INVESTIGATION OF DEFORMATION FIELD AND HYDROGEN PARTITION AROUND CRACK-TIP IN FCC SINGLE-CRYSTAL

In situ fracture experiments and measurements of the hydrogen distribu tion in the immediate vicinity of the crack tip were used to investiga te the effects of hydrogen on a face-centered cubic (fee) single-cryst al fracture process. The techniques used were scanning electron micros copy (SEM), a scribed-grid method with a computer-controlled data acqu isition system, and ion microprobe mass analysis (IMMA). It was observ ed that the general features of plastic deformation are similar in bot h charged and uncharged hydrogen samples under mixed-mode loading cond itions, and in both cases the strain field ahead of the crack tip is b est expressed by an exponential equation. There are also differences. Hydrogen easily enlarges the crack tip opening displacement (CTOD) und er a lower threshold stress-intensity factor, inhomogeneously increase s the localized plastic deformation, and markedly enhances the steepne ss of strain curve near the crack tip. Internal hydrogen increases pla sticity in the immediate vicinity of the crack tip but its effective r ange is smaller when compared with external hydrogen effects. The resu lts show that two peaks of hydrogen concentration appear ahead of the crack tip: one peak is in the immediate vicinity of the crack tip and another peak is located some distance from the crack tip. It is conclu ded that the distribution of dissolved hydrogen with two peaks around the crack tip corresponds to the distribution of strain and stress fie lds, respectively, due to the interaction of hydrogen with dislocation and hydrostatic stress.