Crystallization behavior, Al-Ce intermetallic formation, and microstructure refinement of near-eutectic Al-Si alloys by rare-earth element additions

Near-eutectic Al-Si alloys have low strength and high brittleness because of the presence of many eutectic beta-Si flakes, needle-like Al-Fe-Si intermetallics, and coarse alpha-Al grains. This study disclosed the effects of cerium-rich RE (rare earth) element modification on orientation characters of crystals, formation of Al-Ce compounds, and microstructural refinement to improve the microstructure and mechanical properties of the alloys. The RE addition depressed preferential growth along the close-packed and/or sub-close-packed planes and promoted growth along the non-close-packed planes, in which La and other elements were dissolved into needle-like Al11Ce3 phase. When the temperature decreased, Al11Ce3 was preferentially crystallized from the melts and then devitrified by attaching to the surface of beta-Al5FeSi needles. Moreover, many small Al(11)Ce(3 )particles were precipitated in the matrix and on the Si surface by a T6 heat treatment. Eutectic beta-Si phases were constructed into discontinuous networks, short rods, and even particles by RE additions, which were further transformed into fine nodules following the T6 treatment. alpha-Al grains and primary beta-Al5FeSi needles were simultaneously refined. The addition of 1.0 wt.% REs and subsequent T6 treatment yielded the highest tensile strength, elongation, and hardness of the alloy.