Cottrell’s atmosphere

The non-monotonous dependence of surface layer microhardness on deformation degree results from different mechanisms of nitrogen diffusion in deformed material. In our point of view, under the deformations of 3-8 and 20-30 % the greatest number of mobile dislocations, capable to provide the additional transfer of nitrogen interstitial atoms with Cottrell s atmospheres by the dislocation-dynamic mechanism [6-8], can be formed. 

Although this line of reasoning shows one of the principal features of heterogeneous nucleation, the real situation of nucleation near a dislocation line is much more complex [S. Q. Xiao, P. Haasen (1989)]. The stress field of the dislocation changes the composition of both the matrix and the precipitate, which in turn influences both yp and Agp. In view of this fact, one has to determine whether nucleation near the dislocation occurs before or after the Cottrell atmosphere around the dislocation had sufficient time to form. 

When all the SE s of a solid with non-hydrostatic (deviatoric) stresses are immobile, no chemical potential of the solid exists, although transport between differently stressed surfaces takes place provided external transport paths are available. Attention should be given to crystals with immobile SE s which contain an (equilibrium) network of mobile dislocations. In these crystals, no bulk diffusion takes place although there may be gradients of the chemical free energy density and, in multicomponent systems, composition gradients (e.g., Cottrell atmospheres [A.H. Cottrell (1953)]). 

For a > 1, nucleation is barrierless—i.e., the transformation is controlled solely by growth kinetics. However, for a < 1, a barrier exists. The local minimum of AQ (r) at point A in the plot corresponds to a metastable cylinder of /3 of radius r0 forming along the dislocation line. (In a sense, this is analogous to the Cottrell atmosphere described in Section 3.5.2.) In Eq. 19.54, the metastable cylinder s radius is... 

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