Precipitation in an extruded AA7003 aluminium alloy: Observations of 6xxx-type hardening phases

Abstract

Precipitation behavior in an industrially extruded AA7003 alloy has been studied using Transmission Electron Microscopy (TEM) together with Differential Scanning Calorimetry (DSC). Air Cooling (AC) after solution heat treatment results in quench induced heterogeneous precipitation of both β-Mg2Si and η-MgZn2 phases. Detailed TEM characterisation of resulting nanoscale precipitates after AC, or Water Quenching (WQ), and subsequent artificial ageing demonstrate that η′ and η2 hardening precipitates dominate in T6, whereas the overaged T7 state contains η2 and η1, where the latter accounts for approximately 50% of the relative phase fraction. The T7 state in addition forms 6xxx-type hardening precipitates only after WQ. Results presented here are expected to be relevant for any Si containing 7xxx alloy and open new possibilities for development of hybrid 6xxx- and 7xxx series aluminium alloys. This is discussed with respect to potential influence on mechanical- and corrosion properties. Keywords: Aluminium alloys, Scanning transmission electron microscopy, Heterogeneous nucleation, Differential scanning calorimetry, η-MgZn2, Precipitation