Enhanced fatigue crack propagation resistance in a superhigh strength Al–Zn–Mg–Cu alloy by modifying RRA treatment

• An innovative RRA treatment was designed to enhance fatigue crack resistance.
• An almost complete transgranular crack propagation was found in Paris regime.
• The coarse precipitates in the grain interior determined the transgranular crack path.

A new retrogression and re-aging (RRA) treatment was designed to enhance the fatigue crack propagation (FCP) resistance of a superhigh strength Al–Zn–Mg–Cu alloy in this work. As compared to traditional RRA treatments, a lower retrogression temperature and a longer retrogression and re-aging time were employed. This modified RRA processing obtained a coarse precipitates in matrix, and a slightly increased precipitate free zone (PFZ) width at grain boundary. This led to an almost complete transgranular crack propagation, rather than a partial intergranular crack propagation as traditional RRA tempered samples. This kind of fatigue crack propagation manner gave rise to an ultralow FCP rate, comparable to fatigue resistant 2000 series aluminum alloy, in an Al–Zn–Mg–Cu alloy while keeping superhigh tension strength.