Diffusion couple specimen has been used since several years . These specimens are used to quantify the effect of alloying elements on mechanical properties (hardness) during ageing. In aluminium alloy, the strengthening of the alloy is mostly due to the precipitation of a second phase at a nanoscale2. SAXS characterization can be used to quantify the volume fraction and radius of precipitate.

In 7xxx aluminium alloy, Mg(Zn,Cu) precipitate is used as strengthening phase. For now, the first gradient of content that we made is a gradient of zinc, at iso magnesium and copper. We achieved to create a gradient of zinc from 2.8 to 4.5at% on 4mm. Then specimens has been aged and characterized at the ESRF.

We saw that zinc content do not impact radius size during germination and growth. But zinc content modify the coarsening kinetic of the alloy. Indeed, lower zinc leads to reduce the size of the precipitate after coarsening.

Higher zinc increases the volume fraction so it will increase the mechanical properties until peak aged. But the difference in coarsening rate leads to a higher mechanical property in overaged temper for low zinc. In our case, low zinc goes from a TYS of 530MPa in peak age to a TYS of 510MPa, beside high zinc goes from 580 to 480MPa.

Here the diffusion couple permits to quantify the effect of zinc/copper ratio on coarsening rate and mechanical properties.