Effect of rare-earth La on microstructure and mechanical properties of Al7Si4CuMg alloys prepared by squeeze casting

The effects of rare-earth lanthanum (La) on the microstructure and tensile properties of Al7Si4CuMg alloys prepared by squeeze casting were studied by microscopy technologies, thermal analysis and tensile tests. The results demonstrate that La can refine alpha-Al grains, modify eutectic Si and improve tensile properties of the Al7Si4CuMg alloys. The refinement mechanism of alpha-Al grains with the addition of La has been discussed based on the classical nucleation theory, interdependence model and thermodynamics. These results indicate that the added La increased constitutional supercooling and heterogeneous nucleation cores ahead of the melt/alpha-Al interface and thus La promoted the nucleation of alpha-Al. This refinement effect of La on alpha-Al was further promoted by the squeeze pressure during solidification. Moreover, the formation of twinned Si particles was attributed to the adsorption of La-rich clusters along the < 112 >(Si) growth direction and at the intersection of two {111}(Si) facets. This is consistent with the poisoning of the twin plane re-entrant edge and the impurity-induced twinning modification mechanisms. With the La addition of 0.15 wt.% (squeeze pressure was 75 MPa), the Al7Si4CuMg alloys obtained the finest alpha-Al grains, and optimal mechanical properties with the ultimate tensile strength of 297 MPa and elongation of 2.5%. When La addition was further increased to 0.20 wt.%, the refinement of alpha-Al and modification of eutectic Si were both limited by the occurrence of eutectic reaction L -> alpha-Al + Si + Al-x (La, Cu, Ti), because this eutectic reaction reduced the undercooling and produced the needle-like La-rich particles.