Abstract
Fatigue crack growth tests under constant-amplitude loading conditions in air and in vacuum have been carried out in the moderate growth rate regime. The fracture surfaces were examined at low andhigh magnifications in a scanning electron microscope to identify the different fractographic features. An etchant that attacks the basal crystallographic planes preferentially was used to elucidate the cracking crystallography. The results obtained show that large facets lying in or close to {111} planes are found in air at low R ratio and in vacuum at both the R ratios studied. These facets are formed by micro-void coalescence as shown by the presence of dimples. The other fractographic features identified are pseudointergranular facets, the formation of herringbone patterns and the formation of pseudo-striations. These features are found only in air. Different mechanisms are proposed to explain the observed cracking modes.
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