Energetics of dislocation relaxation associated with cleavage in cadmium-doped zinc crystals as probed by synchrotron topography

Abstract

An almost tenfold increase in the cleavage energy of zinc crystals doped with 0.1 at.% Cd has been previously reported in the literature and attributed to an increase in the intrinsic surface energy. The energetics of the cleavage process as a function of dopant concentration over the range 0.01, 0.03, 0.06 and 0.13 at.% Cd has been systematically measured and correlated with the observed dislocation microstructure induced by crack initiation and growth. The dislocation relaxation zones were determined by non-destructive observations using synchrotron radiation topography. Since the primary cleavage and slip systems are coplanar, previous workers have ignored plastic relaxation and assumed that fracture had to proceed in a purely brittle fashion. Synchrotron topography results indicated extensive plastic relaxation at the crack tip and also downstream during crack propagation. While plastic relaxation accompanied fracture for both pure zinc and cadmium-doped zinc crystals, the topographs indicated a conversion of the plastic relaxation mode from a low energy array akin to a Taylor lattice to a high energy dislocation microstructure which caused severe lattice distortion of the entire cleavage surface. On the basis of these observations a modified Griffith analysis could be used to determine the relative contributions to the cleavage energy due to the intrinsic surface and dislocation relaxation processes. It was concluded that the latter process is the dominant factor in controlling the fracture toughness of these alloys.