Electron microscopic investigations on the formation and propagation of cracks in LiNbO3 single crystals

Abstract

The formation and propagation of as‐grown cracks are investigated in undoped and doped, brittle LiNbO3 single crystals grown by the Czochralski technique using different ex‐situ and in‐situ electron microscopic methods (SEM, CTEM, HVTEM, and HRTEM). In the surroundings of the tips of cracks propagating in thinned (001) foils, dislocation activities are observed at estimated sample temperatures below 0.4 Tm. Ahead of the actual crack tips, a special, narrow and elongated, process zone can be seen in which the atomic processes of cracking take place. The main steps of the processes in this zone are the successive formation and coalescence of crack nuclei through local intermediate amorphization of the crystalline material at about 10% lattice plane deformations measured by optical diffraction techniques. The crack propagation occurs by successive coalescences between the actual crack tip and the next neighbouring crack nucleus. The coalescence, i.e. the final breaking of the amorphized material happens already at decreased deformations of ≈5%.