C–Cr segregation at grain boundary before the carbide nucleation in Alloy 690

The grain boundary segregation in Alloy 690 was investigated by atom probe tomography. B, C and Si segregated at the grain boundary. The high concentration regions for each segregation element form a set of straight arrays that are parallel to each other in the grain boundary plane. The concentration fluctuation has a periodicity of about 7 nm in the grain boundary plane. Before the Cr23C6 nucleation at grain boundaries, the C–Cr co-segregate on one side of the grain boundaries while not the exact grain boundary core regions have been detected. The reasons why grain boundary carbides have coherent orientation relationship only with one side of nearby grain which grain boundary is located at high index crystal plane were discussed.