Direct observation of Cu interphase precipitation in continuous cooling transformation by atom probe tomography

 Qingdong LIU, Jianfeng GU, and Wenqing LIU

Abstract: Atom probe tomography (APT) combined with electron back scatter diffraction and transmission electronic microscopy (TEM) is utilized to characterize the nature of copper precipitation during austenite-ferrite transformation in a continuous cooling high-strength low-alloy steel. The copper precipitation manners in association with the austenite decomposition kinetics are studied. The prevailing microstructure of the continuous cooling steel consists of acicular ferrite (AF), which is formed at an intermediate cooling rate of 10 degrees C/s. Besides, a limited volume of polygonal ferrite (PF) because of fast cooling rate and a trace of retained austenite are detected. Numerous copper-rich phase is found by TEM observation both in highly dislocated AF and dislocation-free PF. Generally, the copper-rich precipitates have comparatively large sizes and are considered to be formed by interphase precipitation during austenite-ferrite transformation. A high number density of nanometre sized copper-rich clusters that are lack of diffraction contrast in conventional TEM observation are detected by APT. These smaller copper-rich clusters, which are usually located between the linear-arranged copper-rich precipitates, are considered to be formed from supersaturated solid solution after the cessation of austenite-ferrite transformation. That means an ageing reaction for Cu precipitation occurs during continuous cooling transformation. The copper-rich precipitates and clusters are both rich in nickel, manganese and iron.