The effect of thin film formation at crack tips on fracture

The embrittlement of a HSLA steel by gaseous chlorine has been studied. At chlorine pressures greater than 1.33 × 10³ Pa, embrittlement is observed to occur via a predominantly intergranular fracture mode. At pressures below this value no effect of the chlorine gas is observed and the fracture proceeds via the ductile mode characteristic of fracture in air or vacuum. Angularly resolved XPS studies indicate the formation of a thin film of FeCl₂ occurs above and below the threshold pressure. The film thickness was found to increase with pressure and exposure and at constant pressure obeys a 1/X = A - B ln t growth law. Calculations made indicate that elastically hard thin films covering a crack tip can severely restrict or prevent dislocation emission in a substrate which is normally unstable to dislocation emission. It is found that for μ_film/μ_sub > 2 and for film thicknesses greater than 2 monolayers brittle behavior is predicted under some conditions for normally ductile behaving substrates. In conjunction with concepts of crack tip shielding, the embrittlement phenomenon is rationalized.