Microstructural evolution and thermal stability of Al–Zn–Mg–Cu–Si–Zr alloy fabricated via spark plasma sintering

We investigated the microstructural evolution and thermal stability of Al–Zn–Mg–Cu–Si–Zr alloy fabricated via high-energy ball milling (HEBM), spark plasma sintering (SPS), and heat treatment at 500 °C. HEBM induces the Al2O3 surface oxide to penetrate the powder interior, and the surface oxide is transformed into MgO particles along the grain boundaries during the sintering process, depleting the Mg solid solution in the matrix. HEBM caused the formation of Mg-free phases and accelerated the transformation of Zr into the Si2Zr phase. Heat treatment promoted the transformation of Zr to the Si2Zr phase in the Zr/Si2Zr coupled particles and caused the coarsening of the secondary phase. The MgO particles present along the grain boundaries suppressed the grain growth of the sintered alloy until fine MgO was transformed into coarse MgAl2O4. The microhardness of the sintered alloy was significantly increased by the application of HEBM, mainly owing to strengthening by grain refinement and oxide dispersion. The Al–Zn–Mg–Cu–Si–Zr sintered alloy maintained high microhardness values even after heat treatment at 500 °C for 168 h, indicating the excellent thermal stability of the alloy compared to the Zr-free Al–Zn–Mg–Cu–Si alloy.