No abstract

Glide velocity measurements were made on isolated screw and 60° dislocations in silicon for the temperature range 775°–925°C. The x-ray topographs which were used to reveal dislocation displacements also gave qualitative information concerning the early stages of dislocation multiplication in highly perfect silicon crystals. Freshly generated dislocations were more mobile than aged dislocations. Pinning points which were tentatively attributed to thermal jogs developed along the lines. The pinning point spacing decreased with increasing temperature as would be expected for a jog formation energy of 1.2 eV. Heating to above 1000°C effectively immobilized all the dislocations present in the crystal. On subsequent loading at 825°C, no motion took place until the stress was high enough to cause catastrophic multiplication when a segment of dislocation did break away. This resulted in the formation of heavy bands of slip. For fresh dislocations the temperature dependence of velocity was analyzed on the basis of a kink pair nucleation and kink propagation model. The measured activation energy for motion of both 60° and screw dislocations was 1.8±0.3 eV.

We have developed two techniques for time-resolved x-ray diffraction from bulk polycrystalline materials during dynamic loading. In the first technique, we synchronize a fast detector with loading of samples at strain rates of ~10(3)-10(4) s(-1) in a compression Kolsky bar (split Hopkinson pressure bar) apparatus to obtain in situ diffraction patterns with exposures as short as 70 ns. This approach employs moderate x-ray energies (10-20 keV) and is well suited to weakly absorbing materials such as magnesium alloys. The second technique is useful for more strongly absorbing materials, and uses high-energy x-rays (86 keV) and a fast shutter synchronized with the Kolsky bar to produce short (~40 μs) pulses timed with the arrival of the strain pulse at the specimen, recording the diffraction pattern on a large-format amorphous silicon detector. For both techniques we present sample data demonstrating the ability of these techniques to characterize elastic strains and polycrystalline texture as a function of time during high-rate deformation.

No abstract