Development of Particle and Grain Uniformity in High-Ductility Al-Zn-Mg Alloys with the Addition of Mg and Cu

The ductility of Al-Zn-Mg-(Cu) alloys was investigated by the microstructure uniformity following the addition of Cu and Mg elements. In the case of Cu-added alloy, additional particles, containing Cu elements, were formed with coarse and irregular sizes. These particles were tens of micrometers long existed along the grain boundaries and were retained even after homogenization. Such a nonuniform particle size distribution could lead to inhomogeneous grain structures because of irregular particle-stimulated nucleation. However, Mg-added alloy has fine and spherical particles with uniform size distribution. While on the deformation, strain energy, such as dislocation, had been stored along the grain boundaries, hence the nonuniform grain-size distribution inhibited uniform deformation during tensile deformation. Furthermore, lower plasticity has occurred from the microvoid including Cu-containing particles, which can induce the unexpected crack initiation. To achieve the high ductility of Al alloy, homogeneously distributed grains and grain boundaries should have existed to improve uniform deformation by the addition of an Mg solute and low Cu content.