Size-Effect Diffuse Scattering in High Voltage Electron Diffraction from Disordered α-CuAl

Conclusion

We have demonstrated that operation near the critical voltage allows a comparatively clear view of the modified SES. The partial interpretation of the experimental results is consistent with reintroduction of size-effect strain into the 16.7 at. % alloy. A detailed explanation of the annealling behaviour would require further investigation of the effects of (e.g.) the consequences for internal oxidation of fast and slow cooling, and the fact that we are dealing with very small specimens under conditions of heavy electron irradiation. For the near future, it should be recalled that the experiments described here were carried out in the convergent beam mode (to minimize thickness averaging) and changing the specimen temperature to induce the degeneracy (to keep the electron-optical conditions fixed). Electron microscopes are now becoming available, however, with reliable high acceleration voltages (up to 400 kV), high-angle double tilt heating stages, image intensifiers and small selected-area illumination spots. It will thus soon become possible to uncover the SES without convolution for many alloy systems near V_C, following continuously their strain fields and ordering kinetics. Since the critical voltage effect gives us a 60°C “window” through which to observe the SES, and this window may be moved up or down by changing the accelerating voltage, an examination of Shirley and Fisher’s list of accessible critical voltage reveals that alloys of several technologically important metals Cu, Al, Fe, Ni, Co, Cr, Pd, V etc. become available for study [14].