ELECTRON-MICROSCOPIC INVESTIGATIONS ON THE FORMATION AND PROPAGATION OF CRACKS IN LINBO3 SINGLE-CRYSTALS

The formation and propagation of as-grown cracks are investigated in u ndoped and doped, brittle LiNbO3 single crystals grown by the Czochral ski technique using different ex-situ and in-situ electron microscopic methods (SEM, CTEM, HVTEM, and HRTEM). In the surroundings of the tip s of cracks propagating in thinned (001) foils, dislocation activities are observed at estimated sample temperatures below 0.4 T-m. 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 amorphizati on of the crystalline material at about 10% lattice plane deformations measured by optical diffraction techniques. The crack propagation occ urs by successive coalescences between the actual crack tip and the ne xt neighbouring crack nucleus. The coalescence, i.e. the final breakin g of the amorphized material happens already at decreased deformations of approximate to 5%.