SEGREGATION OF ATOMS ON THE INTERFACES IN THE RPV MODEL STEEL STUDIED BY APT

Abstract

The segregation of impurity or solute atoms to grain boundaries as well as phase interfaces can either improve or degrade the chemical, physical and mechanical properties of alloys. This phenomenon has been studied widely for iron based alloys, and the analysis method by an atom probe tomography (APT) is a powerful tool for better understanding this problem. The resulting composition changes of grain boundaries and phase interfaces, as well as the precipitation of Cu-rich nanophases, are frequently associated with the phenomenon of embrittlement in ferritic reactor pressure vessel (RPV) steels. The present work was carried out to study the segregation of impurity or solute atoms to grain boundaries as well as phase interfaces in a RPV model steel with higher content of Cu (0.53%, atomic fraction) than commercially available one. The RPV model steel was prepared by vacuum induction melting. The specimens were further heat treated by water quenching at 880 for 30 min and tempering at 660 for 10 h, and finally aged at 370 for 3000 h. The results show that the segregation amount of Ni, Mn, Si, C, P and Mo atoms on grain boundaries are varied. The sequence of segregation tendency for different atoms from strong to weak is C, P, Mo, Si, Mn and Ni, whilst Cu atoms were clearly depleted at the grain boundaries. Si atoms also segregate to the grain boundaries, but it depends on the characteristic of the grain boundaries. The C segregation range at grain boundaries is the widest. According to the width of the composition profiles at the half intensity for different atoms at the grain boundaries, the segregation range of C atoms is 1.5 times wider than