Enhanced fatigue crack propagation performance in a high Mg-containing Al–Mg–Zn–Si alloy via two-step aging: Role of precipitation characteristics in the matrix and grain boundary

Fatigue crack propagation (FCP) performance of a high Mg-containing Al-Mg-Zn-Si alloy is critical for further application. Here we detailedly investigate FCP behavior and fracture morphology of the alloy under selected single and double stage aging regimes with a same strength level of 550MPa and compares their precipitation characteristics including the matrix and grain boundary. The results shows that the alloy with a double stage aging regime of 90°C/24h+120°C/20h has lower FCP rates than the alloy treated by a single stage aging regime of 120°C/24h. SADPs and HREM observations jointly proves all spherical and rod-like phases in the matrix and rod-like phase in precipitate free zone (PFZ) along grain boundary for the two aging regimes are predominantly T' phase. The former one possesses spherical matrix precipitates with smaller size and obviously larger quantity and narrower PFZ width containing scattered rod-like phase except the close size of grain boundary precipitates. A theoretical model which directly correlated FCP rate with MPs (size, volume fraction and number density), PFZ width and grain size was proposed the estimations are preferably consistent with the experimental results for stable expansion zone.