The anisotropic nature of local crack stability in BCC crystals

In a semi-brittle material, dislocation emission from the crack tip and from near tip sources controls the local elastic-plastic tip stress field. As such, this local dislocation emission, which is connected to the intrinsic constraint of crystal plasticity, might also lead to anisotropic features of both the effective surface energy, γ_eff, and the local driving force, G_local. The major objective of the current study is to elucidate some of the local anisotropic aspects involved in the cleavage fracture mode of body centered cubic (BCC) crystals. Special emphasis is given to the effective surface energy recognized here as an anisotropic entity expressed by [γ_eff]φ, λ (here φ, λ designate the specific cleavage plane and the crack growth direction, respectively). These local views become possible through an extensive research activity in hydrogen induced subcritical growth of Fe-3%Si crystals. The experimental program included internal and external hydrogen interactions for different crystal orientations. Attention was given to the study of internal flaw/slow flaw growth in addition to sharp crack extension in prefatigued single edge notched specimens. A detailed phenomenological and theoretical analysis which is related to the local fracture parameters is described, with some application to the current hydrogen interaction problem as well. This case of quasi-static slow crack growth on a cleavage plane enabled the identification of the anisotropic features mainly by: extensive crack tip morphology investigation, the shapes of growing crack fronts or voids, a modified elastic-plastic dislocation simulation at the crack tip and further distinction between the microcrack initiation and propagation stages. Analysis of the above elements as based on experimental results indicates that the crack tip stability and the crack front shape are related to the dislocations and/or slip plane arrangements with anisotropic control of crack initiation and arrest. It is emphasized that crack growth itself need not be anisotropic and, in fact, may be more isotropic.