Two high-purityaluminiumalloysbasedoncompositionAl–3.0Mg–1.0Cu(wt%),onewithadded0.4wt%
Ag, werecomparedupto11daysageingat443Kbymeansoftransmissionelectronmicroscopyand
hardness measurements.Thebasealloyexhibitsaninhomogeneousprecipitatemicrostructurewitha
high densityof fine needle-shapedGuinier-Preston-Bagaryatsky(GPB)zonestogetherwithcoarser
precipitatesofS′-Al2CuMg androdsofthestructurallyunknownZ-phase.TheS′ phase ispreferably
formed ondislocations.TheadditionofAghasastrongeffect,leadingtoahomogeneousdistribution
with fine Ag-containingicosahedralquasi-crystallineprecipitates(iQC).BoththeGPBzonesinthebase
alloy andtheiQCphaseintheAgaddedalloysurviveevenafterlongtermageing.Agisfoundtosuppress
the formationoftheS′ phase. ItissuggestedthattheZphaseisanapproximationphasetothequasi-
crystalline phaseasisthecasefortheT-phase,implyingtheyarebasedonsimilar(Bergman)clusters