Precipitation phenomena in Al-Zn-Mg alloy matrix composites reinforced with B4C particles

To provide insight into precipitation phenomena in age-hardening Al-Zn-Mg(-Cu) matrix composites, an Al 7075 alloy composite reinforced with B4C particles was selected as a model system. The bulk composites were fabricated via plasma activated sintering and followed by a peak aged (T6) heat treatment. Two types of Al matrix zones were identified in the composite: (1) the regions in the vicinity of the matrix/reinforcement interface, defined as matrix plastic zone (MPZ) hereafter, and (2) the regions away from the matrix/ reinforcement interface, simply defined as matrix hereafter. The precipitation behavior in the MPZ was characterized and compared to that in the matrix. The MPZ contained a high density of dislocations. The number density of GP zones in the MPZ is lower than that in the matrix while the average size of the GP zones in MPZ is coarser. In addition, semi-coherent platelet eta' precipitates were observed but only in the MPZ. The dislocations and the Al/B4C interfaces provide more heterogeneous nucleation sites for the eta' precipitates in the MPZ. The growth and coarsening of the eta' precipitates caused rapid depletion of Mg and Zn solute atoms in the MPZ.