Structure factors and the critical voltage effect in ordered b.c.c. (B2) alloys: II. The analysis of critical voltage measurements in α'FeCo, β'CoAl and β'NiAl

Abstract

For pt.I, see ibid., vol.13, p.1581 (1983). The critical voltage effect in high-voltage electron diffraction has been used to measure the 100 low-angle structure factors of the ordered b.c.c. alloys alpha 'FeCo, beta 'CoAl and beta 'NiAl as functions of composition and temperature and the 100 low-angle structure factors of beta 'CoAl and beta 'NiAl as functions of composition at room temperature. From these measurements values of the 100 and 110 low-angle atomic scattering factors and the atomic mean-square displacements (MSD) were obtained. The results indicate that more accurate critical voltage measurements and improved measurements of the 220 and 200 structure factors are required to obtain values of the 100 and 110 atomic scattering factors accurate enough to examine the electron charge distributions. A simple model was successfully used to analyse the MSD values of alpha 'FeCo and beta 'CoAl. For beta 'NiAl, the presence of the omega phase introduced complications to the MSD analysis, but with a minor modification to the model, the static displacements due to the diffuse omega were evaluated, and good agreement with the results of other workers was obtained. Studies on Al-rich beta 'NiAl alloy indicated that both Al antistructure atoms and diffuse omega are present in these.