A Comprehensive Review of θ-Series Precipitates in Aluminum Alloys

This review systematically synthesizes the research progress on θ-series precipitates. It traces the historical evolution of θ-series precipitate research, from the accidental discovery of age hardening in Duralumin to the atomic-scale insights enabled by advanced electron microscopy and computational methods. The precipitation sequence (supersaturated solid solution → GP zones → θ″ → θ' → θ), transformation mechanisms, and interfacial characteristics of θ'/Al are comprehensively analyzed, with special attention to ongoing controversies such as the structure of GP zones and the pathways of θ″ → θ' transition. Furthermore, the review discusses how alloying elements regulate θ' stability through interfacial segregation, vacancy interactions, and co-precipitation effects. Critical unresolved challenges are highlighted, including the kinetic limitations of θ' coarsening and the need for mechanistic studies on multi-element microalloying. This synthesis aims to provide a foundation for future research toward designing high-performance age-hardenable aluminum alloys.