Microstructure development, phase reaction characteristics and mechanical properties of a commercial Al-20%Mg<inf>2</inf>Si-xCe in situ composite solidified at a slow cooling rate

The microstructure, phase reaction characteristics and mechanical properties of fabricated Al–20%Mg2Si in situ composite with different contents of cerium have been investigated using optical microscopy, scanning electron microscopy, X-ray diffraction, thermal analysis and hardness tests. The results show that addition of Ce not only refined Mg2Si reinforcement particles but also changed the morphology of eutectic Al–Mg2Si, Al5FeSi (β) intermetallic and Al5Cu2Mg8Si6 (Q) + Al2Cu (Ɵ) phases. It was found that 0.8 wt% Ce is the optimum concentration to transform the phases into refined structures. The structure of the skeleton of Mg2SiP changed to a polygonal shape with uniform distribution and decrease in size from 124 μm to 60 μm and increased in density from 12 to 45 particles/mm2. Flake-like Mg2SiE transformed into a rod-like morphology. In addition, the aspect ratio of needle-like β structures reduced from 40.5 to 22.9, accompanied with an increase of solid fraction for Q + Ɵ phase. Ce addition increased the nucleation temperature of Mg2SiP and β phases; however, it had an opposite effect for the Mg2SiE and Q + Ɵ phases. The composite hardness increased from 61.32 to 74.15 HV because of refinement of the microstructure. The refining mechanism of Mg2SiP and Mg2SiE phases is discussed in the current study, and formation of new Ce compounds is believed to be responsible for the refinement effect