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Models of Etch-Pit Formation

From the geometrical arrangement of atoms around dislocations, it is Immediately obvious that screw dislocations should yield etch pits with slope, m, given by  [Pg.77]

The change in the free energy, AG, for the nucleation of a diskshaped cavity of depth, h, and radius, r, at the site of a dislocation is given by  [Pg.79]

The behavior of the free-energy curve is determined by taking the first two derivatives of A G, and equating them to zero. The first derivative, d AG ,/dr, gives a quadratic equation having two roots  [Pg.79]

Thus for 1, there is an energy barrier for etch-pit nucleation at a dislocation site but the value of this barrier is less than that at the perfect surface. For = 1, there is a double root  [Pg.80]

When 1, there is no real root, which means that there is no nucleation barrier for etch-pit formation. Thus, since increases with A/ (Eq. 78), there is a particular value of A above which dislocations open up spontaneously, Thecritical value of this A is obtained from Eq. 78for = 1, i.e., [Pg.80]


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