Composition optimization for Al-Zn-Mg-Cu alloys based on thermodynamics and first-principles calculations

Abstract Composition optimization of Al-Zn-Mg-Cu alloys were systematically investigated by considering the precipitation strengthening and solid solution strengthening, based on thermodynamics and first-principles calculations. The calculated equilibrium phase diagram of Al-Zn-Mg-Cu alloy showed that Al-xZn-yMg-zCu (x=4.8∼6.3 wt.%; y=1.1∼2.6 wt.%; z=1.6∼3.1 wt.%) alloys might contain five precipitates, including η-MgZn2, θ-Al2Cu, S-Al2CuMg, τ-Al2Mg3Zn3 and τ-Al6CuMg4. The first-principles calculated results indicated that θ-Al2Cu and S-Al2CuMg precipitates might be the dominated strengthening precipitates for the alloys. Then, the supercell models of the five solid solutions (Al67Zn2Mg2Cu, Al83Zn2Mg2Cu2, Al68ZnMg2Cu, Al64Zn3Mg4Cu and Al79Zn2Mg2Cu) were established according to the contents of θ-Al2Cu and S-Al2CuMg phases in the designed compositions, and followed the structural, elastic and electronic properties of these solid solutions were calculated. The calculated results showed that Al79Zn2Mg2Cu (Al-5.8 wt.%Zn-1.6 wt.%Mg-2.1 wt.%Cu) might present the optimal mechanical properties.