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 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 undeformed 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 morphology and orientation. Our study provides new insights into understanding the industry bulk alloys’ microstructures and properties.

Al-Cu合金是工程铝合金物理冶金中历史悠久的模型合金体系。然而，这种简单合金中发生的一些基本相变现象仍未得到充分理解。其中，例如其关键硬化θ′相的形成机制仍然是个谜。有强有力的证据表明，θ′-沉淀物可以通过不同的高温沉淀途径形成，而其通过自 1938 年以来众所周知的常规途径形成的机制仍有待澄清。使用最先进的电子显微镜，我们在这里报告了 θ'-沉淀物的二次高温沉淀途径。结果表明，在高温（≥ 250 °C）下，在名为 θ' _S-HTP 的二次高温前驱体的引导下，未变形块体 Al-Cu 合金中可以形成非常细小的 θ' 沉淀物。有趣的是，通过Sc微合金化，亚稳θ′ _S-HTP 析出物的存活率急剧增加，并且形成的θ′-析出物变得更加细小，显着提高了合金的强度和热稳定性。研究还表明，θ′ _S-HTP 沉淀物可以在遗传上进化为 θ′-沉淀物，而无需改变其形态和方向。我们的研究为了解工业块状合金的微观结构和性能提供了新的见解。