Using an Sb-doped Ni-Cr steel as a model material, the micromechanisms of grain-boundary segregation and embrittlement were studied. Acoustic emission and electron-microscopy observations, coupled with an elastic-plastic finite-element stress analysis and grain-boundary-segregation measurements using selected-area Auger-electron spectroscopy were used to determine the critical local stress for fracture of a grain boundary in notched four-point bend specimens tested in air and hydrogen gas. Tensile tests of unnotched specimens were also utilized. The results indicate that, in both air and hydrogen, brittle fracture in this steel initiates in grain boundaries with the largest inclusions and highest Sb concentrations and that the volume of highly-stressed material plays an important role. Increased grain-boundary concentrations of Sb and hydrogen each had the effect of lowering the stress necessary for nucleation and unstable propagation of the microcracks. The results are interpreted in terms of a probability-based intergranular-fracture criterion which accounts for the effects of grain-boundary impurity concentration distributions, grain-boundary inclusion-size distributions, and the effects of specimen geometry and local stress fields on the measured intergranular-fracture stress. The results of the hydrogen experiments indicate that the principal effect of hydrogen is a reduction of cohesion, rather than enhancement of plasticity. Grain-boundary segregation observations indicate that antimony and nickel cosegregate to the prior-austenite grain boundaries in these steels. Their segregation behavior agrees qualitatively and semiquantitatively with the predictions of Guttmann's thermodynamic model of segregation in multi-component systems. Also, it was found that grain-boundary microstructural effects, in particular, the martensitic-lath substructures in this steel, are a major source of compositional variability from point-to-point on fracture surfaces.
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