Abstract: The Al-Cu alloy is a historical model alloy system in the physical metallurgy of engineering aluminum al-loys. 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 harden-ing θ'-phase remain mysterious. There is strong evidence that θ'-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 θ'-precipitates. It is demonstrated that led by a secondary high-temperature precursor, named θ' S-HTP, very fine θ'-precipitates can form in the unde-formed bulk Al-Cu alloys at elevated temperatures (≥ 250 °C). Interestingly is that with Sc-microalloying the surviving rate of meta-stable θ' S-HTP precipitates increases drastically and the formed θ'-precipitates become much finer, significantly enhancing the alloys' strength and thermal stability. It is also revealed that a θ' S-HTP precipitate can genetically evolve into a θ'-precipitate without having to change its mor-phology and orientation. Our study provides new insights into understanding the industry bulk alloys' microstructures and properties.

Key words: Aluminum alloy, Phase transformation, High-temperature precipitation, Electron microscopy, Strength