Ductile-brittle transition temperature shift controlled by grain boundary decohesion and thermally activated energy in Ni-Cr steels

Temper embrittlement induced by segregation of metalloid solutes to grain boundary (GB) was evaluated by a shift of the ductile-brittle transition temperature (DBTT). DBTT was found to be linearly correlated with the amount of metalloid on the GB (Xgb) for both dynamic and static displacement rates (dδ/dt) in high and medium hardness steels. Recent first-principles calculations have determined the GB embrittling potency (Δep) of segregated Sb, Sn and P. In both high and medium hardness steels, the slope (α) of DBTT vs. Xgb was found to be linearly dependent on Δep regardless of the segregated solutes. In high hardness steels, the slope is independent of dδ/dt, while in medium hardness steels the α is dependent on dδ/dt. An Arrhenius plot of dδ/dt vs. the reciprocal DBTT was used to drive the thermal activation energy (Eact), which represents a barrier to plasticity. It was found that Eact correlates to a reduction in the GB fracture surface energy. The Eact depends strongly on GB decohesion in high hardness steels but only weakly depends on it in medium hardness steels.