Faceted fatigue fracture and its relation to the crystallographic slip systems in Cu-16 at. Pct al single crystals

Abstract

In order to explore the relationship between the characteristic linear marks on the fracture surfaces of fatigued planar slip materials and the activity of cross slip and secondary slip systems, fatigued single crystals of Cu-16 at. pct Al alloy with a common single slip orientation were examined by scanning electron microscopy (SEM). This paper documents and explains the fractographic details of stages I and II crack propagations as a function of crack depth, strain amplitude, and loading history in a planar slip metal. The linear marks on the fracture surfaces in stage I for the specimen cycled at low strain amplitude are found mostly parallel to the traces of the cross slip plane, which is the plane on which propagation occurs to link up closely parallel cracks on the primary slip plane. With increase of crack depth, the contribution of conjugate slip for the linking up of parallel primary cracks increases. By geometrical consideration of the secondary slip planes with respect to primary cracks, the cross slip plane was found to be the most favored for the linking up of parallel primary cracks. This linking-up phenomenon produces the dominant fracture surface morphology. The fracture surfaces in the final stages show the typical features of ductile fracture in striations or catastrophic rupture. This result indicates that the observation of crystallographic traces on the fracture surfaces in stages I and II need not be associated with a brittle cleavage mechanism of fatigue fracture. The present authors support a ductile, plastic mechanism of crack propagation in both stages I and II.