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

Al-Cu binary alloys are important and interesting industry materials. Up to date, the formation mechanisms of the key strengthening precipitates, named theta'-phase, in the alloys are still controversial. Here, we report that for non-deformed bulk Al-Cu alloys the theta'-phase actually has its own direct precursors that can form only at elevated aging temperature (> ca. 200 degrees C). These high-temperature precursors have the same plate-like morphology as the theta'-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 theta'-phase (Al4Cu2=Al2Cu). There are evidences that because of their genetic links in structure, such a high-temperature precursor may transform to the theta'-phase without having to change their morphology and interface structure. Our study reveals a well-defined and previously hidden precipitation scenario for the theta'-phase to form in Al-Cu alloys at an elevated aging temperature. (C) 2021 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

The theta'-phase in Al-Cu alloys has its own high-temperature precursors with similar morphology but different structures, which can form at elevated aging temperatures and eventually transform into the theta'-phase. This study provides a clear precipitation scenario for the formation of the theta'-phase in Al-Cu alloys at high aging temperatures.