2000 Volume 40 Issue 8 Pages 809-818
The segregation mechanism of alloying elements in prior austenitic grain boundaries of embrittled 5% Ni steels has been investigated by AES in previous studies, while that of improved 5% Ni steels has not been clarified yet. In the present study, the segregating behavior of alloying elements in improved 5% Ni steels has been elucidated for the first time by applying FE-TEM with respect to the effects of soluble Al on the suppression of temper embrittlement and hydrogen embrittlement in the steels.
It is consequently made clear by applying FE-TEM-EDS that alloying elements tend to segregate coincidently to grain boundaries, showing the asymmetric segregation according to solute interactions. The observed behavior is newly defined as the divorced coincident segregation, which is considered to result from the moderate site competitions in the deep potential well of McLean's equilibrium segregation. The divorced coincident segregation was investigated further along grain boundaries, where the periodic segregation of Al and Si was found to vary alternately along grain boundaries analyzed.
The beneficial effect of soluble Al on the two types embrittlement is attributed to the mechanism of spot toughening through the periodic decrease of solutes in grain boundaries. The principle of the divorced coincident segregation proposed in 5% Ni steels can be applicable not only to ordinary steels but also to other poly-crystalline alloys in improving their properties.