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Abstract

The cohesion of grain boundaries (GBs) is dependent on their chemical composition and structure and greatly influences the mechanical properties of the material. It is well known that H prefers segregating to the GBs (and other defects). Hydrogen enchanced decohesion is one important mechanism of Hydrogen embrittlement. Therefore it is necessary to understand the effects of the impurities and alloying additions that segregate to the GB on the cohesion of GBs. Mn and C being important additions in iron alloys, were chosen for the study.
The embrittling effects of H in the presence of alloying elements Mn and C at the Σ5(310)[001]36.9 ◦ symmetrical tilt grain boundary in body centered cubic Fe was investigated by performing spin polarized DFT calculations. The calculations were carried out with respect to the effect of a varying number of Mn, C and H atoms at different segregation sites of the GB. The impact of segregation of Mn, C and H on the GB energy, work of separation and theoretical strength has been studied. The mechanical properties were characterised by the uni-axial tensile tests performed perpendicular to the GB. Theoretical strength was determined from the energy-displacement data fitted using the universal binding energy relationship. The work of separation was split into mechanical and chemical contributions. The thermodynamic model of Rice and Wang was used to determine the (cohesion enhacing or weakening) nature of the segregated elements. Mn was found to be a cohesion enhancer at the considered GB. C exhibited an enhancement of the GB cohesion. The influence of H on strength of the GB in pure Fe is negligible. However there is a pronounced reduction of cohesion and strength in the three component Fe-C-H system, when H replaces the cohesion enhancing C. In contrast, in the presence of Mn, H itself shows a detrimental effect on the cohesive properties of the GB. Co-segregation of H and C in the presence of Mn was also studied. The segregation energies show that Mn attracts H to the GB but repels C making the alloys with high Mn content more vulnerable to H embrittlement. With this work we could demonstrate that ab-initio calculations are an important tool to understand the influence of chemical composition on H embrittlement.