EFFECTS OF MN ADDITIONS ON THE P-EMBRITTLEMENT OF THE FE GRAIN-BOUNDARY

To achieve an electronic level understanding of intergranular embrittl ement and its control in steel, the first principles full potential li nearized augmented plane wave method and the atomic force approach are used to investigate the effect of Mn additions and P impurities on th e energetics and underlying electronic properties of both the Fe grain boundary (GB) and the corresponding intergranular fracture surface (F S). The calculated binding-energy difference is +0.17 eV/adatom for P in the P/Fe binary system, in agreement with its observed embrittlemen t potency. The Mn is also found to contribute a direct embrittling eff ect of +0.20 eV/adatom, associated with stronger Mn-Fe chemical bondin g in the FS environment. The computed binding-energy difference for P in the (P+Mn)/Fe ternary system is increased to +0.40 eV/adatom, consi stent with experimental evidence that Mn facilitates P embrittlement i n the grain boundary. The origin of the Mn enhanced P embrittlement is attributed to the strengthened in-plane P-Mn interaction, which makes the P impurity interact more isotropically with the surrounding Mn an d Fe atoms in the GB and FS.