A secondary high-temperature precursor of the Θ'-phase in Al-Cu-( Sc ) alloys

The Al-Cu alloy is a historical model alloy system in the physical metallurgy of engineering aluminum alloys. Nevertheless, a few fundamental phenomena of phase transformation occurring in this simple alloy are still not adequately understood. Among all, for instance, the formation mechanisms of its key hardening 0 '-phase remain mysterious. There is strong evidence that 0 '-precipitates can form from a different high-temperature precipitation pathway, while their formation mechanism via the conventional pathway well-known since 1938 remains to be clarified. Using state-of-the-art electron microscopy, here we report a secondary high-temperature precipitation pathway of 0 '-precipitates. It is demonstrated that led by a secondary high-temperature precursor, named 0 ' S-HTP , very fine 0 '-precipitates can form in the undeformed bulk Al-Cu alloys at elevated temperatures ( >= 250 degrees C). Interestingly is that with Sc-microalloying the surviving rate of meta-stable 0 ' S-HTP precipitates increases drastically and the formed 0 '-precipitates become much finer, significantly enhancing the alloys' strength and thermal stability. It is also revealed that a 0 ' S-HTP precipitate can genetically evolve into a 0 '-precipitate without having to change its morphology and orientation. Our study provides new insights into understanding the industry bulk alloys' microstructures and properties. (c) 2024 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.