First-Principles Study on the Grain Boundary Embrittlement of Metals by Solute Segregation: Part II. Metal (Fe, Al, Cu)-Hydrogen (H) Systems

The microscopic mechanism of grain boundary (GB) embrittlement in metals by hydrogen segregation (trapping) has been not well understood for many years. From first-principles calculations, we show here that the calculated cohesive energy of bcc Fe Sigma 3(111) and fcc Al(Cu) Sigma 5(012) symmetrical tilt GBs can be significantly reduced if many hydrogen atoms segregate at the GBs. This indicates that the reduction of the cohesive energy of the GB may cause the hydrogen-induced GB embrittlement in Fe, Al, and Cu. Considering the mobile effect of hydrogen during fracture, especially for the Fe system, more hydrogen atoms coming from solid solution state can segregate on the gradually formed two fracture surfaces and reduce further the cohesive energy. We suggest a new idea about the upper and lower critical stresses observed in the constant-load test of hydrogen-induced delayed fracture in high-strength steels; the upper critical stress is determined by the amount (density) of immobile hydrogen atoms segregated at the GB before fracture, and the lower critical stress is determined by the total amount (density) of immobile and mobile hydrogen atoms, the latter of which segregate on the two fracture surfaces during fracture.