• • A 100 MPa increase in tensile strength (from precipitate content variation) produced no measurable gain in fatigue strength, indicating that alloy designers cannot assume fatigue performance scales with tensile metrics in 7xxx systems.
• • The dual-edged strengthening mechanism—enhanced plastic deformation resistance versus strain localization—creates a plateau in fatigue strength for intermediate yield strengths (approximately 500–600 MPa), as evidenced by the first-increasing-then-declining trend.
• • Precipitate density was isolated as the sole strengthening variable while maintaining consistent microstructure and damage mechanisms, eliminating confounding factors such as grain size, defects, and dislocation slip modes that previously obscured intrinsic strength–fatigue correlations.
• • The established yield–fatigue strength relationship provides a predictive tool for alloy development, showing that fatigue strength peaks at an optimal yield strength before declining, with direct implications for aerospace and transportation component design where fatigue life is critical.