Examination of dislocation substructures in fatigued Cu using electron channelling contrast imaging

For flaw-free smooth copper single crystals, nucleation of fatigue cracks is generally associated with PSBs emerging at the free surface once stress saturation has been established. In the early stage of crack growth fatigue cracks run along PSBs (stage I). After a few tens of microns the crack deviates from the PSB so as to propagate normal to the loading axis and becomes a stage IT crack as defined by Forsyth [1]. There have been few TEM studies of the dislocation substructures near fatigue cracks, because of the difficulty of specimen preparation [2,3]. The SEM based electron channelling contrast imaging (ECCI) technique makes use of the variation in the back scattered electron (BSE) yield with the crystallographic direction of the incident beam so as to reveal the distribution of lattice defects close to the sample surface [4]. ECCI is analogous to diffraction contrast imaging in the TEM, but with the considerable advantage that bulk specimens can be examined albeit at lower spatial resolution. This paper demonstrates the utility of the ECCI technique through examples from an investigation of fatigue crack initiation and early growth. We present ECCI images showing dislocation arrangements in the vicinity of fatigue cracks in bulk specimens. The ECCI technique is finding increasing use in fatigue studies [5-7]