The mobility of dislocations in high purity aluminum

Abstract

This thesis presents the results of measurements of the velocities of edge and mixed dislocations in aluminum as a function of temperature and applied shear stress. All tests were conducted on 99.999% purity aluminum single crystals. Dislocation velocities were determined by observing the positions of dislocations by the Berg-Barrett X-ray technique before and after applying a stress pulse. Torsion stress pulses of microsecond duration were applied by propagating torsional waves along the axes of cylindrical crystals. Resolved shear stress up to 16 x 106 dynes/cm2 were applied at temperatures from -150°C to 70°C. Measured dislocation velocities ranged from 10 to 2750 cm/sec. The velocities measured are believed to be characteristic of single straight dislocations moving through essentially perfect crystals, where the velocity is not significantly influenced by dislocation curvature, impurities or dislocation-dislocation interactions. The results of this study indicate that the velocity of edge and mixed dislocations is linearly proportional to the applied resolved shear stress, in the temperature range studied. Dislocation velocity increases as temperature is decreased. These results are compared to the predictions of theories which treat the interaction between moving dislocations and the lattice (phonon interactions). The theoretically predicted variation of dislocation velocity with temperature and stress agrees fairy well with the experimental results, but the predicted velocities are about six times less than the experimental velocities. Possible reasons for this discrepancy are discussed.