Al-4.5Si-1Cu-0.3Mg(-1Fe) (wt%) alloys fabricated by a deformation-semisolid extrusion (D-SSE) process have been investigated by transmission electron microscopy, down to the atomic level. T5 and T6 heat treatments were conducted to understand the age-hardening behavior of the alloys. Disordered Mg-Si(-Cu) precipitates with strong Cu enrichments at their interfaces with the Al matrix have been observed in the overaged conditions of both heat treatments and in the peak hardness of the T6 condition, but only Cu-containing atomic clusters were detected in the peak hardness of the T5 heat treatment. Despite having a lower bulk precipitate number density at comparable precipitate size and volume fraction, hardness in the T6 condition was higher in the alloy with highest Fe content due to the extra contribution from the precipitates nucleated on fragmented β-Al5FeSi particles and grain boundaries. Many of these precipitates were Q'-phase, and two new coherent interfaces with the Al matrix are reported for this phase. •Fragmented β-Al5FeSi particles and grain boundaries act as nucleation sites for the Q' phase.•Two new types of interfaces along the coherent Al direction of the Q' phases have been found.•Cu-containing atomic clusters have been found in the peak hardness of an artificially aged T5 condition.