Probing rare earth segregation in steels

The challenge of characterizing the precise sites of rare earth (RE) atoms has limited our understanding of microalloying and the design of RE steels with superior performance in recent decades. Electron microscopy with near-atomic resolution can be used to identify RE retention or trapping in alloys but fails to provide reliable results in steel due to the extremely low solid solubility of RE atoms. Here, we probed the segregation and clustering of RE atoms by identifying the location of RE oxides via the in situ oxidation of these extremely active atoms in dendritic regions, ultimately providing direct visualization of the distribution of RE atoms. First-principles calculations confirmed that dendritic interfaces can serve as trapping sites for solute RE atoms. These findings effectively solve the problems caused by the use of traditional techniques to characterize RE atoms in steels and provide the first evidence for a solid solution of RE atoms.