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Chapter VII Reactivity at Dislocation….
The nature of the etch pits depends on the character of the dislocation and on
exact composition of the etchant. Edge and screw dislocations etch slightly
different, the former produce deeper pits. The etching is inhibited by some
segregated impurities at dislocations, therefore, aged and fresh dislocation etch
differently. Gilman et al [9] have studied etch pits on the lithium fluoride crystals
formed by etching in dilute aqueous solution of FeF3. They have suggested that
the faster nucleation rate at edge dislocations is due to their higher energy,
whereas the low nucleation rate at dislocations with segregated impurities is
because of lowering in the dislocation energy.
By proper choice of slip traces one can chose emergence points of screw and
edge dislocations separately and check whether both types of dislocations are
etched. For cleaved specimen of alkali halide, the 45 o traces are due to edge
dislocations while the bands parallel to the crystal edges contain screws inclined
45 o with respect to the surface. These circumstances then permit investigation of
the effect of the inclination of the dislocations with respect to the etched face. It is
not certain that all pits corresponds to dislocations, for instance, precipitates may
cause etching. It has been shown that precipitates in LiF crystals cause the
formation of terraced pits [35]. Fission tracks can also be etched [36]. There are
certain review papers [15, 37, 38] and books [39-41] available on etching study.
The book written by Heimann describes the intricasies of art of etching study and
science [42].
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