Atom probe tomography was utilized to investigate Cu precipitation in a high-strength low-alloy steel isothermally aged at 500 ℃ for 1,4,16,and 64 h after water-quenching from 900 ℃.With prolonged aging time,the Curich precipitates (CRPs) increased in size and decreased in number density,and gradually evolved from spheroidal to elliptical in morphology.The small CRPs were rich in a high amount of Fe and a certain amount of Ni and Mn at their early nucleation stage.The large CRPs with increased size due to extensive aging contained less Fe and more Cu at their later growth stage.Additionally,Ni and Mn were both readily to segregate at the CRP/matrix heterophase interfaces,and Mn was higher in content than Ni in the precipitate interior especially when the CRPs were large in size.