The plate-shaped θ’(Al2Cu)precipitate acts as one of the primary strengthening phases in Al-Cu alloys.The interface,especially the semicoherent interface,between Al-Cu solid solution(αAl)and θ’phase con-tains a lot of clues about phase transformations.Thus,these interfacial structures in an Al-Cu alloy after high-temperature and longtime aging have been analyzed in detail using atomic-scale high-angle annu-lar dark-field scanning transmission electron microscopy and first-principles calculations in this work.It was found that the lateral growth of θ’precipitates is subjected to a combination of several major mech-anisms under this aging condition.Except for some common intermediate phases,two novel and striking structures were observed on the interface,which implies two alternative atomic diffusion mechanisms for θ’precipitate growth.For one condition,the atomic diffusion from αAl to θ’phase transformation adopts an interstitialcy mechanism based on additional Al atoms.For the other condition,the diffusion is carried out through Al atoms.Both mechanisms are distinctly different from the previous theory based on direct diffusion of Cu atoms.The first-principle calculations also confirm that these newfound diffusion processes are energetically favored.