The influence of partially recrystallized grain structures on hydrogen-environmentally induced cracking (H-EIC) behavior of AA7085 alloy in humid air

The effect of partially recrystallized grain structures on the Hydrogen (H) - Environmentally Induced Cracking (H-EIC) behaviour of AA7085 alloy has been systematically investigated in thick plates, re-processed by hot rolling within the range of 300–470 °C to generate different levels of recrystallization. In-situ optical monitoring was employed with four-point bend tests, performed in warm-humid air (50 % RH & 70 °C), to study the H-EIC crack growth rates with high temporal resolution. Increasing the fraction of recrystallized grains from 2 % to 20 % led to an increase in crack path tortuosity, which reduced the overall crack propagation velocity. However, at 40 % recrystallization, crack growth rates increased. These effects are attributed to influence of recrystallization on the effective crack tip stress intensity (KIeff), introducing microstructural complexity. The inverse segregation of Zr in thick 7xxx plates is shown to have an important influence on the recrystallized grains that develop during marginal recrystallization conditions, which results in a morphology that is constrained by the inter-dendritic regions inherited from the cast microstructure. At lower levels of recrystallization, isolated recrystallized grains of a very different size and shape act as obstacles to the intergranular cracks leading to severe crack deflections, greater bifurcation, and crack wake ligaments. With higher levels of recrystallization, the crack path can follow inter-connected bands of recrystallized grains which, on average results in less strong deflection events and crack-wake ligaments and consequently a lower reduction in KIeff at the crack tip and a shorter crack path through the material.