Measurement and Theoretical Calculation Confirm the Improvement of T7651 Aging State Influenced Precipitation Characteristics on Fatigue Crack Propagation Resistance in an Al–Zn–Mg–Cu Alloy

Precipitation characteristics influencing fatigue crack propagation contained matrix precipitate, grain boundary precipitate and precipitate free zone for Al–Zn–Mg–Cu alloys. Over-aging treatment could effectively regulate precipitation and then to be able to change fatigue crack propagation behavior compared with the peak aging state. In the current work, typical T651 and T7651 aging tempers of the alloy were extracted via hardness, electrical conductivity and mechanical properties under one-step and two-step aging treatments. Fatigue crack propagation (FCP) rate under them was tested and corresponding precipitation characteristics and fracture morphology were observed. The results indicated that fatigue crack propagation resistance for the T7651 temper possessed an obvious improvement on the side of that for the T651 temper, which was also supported by fracture appearance, including tearing ridge, tearing dimple and fatigue striation. The precipitation observation showed that the T651 alloy contained GPI zone, GPII zone and ηʹ phase while the T7651 alloy possessed ηʹ phase and η phase. Compared with the T651 temper, matrix precipitate for the T7651 temper distinctly owed an expanding of size distribution and an enlargement of average size while cuttable phase still remained the dominance for both tempers. Grain boundary precipitate and precipitate free zone manifested no obvious difference between the two aging tempers. Cut and bypass mechanisms of dislocation–precipitate interactions were used for explanation and it revealed the reinforced cuttable phase was in favor of enhancing fatigue crack propagation resistance. A theoretical model which directly correlated FCP rate with matrix precipitate characteristics was employed to calculate FCP rate by substituting quantitative precipitate characteristics and the calculation results were vaguely consistent with the experimental measurement, which proved its reliability and feasibility.