Scandium　(Sc)　and　silicon　(Si)　are　effective　for　modifying　Al　based　alloys,　while　their　combined　effect　may　be　limited　by　some　Si-Sc　interaction,　which　was　rarely　reported.　In　this　work,　Al　alloys　with　varying　Si　and　Sc　additions　were　prepared　for　microstructure　characterization　and　tensile　testing.　The　results　show　that　Sc　refines　the　alpha-Al　grain　via　a　heterogeneous　nucleation　induced　by　in-situ　formed　primary　Al3Sc.　A　network　AlSi2Sc2　phase　was　formed　via　a　eutectic　reaction　and　a　bulk　AlSi2Sc2　was　generated　from　a　peritectic　reaction.　Both　AlSi2Sc2　competitively　consumed　the　Sc　solute　in　the　same　alloy,　weakening　the　refining　effectiveness　of　Al3Sc.　The　Sc　also　refined　the　eutectic　Si　to　more　ordered　fiber/plate-like　structure,　and　a　proper　ratio　of　Si　to　Sc　could　avoid　the　AlSi2Sc2　formation.　When　increasing　Sc　from　0.3　to　1.0　wt%　in　Al　alloys　containing　6　wt%　Si,　the　ultimate　tensile　strength　(UTS)　and　yield　strength　(YS)　increased,　but　the　EL　decreased　due　to　the　presence　of　AlSi2Sc2　phase.　The　UTS　and　YS　can　increase　along　with　a　41%　raise　in　EL　by　a　strategy　combining　higher　Sc　(＞　0.6　wt%)　and　a　lower　Si　(＜　4　wt%)　with　Al3Sc　in　a　dominant　role　in　soli-dification　reactions　without　AlSi2Sc2　formation.　The　mechanisms　of　microstructure　modification　with　Si-Sc　interaction　are　also　discussed.(c)　2022　Elsevier　B.V.　All　rights　reserved.