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Both microstructure sizes and phase distributions influence the mechanical performance of multi-phase alloys.Herein,we investigated the microhardness enhancements of two Al-Cu-Ge alloys with enlarged undercoolings,corresponding to their microstmctural evolutions.The similar range undercoolings for the two alloys were obtained by a glass fluxing method,and comparatively minimum undercoolings were achieved by differential scanning calorimetry.As undercooling increased to 147 K(0.19TL)and 144 K(0.18TL)for Al70Cu10Ge20 and Al80Cu10Ge10,their microhardness increased to 202 HV and 157 HV,respectively,at different rates.For an Al70Cu10Ge20 alloy,the microstructure refinements of primary CuAl2 dendrite and(Al)+(Ge)pseudobinary eutectic,especially the former,dominate its large microhardness increase with high-level undercoolings.By comparison,for an Al80Cu10Ge10 alloy with a slight composition difference,a hardness drop and a limited refinement hardening effect are observed due to the early formation of primary(Al)dendrite.As undercooling increases,an alternative hardening owing to the reduced amount of(Al)dendrite as well as the larger amount and more homogenous distribution of(Al)+CuAl2 pseudobinary eutectic phases is presented.The mechanical performance of the two alloys well matches the combined values of their composing structures,following the rule of mixtures and according to their microstructure evolutions.