CRACK-TIP STRAIN FIELDS AND FRACTURE MICROPLASTICITY IN HYDROGEN-INDUCED CRACKING OF FE-3WT-PERCENT-SI SINGLE-CRYSTALS

The development of a sound physical view regarding crack quasi-stability during subcritical crack extension still remains challenging. It is evident that further progress requires exploration of the refined micromechanical processes of fracture. The present study emphasizes the experimental findings which indicate a crack arrest potential in terms of flexible dislocation activity available in semibrittle b.c.c. single crystals. This is particularly demonstrated in Fe-3 wt%Si single crystals with subcritical crack extension induced by sustained loading and external hydrogen interaction. Pre-fatigued crystals oriented in (001)[010] and loaded above 18 MPa m1/2 in 1 atm of gaseous hydrogen resulted in slow crack growth. Since a fine-scale localized approach was adopted, observations were conducted with the use of selected-area channelling patterns in a scanning electron microscope. This provided a 5 mu-m spatial resolution and a submicrometre depth resolution with regard to strain measurement. The current investigation produced two major results. (1) The crack-tip strain distribution under plane-stress and plane-strain conditions agree well with the continuum mechanics formulations. (2) The tracking of microplasticity near the fracture surface, as well as fracture surface topology, demonstrates that inhomogeneous anisotropic deformation has an integral role in the crack growth process. Analysis of the above aspects indicates a consistency in the alternate events of crack extension and arrest. In particular, the dynamic nature of crack initiation and arrest is included in the framework of a global propagation-controlled cleavage process. This is strongly supported by current experimental results.