Origin of the influence of Cu or Ag micro-additions on the age hardening behavior of ultrafine-grained Al-Mg-Si alloys

The age hardening behavior of ultrafine-grained Al-Mg-Si alloys with micro-additions of Cu and Ag has been investigated with a special emphasis on the relationship between microstructural features and hardness evolutions. Using transmission electron microscopy, it is shown that the higher hardness and thermal stability induced by a small amount of Cu is due to clustering during grain refinement by severe plastic deformation. It is also demonstrated that the addition of Ag is beneficial for the thermal stability of the ultrafine grain structure as it segregates along grain boundaries and significantly reduces their mobility.