Aluminium alloys used in aerospace and automotive applications typically contain dispersoids particles that are used to control the grain structure during thermomechanical processing. Modern 7xxx (Al-Zn-Mg-Cu) alloys usually contain zirconium (Zr) for the purpose of forming dispersoids during homogenization heat treatment. These dispersoids are usually assumed to have an ideal Al3Zr stoichiometry and take two crystal forms, an L12 metastable phase (which is dominant for commercial homogenization treatments) and a D023 equilibrium phase.
We have applied high resolution electron microscopy and composition mapping to investigate the dispersoids in AA7010, a commercial 7xxx alloy. We find that the dispersoids are far from the ideal stoichiometric composition, and contain significant quantities of Zn (up to 15at%) as well as excess Cu (located mainly in the interface region). Both Zn and Cu are predicted by atomistic simulation to substitute onto the Al sublattice in the dispersoid phase, giving a composition (Al,Zn,Cu)3Zr. Furthermore, the equilibrium form of the dispersoid phase is not the expected D023 structure from the binary Al-Zr system, but fits a tI10 Ni4Mo structure type, which allows incorporation of higher levels of Zn onto the Al sublattice.
Both types of dispersoid have been observed to act as heterogenous nucleation sites for the major precipitate phase in AA7xxx (-MgZn2) and an unidentified Cu rich (but Mg free) phase in the case of the larger dispersoids. Calculations of the amount of age hardening solute removed into the dispersoids reveal that the direct effect on ageing potential is small, but the effect on quench sensitivity could be significant.