Effects of Mn additions on the P embrittlement of the Fe grain boundary

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Abstract
To achieve an electronic level understanding of intergranular embrittlement and its control in steel, the first principles full potential linearized augmented plane wave method and the atomic force approach are used to investigate the effect of Mn additions and P impurities on the energetics and underlying electronic properties of both the Fe grain boundary (GB) and the corresponding intergranular fracture surface (FS). The calculated binding-energy difference is +0.17 eV/adatom for P in the P/Fe binary system, in agreement with its observed embrittlement potency. The Mn is also found to contribute a direct embrittling effect of +0.20 eV/adatom, associated with stronger Mn-Fe chemical bonding 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, consistent with experimental evidence that Mn facilitates P embrittlement in 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 and Fe atoms in the GB and FS.