on the other hand, has less impact on material properties. The underlying mechanism of those leverage effect is analyzed by the bonding electron density and lattice distortion around the major alloying elements. Our results are in good agreement with experimental observations and clarify the controversaries of those element pairs on the material properties. The weight of each lever could provide a practical guidance to the optimization of the alloy composition of 7xxx Al-Zn-Mg-Cu alloys, and also shed lights on the development of other advanced alloy systems.