The Problem of Nucleation in a Concentration Gradient Field

In Chapter 4, different modifications of nucleation theory in a concentration gradient field are described. Using the thermodynamic approach, we have introduced the notion of a critical concentration gradient above which nucleation becomes thermodynamically prohibited. Different microscopic schemes (nucleation modes) have been applied to the description of the nucleation mechanism. 

Up to recent times, there have not been any experiments allowing confirmation a specific nucleation mode. This peculiarity is caused by concentration gradients (for the first phase) predicted by the theory, which are too high, and, thus, one needs to investigate diffusion zones where the characteristic thickness does not exceed several nanometers. In order to do so we must carry out the analysis with spatial resolution of about interatomic distance. In marginal cases, the critical thickness of the diffusion zone may be less than the lattice parameter of the expected phase. In such situations, thermodynamic suppression of nucleation due to the reduction of the driving force is not likely to take place at all. 

One of the modern hi -accuracy experimental methods is tomography atom probe (TAP). It enables the investigation of reactions in thin metallic diffusion couples by means of spatially resolved chemical analysis with a local accuracy up to interatomic distance [11, 12]. This technique was also applied to reactive diffusion in the thin film system Al-Co within the temperature range of 200-400 °C [6]. In accordance with the previous results obtained by the authors [13, 14] by DSC, the first phase to nucleate and grow in the temperature interval from 200 to 400 °C is AI9C02. The authors of [6] have established that in the diffusion zone between A1 and Co formation and growth of a solid solution layer occurs first, and nucleation of AI9C02 particles is never observed before the diffusion zone reaches about 3-4 nm thickness. Thus, the appearance of the product phase is controlled by the width 

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