Download (4Mb) - Etheses - Saurashtra University

Chapter VII Reactivity at Dislocation….
dissolution rate Vd at the defect. If the height of these steps is h, then Vd/h steps
are produced in unit time. The model takes no account of the square shape of
the steps and hence that of the pit they comprise, and it is thought to be a two-
dimensional model. Following nucleation, the steps move out by removal of
atoms [23] and can have velocity Vs. The Vs may be influenced by the
superimposed diffusion field, particularly close to the defect source. It has been
predicted that a series of steps emanating from a source will, after sufficient
distance travel, achieve a uniform velocity Vs [12, 24]. This suggests that the
velocity, Vs at a sufficient distance from the source, is independent of the rate of
production of steps and hence is independent of Vd. If the steady state is
achieved well inside the pit, its slope and rate of widening will be independent of
the type of defect. Ives and Hirth [25] applied this analysis to etch pits in lithium
fluoride, but no definitive verification of the parametric dependence of Vs was
possible.
The possibility of certain special situations arises when a pit is formed at the
intersection of a dislocation which has a component of its Burgers vector
perpendicular to the surface [26]. No nucleation event is necessary in this case
because this type of dislocation produces a step, which commences at the point
of intersection with the surface. Various authors have given account of this
situation and proposed spiral step in growth [12] and in evaporation [19], which at
sufficient radius can achieve steady state spacing between successive turns.
Ives and Hirth [25] have assumed that Vs is independent of the distance from the
source, then, since Vd is a measure of the rate of production of steps and Vs
254