Effects of anisotropy and slip geometry on fatigue fracture of Cu/sapphire bicrystals

Interfacial fatigue cracks were propagated in Cu/sapphire bicrystals with (110)Cu∥(101̄0)Al2O3/[001]Cu∥[0001]Al2O3 to study the effect of anisotropy and slip geometry in the fracture process. Compact tension specimens with two different crystallographic crack growth directions were loaded to ΔKIa≃2 Mpa m1/2 with R0. Interfacial cracks grew preferentially along 〈110〉Cu and less favorably along 〈001〉Cu. Large areas of the copper fracture surface were relatively featureless for crack growth along 〈110〉Cu, whereas well defined striations could be observed for the second direction. A refined elastic analysis of the anisotropic near-tip fields for the interfacial crack revealed that the preferential crack growth direction had the highest energy release rate and mode I crack tip opening displacement. The second direction corresponded to a minimum mode II mix. Similar correlations were found in other Cu/sapphire bicrystal experiments described in the literature. Dislocation nucleation from the interfacial crack tip is also analyzed.