The Segregating Behavior of Alloying Elements Based on the Divorced Coincident Segregation in 5% Ni Steels by the Application of FE-TEM.

The segregation mechanism of alloying elements in prior austenitic grain boundaries of embrittled 5% Ni steels has been investigated by AES in previous works, while that of improved 5% Ni steels has not been clarified yet. In the present study, the segregating behavior of alloying elements in the improved 5% Ni steels has been elucidated for the first time by the application of FE-TEM with an emphasis on the effects of soluble Al on the suppression of temper embrittlement and hydrogen embrittlement. It is consequently made clear by the application of FE-TEM-EDS that alloying elements tend to segregate coincidently to grain boundaries, occupying the opposite side of grain boundaries each other according to their interactions. The observed behavior is newly defined as the "divorced coincident segregation" in the present study, and is considered to result from the moderate site competitions in the deep potential well of McLean's equilibrium segregation. The results also show that the amount of {Al-(Si+Cr+Mn)} segregated in grain boundaries governs the resistance to the two types of embrittlement in 5% Ni steels under the condition of low impurities such as N, P and Sn.