Image formation from alloys in the field‐ion microscope

Abstract

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.

Field‐ion images of dilute solid solutions

“…The theory of Southworth & Ralph (1969) provides a means for calculating the field required to evaporate an atom (solute or solvent) from within a bounded -(210] net as well as the field required to evaporate an atom (solute or solvent) from a kink site on the boundary of a (210: net. If the field to evaporate a solute atom from within a bounded 1210) net is less than the field to evaporate a solvent atom from a kink site on the boundary of the net then the process of preferential evaporation is said to occur.…”

“…As a result of this preferential evaporation vacant-site contrast will be observed within a bounded (210) net. However, vacant-site contrast may have its origin in another process, that is, selective ionization (Southworth & Ralph, 1969). In this case the solute atom remains within the (210: net but, due to charge redistribution, the electric field above the solute atom is decreased sufficiently to prevent ionization of the imaging gas and the atom is not imaged in the microscope.…”

“…Unfortunately, the heat of solution is required to complete the calculations based on the theory of Southworth & Ralph. Accurate values of heat of solution for the alloys studied are not known, and in the present calculations (see Table 3 ) these have been set equal to zero, while the values of the other parameters have been taken from Southworth & Ralph (1969). In the alloys studied, setting the heat of solution equal to zero is unlikely to lead to erroneous conclusions.…”

“…The earlier theories of image formation with alloy specimens were not sufficiently precise to allow great confidence to be placed in any predictions based on them. Recently, Southworth & Ralph (1969) have refined these earlier theories and imply that confident predictions can now be made. The present contribution compares observations of the imaging of two platinum-base and two gold-base alloys with the behaviour predicted by the theory of Southworth & Ralph.…”

Hydrogen‐ion image features of tempered martensite

“…However, little attention has been given to image features in these materials and variations in these features with heat treatment and phase composition. Image contrast features for many other alloy systems, especially solid solutions, have been investigated previously (Ralph, 1968 ;Machlin, 1969 ;Southworth & Ralph, 1969a, b;Miiller & Tsong, 1969;Bowkett & Smith, 1970).…”