Effects of anisotropy and slip geometry on fatigue fracture of Cu/sapphirebicrystals

Interfacial fatigue cracks were propagated in Cu/sapphire bicrystals with ( 110)Cu parallel to (10 (1) over bar0)Al(2)O(3)parallel to [0001]Al2O3[0001] Al2O3 to study the effect of anisotropy and slip geometry in the fracture p rocess. Compact tension specimens with two different crystallographic crack growth directions were loaded to Delta K1a similar or equal to 2 Mpa m(1.2 )with R(.)0. Interfacial cracks grew preferentially along (110),,, and less favorably along < 001 > (Cu). Large areas of the copper fracture surface w ere relatively featureless for crack growth along < 110 > (Cu), whereas wel l defined striations could be observed for the second direction. A refined elastic analysis of the anisotropic near-tip fields for the interfacial cra ck revealed that the preferential crack growth direction had the highest en ergy release rate and mode I crack tip opening displacement. The second dir ection corresponded to a minimum mode II mix. Similar correlations were fou nd in other Cu/sapphire bicrystal experiments described in the literature. Dislocation nucleation from the interfacial crack tip is also analyzed. (C) 2001 Elsevier Science B.V. All rights reserved.