Dislocation distributions associated with fatigue cracking on cleavage planes

A attempt is made to answer the following question: does the dislocation distribution control the intrinsic fatigue resistance of b.c.c. metals? To evaluate this, experiments involving hydrogen-induced fatigue cracking of several Fe-3wt.%Si single crystals oriented in the 〈100〉 direction were performed. Microminiature samples of the round compact type were loaded under constant ΔK with alternating blocks of four constant test frequencies ω to produce different single-cycle crack advances. Hydrogen apparently has the unusual effect of requiring more than 1 cycle to advance the crack at high frequencies but only part of a cycle to advance the crack at low frequencies. To estimate the deformation gradients, both direct transmission electron microscopy and channeling were used. Low energy dislocation configurations mostly consisted of dislocation dipoles on {110} and {121} planes. Both the local hardening on slip bands from dipole configurations and the thermal component of the flow stress from the strain rate sensitivity of FeSi increased with increasing ω. It is proposed that these combined effects can explain the increases in da/dt with increasing ω in terms of a hydrogen embrittlement model.