Previous studies of solid-solution alloys with the field-ion microscope are reviewed, and it is pointed out how their image contrast must be determined by the different response of the atomic species towards field ionization and field evaporation. The former effect, termed selective ionization, describes the relative brightness of the image points arising from the different atomic species. It is pointed out that this process is secondary to considerations of selective evaporation since the latter determines whether both species can occupy potential imaging sites in any given region of the image.Selective evaporation describes the relative fields required to evaporate the two species of atom from the specimen surface. In general the solute species will either be preferentially evaporated from the surface defined by the solvent species, or will be preferentially retained onto it. Consideration of the mechanism of selective evaporation shows that its degree may be expressed by a simple parameter calculated from known thermodynamic and atomic-bonding data. In the later sections the model is applied to reported field-ion data from solidsolution alloys, excellent correlation being obtained.

A number of fundamental definitions of magnification in the field-ion microscope are derived in terms of parameters determined by the projection geometry. These expressions for magnification are then compared with analogous ones derived using an empirical approach. The precise dependence of magnification on emitter radius is discussed, and a particular example of an asymmetric emitter is worked through in detail.

No abstract

No abstract