Abstract
Al-Cu binary alloys are important and interesting industry materials.Up to date,the formation mecha-nisms of the key strengthening precipitates,named θ'-phase,in the alloys are still controversial.Here,we report that for non-deformed bulk Al-Cu alloys the θ'-phase actually has its own direct precursors that can form only at elevated aging temperature(＞ca.200℃).These high-temperature precursors have the same plate-like morphology as the θ'-phase precipitates but rather different structures.Atomic-resolution imaging reveals that they have a tetragonal structure with a = 0.405 nm and c = 1.213 nm,and an average composition of Al5-xCui+x(0≤x＜1),being fully coherent with the Al-lattice.This precur-sor phase may initiate with a composition of Al5Cu and evolve locally towards Al4Cu2 in composition,eventually leading to a consequent structural transformation into the θ'-phase(Al4Cu2=Al2Cu).There are evidences that because of their genetic links in structure,such a high-temperature precursor may transform to the θ'-phase without having to change their morphology and interface structure.Our study reveals a well-defined and previously hidden precipitation scenario for the θ'-phase to form in Al-Cu alloys at an elevated aging temperature.
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