Solute segregation and intergranular brittle fracture in steels

A method for determining the local tensile stress σ* required to initiate brittle fracture at a grain boundary as a function of the concentration of segregated impurity on that boundary is presented. This method has been applied to Ni-Cr steels doped with Sb, Sn, or P, and the embrittling effects have been compared. In order to determine the relationship between the measured changes in σ* and the changes in the cohesive energy γ, a fracture theory has been developed which is the analogue of the Griffith theory for a deformable crystalline solid. In this theory the processes of bond breaking and dislocation emission at the crack tip are treated as concomitant so that a relationship between the local plastic work at fracture and γ is established. This gives criteria for unstable micro crack extension in terms of the relevant plastic properties of the crystal and of the cohesive energy γ. The relationship of the foregoing to the fracture toughness K _1C is discussed. The physical basis of embrittlement is further discussed using inputs from other fields of solid state research. The hypothesis developed here treats embrittlement in terms of volumetric and chemical, or bonding, effects. The embrittling behaviour of hydrogen fits into this framework in a natural way.