Role of retrogression and reaging in suppressing hydrogen-induced transgranular cracking in 7xxx Al alloys

Retrogression and reaging (RRA) treatment is a distinctive approach to counteract hydrogen embrittlement in 7xxx Al alloys without compromising strength. However, the mechanistic understanding of RRA has long centered around grain-boundary-related phenomena, while the roles of grain-interior microstructures in hydrogen embrittlement have remained unclear. Here, we demonstrate the beneficial effect of RRA in reducing transgranular cracks in a specially selected high-Zn alloy that exhibits fully quasi-cleavage cracking, thereby enabling the precise decoupling of grain-interior and grain-boundary mechanisms. Using synchrotron radiation 3D observations, we identify a significant delay in the initiation of transgranular cracks in RRA-treated specimens as compared to those in the peak-aged condition. Atomic-resolution electron microscopy observations confirm the grain-interior partial transformation from the coherent η’ phase to the semi-coherent η phase. It is proposed that the increased fraction of semi-coherent η/Al interfaces reduces hydrogen coverage at coherent interfaces, thereby suppressing transgranular hydrogen-induced debonding on the slip planes.