A hidden precipitation scenario of the θ′-phase in Al-Cu alloys

Abstract

Al-Cu binary alloys are important and interesting industry materials. Up to date, the formation mechanisms 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 °C). 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-xCu1+x (0 ≤ x <1), being fully coherent with the Al-lattice. This precursor 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.