Microstructural evolution of spray-formed Al-11.5Zn-2.0Mg-1.6Cu alloy during hot-extrusion and heat-treatment

Al-11.5Zn-2.0Mg-1.6Cu alloy was synthesized by spray forming (SF) technique followed by hot extrusion and heat-treatment. Its microstructural evolution was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) in the whole process. The curve of hardness as a function of aging time was obtained at 120 °C. Test results indicate that the grain morphology is equiaxed and uniform in the central region of the spray-formed billet, but fine and irregular in the bottom and top regions. Both the grain boundary and the intragranular phases are identified as MgZn2 intermetallics. Hot extrusion promotes the refinement of the microstructure. Grain boundary phase disappears, meanwhile, η-phase (MgZn2) and Al3Zr particles precipitate from the matrix after extrusion. The alloy reaches its maximum hardness after being aged at 120 °C for 14 h, associated with a massive precipitation of intermediate η'-phase (MgZn) in the matrix.