HYDROGEN-ASSISTED STABLE CRACK-GROWTH IN IRON-3 WT-PERCENT SILICON STEEL

Observations of internal hydrogen cleavage in Fe-3Si are reported. Hyd rogen-assisted stable crack growth (H-SCG) is associated with cleavage striations of a 300 nm spacing, observed using scanning electron micr oscopy (SEM) and atomic force microscopy (AFM). High resolution SEM re vealed finer striations, previously undetected, with a spacing of appr oximately 30 nm. These were parallel to the coarser striations. Scanni ng tunnelling microscopy (STM) also showed the fine striation spacing, and gave a striation height of approximately 15 nm. The crack front w as not parallel to the striations. Transmission electron microscopy (T EM) of crack tip plastic zones showed {112} and {110} slip, with a hig h dislocation density (around 10(14) m(-2)). The slip plane spacing wa s approximately 15-30 nm. Parallel arrays of high dislocation density were observed in the wake of the hydrogen cleavage crack. It is conclu ded that H-SCG in Fe-3Si occurs by periodic brittle cleavage on the {0 01} planes. This is preceded by dislocation emission. The coarse stria tions are produced by crack tip blunting and the fine striations by di slocations attracted by image forces to the fracture surface after cle avage. The effects of temperature, pressure and yield strength on the kinetics of H-SCG can be predicted using a model for diffusion of hydr ogen through the plastic zone. Copyright (C) 1996 Acta Metallurgica In c.