Effect of quench- and age-induced grain boundary η-phase precipitates on hydrogen environmentally induced cracking (H-EIC) behaviour of AA7085 alloy in humid air
摘要
The influence of η-phase quench-induced (Q-GBPs) and age-induced (A-GBPs) grain boundary precipitates on hydrogen environment-induced cracking (H-EIC) in AA7085 high-strength Al–Zn–Mg–Cu aluminium alloy was investigated. GBP composition, size, density, and area fraction were systematically varied by adjusting the quench rate (6–200 °C s−1) after solution treatment. High cooling rates (200 °C s−1) suppressed Q-GBP nucleation, producing uniform A-GBP coverage after T76 ageing. Low cooling rates (6 °C s−1) generated coarse, widely spaced Q-GBPs with higher Cu content and solute-denuded zones, with A-GBPs forming in the intervening regions. Medium cooling rates (40 °C s−1) produced numerous finer Q-GBPs, reducing the overall A-GBP fraction. The effect of these distributions on H-EIC stages: crack initiation, short crack growth, and long crack growth, was deconvoluted using in situ optical monitoring of four-point bend tests in humid air (50% relative humidity, 70 °C). High cooling rates (200 °C s−1) led to rapid initiation and a tenfold increase in crack growth rates compared to low cooling (6 °C s−1). At 40 °C s−1, initiation time (40 h) was similar to the low cooling condition, but the crack growth rate increased fivefold, remaining below that of the high cooling rate. These findings highlight the highly reactive nature of age-induced grain boundary precipitates markedly increased the susceptibility to H-EIC, whereas coarse, dendritic Q-GBPs with precipitate-free regions and reduced A-GBP coverage mitigate it. Overall, the results underscore that unambiguous identification of the grain boundary precipitate origin is crucial for accurately assessing the EIC performance of these alloys.