The　aging　hardening　behaviors　of　Al-0.12Sc　and　Al-0.12Sc-0.2Zr　(wt.%)　alloys　with　the　addition　of　Si　range　from　0.05　wt%　to　0.3　wt%　were　investigated　by　microhardness　and　transmission　electron　microscopy.　The　results　indicate　that　content　of　similar　to　0.15%　Si　is　optimal　and　Sc-Si　clusters　forming　at　the　early　ageing　stage　hinder　the　evolution　process　of　Al3Sc.　Upon　aging　isochronally,　Al-Sc-(Zr)-Si　alloy　shows　two　stages　hardening.　The　first　strengthen　stage　occurs　because　of　the　formation　of　Sc-Si　clusters,　and　peak　hardness　of　the　second　stage　owes　to　(Al,Si)(3)Sc　evolving　from　the　clusters.　During　aging　isothermally　at　275　degrees　C,　Al-Sc-Si　and　Al-Sc-Zr-Si　show　similar　hardening　characteristics.　But　ageing　at　225　degrees　C,　the　Sc-Si　clusters　are　relatively　stable,　the　second　stage　increase　in　the　Al-Sc-(0.15,　0.3%)Si　alloys　have　no　appearance.　Clustering　behavior　of　Sc　and　Si　does　not　be　changed　by　the　addition　of　Zr.　The　Al-Sc-Zr-Si　alloy　exhibits　better　thermal　stability.　Because　partition　of　Zr　to　the　preceding　formed　(Al,Si)(3)Sc　reduces　the　coarsening　rate　of　the　precipitates.　(C)　2020　Elsevier　B.V.　All　rights　reserved.