The　ageing　behaviour　and　phase　transformation　of　a　range　of　Si-microalloyed　low　Mg/Zn　ratio　Al-5.0Zn-1.0Mg-Cu　alloys　were　investigated　at　a　temperature　of　150°C　by　using　TEM　and　microhardness　tests.　Adding　Si　had　a　important　influence　on　the　proportion　of　precipitates　and　microhardness.　As　Si　content　increased,　the　precipitated　phase　underwent　a　gradual　transition　from　the　T+η　precipitates　to　the　fine　GPB-II　precipitates.　Concurrently,　the　microhardness　exhibited　a　notable　enhancement　from　120　HV　to　150　HV　when　the　alloys　were　subjected　to　150°C　aging.　Existence　of　small　and　even　GPB-II　phases　in　the　Si-adding　alloy　was　identified　as　underlying　cause　of　this　result.　This　GPB-II　phase　exhibited　a　bilayer　microstructure.　In　core　region　predominantly　comprise　Zn,　Si　and　Mg,　and　the　shell　region　primarily　consist　of　Zn　and　Cu.　This　structure　effectively　inhibited　phase　growth,　maintaining　the　phases　at　a　smaller　scale　and　enhancing　the　alloy＇s　hardening　effect.　©　2024　Institute　of　Physics　Publishing.　All　rights　reserved.