Diffusion Decomposition of Nanocrystalline Alloys at Generation of Non-Equilibrium Vacancies on Grain Boundaries

Influence of HPT Deformation on the Structure and Properties of Amorphous Alloys

“…The decomposition of the amorphous phase with further consolidation of the main element in nanofields is observed prior to nanocrystallization. The phenomenon of a similar decomposition of alloys during ball milling was recently discussed in [79]. As was shown in [79], HPT processing can induce local redistribution of elements in alloys, as well as changes in the concentration of the main element in nanofields due to the formation of vacancy flows.…”

Modeling Enhanced Diffusion Mass Transfer in Metals during Mechanical Alloying

“…Similar deformation mechanisms operate at superplastic deformation of micron and submicron grained alloys at elevated temperatures where accommodation of grains takes place via grain boundary diffusion (Kaibyshev, 2002) and vacancies arising in the boundary may penetrate into grains. However, as noted in (Shtremel', 2007), a mechanism via which disclinations can generate vacancies is not described in , and estimates for the vacancy generation rate are not presented in (Gapontsev et al, 2000;Gapontsev et al, 2002;Gapontsev et al, 2003). Besides, the interaction of vacancy flux in a grain with edge dislocations, which can substantially reduce the vacancy concentration, is not considered, i.e.…”

“…It should be noted that cyclic process of formation and decomposition of an amorphous or intermetallic phase was observed during prolonged ball milling in certain systems (El-Eskandarany et al, 1997;Courtney & Lee, 2005). In these models, grain boundaries (Gapontsev et al, 2000;Gapontsev et al, 2002;Gapontsev et al, 2003) or disclinations (triple grain junctions) can act as vacancy sources when the deformation proceeds via grain boundary sliding and rotational modes. This corresponds to a situation when the size of grains in the particle has reduced to nanometric.…”

“…Thus, models Gapontsev et al, 2000;Gapontsev et al, 2002;Gapontsev et al, 2003) can be considered as incomplete and relating to a distant stage of MA where nanorgains of a solid solution or intermetallic compound have already been formed via a certain physical mechanism which was not considered in these works. An idea of solid solution formation during MA by the "shear-drift diffusion" (Foct, 2004) or "trans-phase dislocation shuffling" (Raabe et al, 2009; has been proposed, which the authors of these works base upon the certain outcomes of atomistic simulations (Bellon & Averback, 1995).…”

“…However, this term was not analyzed in detail. Models for diffusion demixing of a solid solution or intermetallic compound in the course of MA have been developed Gapontsev et al, 2000;Gapontsev et al, 2002;Gapontsev et al, 2003) which consider the formation of non-equilibrium vacancies in grain boundaries and their diffusion into grains. The vacancy flux directed into grains brings about an oppositely directed diffusion flux of solute atoms, which ultimately results in demixing of this stable or metastable phase.…”