Influence of the combined additions Mn, Cr or Ni on the formation of dispersoids and mechanical properties of Al-Mg-Si-Cu alloys

The effect of individual and combined additions of different transition metals (TMs), Mn, Cr, and Ni, on the precipitation of dispersoids and the final mechanical properties of Al-Mg-Si-Cu alloys was systematically studied by scanning electron microscopy, transmission electron microscopy and tensile testing. The various TM additions examined are found to induce notably different dispersoid precipitation behaviors. The combined addition of Mn, Cr produces the highest number density of dispersoids among the three TM -added alloys examined, due to the incorporation of Cr promoting the precipitation of the alpha-Al(Mn, Cr, Fe)Si dispersoid. By contrast, the combined addition of Mn, Ni slightly suppresses the precipitation of the alpha-Al(Mn, Ni, Fe)Si dispersoid compared with individual Mn addition. The presence of a fine dispersoid microstructure can moderately increase the strength, and strongly increase the elongation, especially for the case of the combined Mn, Cr added alloy, where the elongation is increased almost threefold compared with the base alloy. An excellent strength -elongation combination, with an ultimate tensile strength of 421.9 +/- 2.2 MPa and an elongation of 18.2 +/- 3.6, is achieved for this alloy. This property is superior to most reported 6xxx alloys. The main reasons for this result are the presence of a high number density of dispersoids promoting grain refinement, providing high Orowan hardening, and reducing stress concentration on the grain boundary. The findings of this work are expected to provide guidance for developing new Al-Mg-Si-Cu alloys with excellent comprehensive mechanical properties.