The initial stages of the deformation-induced crystalline-to-amorphous transformation were observed using high-resolution electron microscopy. It was found that deformation-induced amorphization initiated along dislocation core regions in the interior of grains. It was proposed that the energy increase due to the accumulation of defects was high enough to drive the transformation. The energies of 60º and 90º dislocations had been calculated to be of the order of 2eV/Å by using various potentials. The increase in energy due to dislocations in one mole volume was 3.84 x 10-13kJ/mol. It was seen that the rise in energy, due to an increase in dislocation density, could be appreciable. Thus, a dislocation density of 3.2 x 1013/cm2 could increase the energy to about 12.24kJ/mol. This was comparable to the crystallization energy (11.9kJ/mol) for fully amorphous material which was prepared by ion implantation. A dislocation density of 3.2 x 1013/cm2 implied that there was 1 dislocation in every 1.8nm-square region. This was barely achievable in heavy plastically deformed metals. However, the deformation during ball-milling was inhomogeneous, and it was thought possible that a high density of dislocations might be attained in local regions of a grain. The amorphous phase fraction increased linearly with increasing dislocation density. Apart from 60º and 90º dislocations, many other defects (stacking faults, twins, point defects) might also contribute to an increase in energy. Partial, rather than full, amorphization could be explained in terms of the ease of defect generation during ball-milling. Dislocations were easier to generate. and piled up in larger grains than in smaller ones; according to the Hall-Petch relationship. For a grain size of less than 10nm, it was hardly possibly to generate dislocations since a stress which exceeded the maximum pressure generated by ball-milling was needed. Another possibility was that the initially generated amorphous phase formed a protective layer around the crystalline particle; thus making it difficult for further deformation to occur.

Deformation-Induced Amorphization in Ball-Milled Silicon J.Y.Huang, H.Yasuda, H.Mori: Philosophical Magazine Letters, 1999, 79[6], 305-14