Improving mechanical properties of an Al-Zn-Mg-Cu alloy during high stress creep aging via microalloying with Yb

Abstract

The performance of traditional Al-Zn-Mg-Cu alloys degrades rapidly under high stress, resulting in insufficient creep aging time and inadequate creep strain. A combination of electron and atomic microscopy characterization, along with first-principles calculations, was used to investigate the effect of Yb on the creep aging behavior and mechanical properties of Al-Zn-Mg-Cu alloys under high stress. The mechanism by which Yb extends the creep aging window was elucidated. First-principles calculations revealed that Yb tends to replace Al2 atoms in the η′ phase and Mg atoms in the η phase, raising the formation energies of η′ and η phases and inhibiting the coarsening of the precipitates. A trace addition of 0.3 wt% Yb induces a new primary phase of Al₃(Yb, Zr), and further refine the grain size to 30.48 ± 2.3 μm. Under high-stress creep aging, the peak strength of 0.3 Yb alloy was achieved at 4 h, with a yield strength of 667.7 MPa. The yield strength of 0.3 Yb alloy remained stable, maintaining 628.2 MPa even after aging time of 8 h. The addition of Yb extended the high-stress peak aging time from 2 h to 4 h, prolonging the peak aging time. The 0.3 Yb alloy exhibited a "2–8 h aging strengthening peak region," demonstrating better performance stability, reduced fluctuation, and a yield strength above 600 MPa. This contrasts with the Yb-free alloy, which only showed a transient peak aging region (2 h). The addition of Yb effectively increased the formation energies of the η′ and η phases, suppressed the coarsening of the η′ to η phase, and extended the process window for the synergy of creep strain and aging precipitation.