A multiscale investigation of hardening behavior in dispersoid-modified AlZnMg alloys

While dispersoid-modified Al–Zn–Mg alloys have improved thermal stability compared to their unmodified variants, they generally exhibit a reduced age-hardening potential. In the current work, Al–Zn–Mg alloys with Hf and Zr additions below 1 wt% were systematically studied with respect to the influence of the induced Hf–Zr-rich Al3X dispersoids on the Mg–Zn precipitation hardening response. A multiscale analysis was applied using correlative instrumented indentation, electron microscopy and atom probe tomography to derive the microstructure-property relationships in these alloys, with a focus on the precipitation behavior during the aging process. The results are compared to a reference dispersoid-free Al–Zn–Mg alloy subjected to the same aging treatment. A heterogeneous microstructure consisting of dispersoid-rich dendritic regions surrounded by dispersoid-free interdendritic regions was identified, with coarser Mg–Zn precipitation in the former. Via indentation mapping, we show that these local composition gradients correlate with spatial fluctuations in hardness. Related quantitative analysis indicates that the observed reduced macroscopic hardening potential during a 140 °C aging treatment of the dispersoid-modified alloys results from the coarser precipitates in the dispersoid-rich regions.