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

Abstract 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 ( X _gb ) for both dynamic and static displacement rates (d δ /d t ) in high and medium hardness steels. Recent first-principles calculations have determined the GB embrittling potency (Δ e _p ) of segregated Sb, Sn and P. In both high and medium hardness steels, the slope ( α ) of DBTT vs. X _gb was found to be linearly dependent on Δ e _p regardless of the segregated solutes. In high hardness steels, the slope is independent of d δ /d t , while in medium hardness steels the α is dependent on d δ /d t . An Arrhenius plot of d δ /d t vs. the reciprocal DBTT was used to drive the thermal activation energy ( E _act ), which represents a barrier to plasticity. It was found that E _act correlates to a reduction in the GB fracture surface energy. The E _act depends strongly on GB decohesion in high hardness steels but only weakly depends on it in medium hardness steels.