Microstructural evolution and corrosion behavior of Al-20Mg2Si composites with trace La addition

Trace La element was introduced in Al-20Mg2Si composites to regulate the microstructure and corrosion performance. The modification mechanism of trace La and the corrosion behavior of the newly developed composites were systematically investigated by thermal analysis, microstructural observations and electrochemical tests. Our study reveals that the addition of only 0.10% La can refine the size of the primary Mg2Si phase from 77.6 to 26.7 µm, modify its morphology from a coarse dendrite to a fine truncated octahedron and reduce the corrosion current density from 5.96 μA cm−2 to 3.03 μA cm−2. Trace La refines the primary Mg2Si phase size by forming a tiny LaAlSi phase which can provide effective heterogeneous nuclei for the primary Mg2Si phase during solidification. Excess La atoms can selectively adsorb on the {100} facets of the primary Mg2Si phase and suppress the preferential growth of the Mg2Si phase, eventually resulting in the morphological transition of the primary Mg2Si phase from dendrite to truncated octahedron shape. The enhanced corrosion resistance of the La micro-alloyed Al-20Mg2Si composites can be attributed to the formation of stronger protective surface oxide films and finer primary Mg2Si phase. The protective oxide films can effectively hinder the penetration of the corrosive medium and the finer primary Mg2Si phase can weaken the localized corrosion tendency of the Al-Mg2Si composites.