Effects of hydrogen trapping particles on environmentally assisted fatigue fracture of Al-Zn-Mg alloy

• Degradation in the fatigue resistance of Al-Zn-Mg has been observed under humid conditions. • Hydrogen partitioning control is applied to mitigate environmental fatigue behaviour. • The conventional η phase precipitates were partly replaced with the nanoscopic T phase particles. • The T phase contains strong hydrogen-trapping sites that absorb hydrogen without embrittlement. • Restoration of fatigue fracture resistance is achieved through T-phase dispersion. The present study investigates the environmental fatigue fracture behaviour of Al-Zn-Mg alloys. In comparison with conventional fatigue testing in laboratory air, the fatigue life in the high cycle fatigue region was significantly diminished by a high humidity environment. However, when a portion of the η -MgZn 2 , which is an ordinary precipitation phase, was replaced with T-(Al 2 Mg 3 Zn 3 or Mg 32 (Al, Zn) 49 ), the fatigue life was recovered by several tens of percent. The dispersion of intermetallic compound particles containing Mn, in addition to the T phase, resulted in a further recovery of the fatigue life. In some cases, corrosion products were observed at the crack initiation point, while in other cases, fatigue cracks initiated directly from surface without any signs of corrosion. The dispersion of T phase and Mn-bearing intermetallic compound particles was found to be particularly effective in the latter case. In order to investigate the reasons why hydrogen affects fatigue properties, 3D direct measurements were conducted of the localised plastic strain due to hydrogen, and Crack-tip opening displacement (CTOD) mapping along a crack front line, and these were used to provide insight. Furthermore, the fatigue crack propagation behaviour in each material and condition was analysed using consecutively acquired 3D images. The relationship between the previously reported hydrogen embrittlement and stress corrosion cracking behaviour of Al-Zn-Mg alloys, and a nanoscopic damage mechanism was then introduced, and the similarities with the environmentally assisted fatigue fracture behaviour in this study were discussed. Nanoscopic mechanism of environmentally assisted fatigue fracture and the principle of its suppression method were also discussed.