ELFT HÍRADÓ

[ERDELYI Zoltan <zerdelyi AT dragon.unideb.hu>]

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* Debreceni Egyetem, Szilárdtest Fizika Tanszék *
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* SZEMINÁRIUM *
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Időpont: 2012. február 24., péntek, 11:00 óra
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Helyszín: Szilárdtest Fizika Tanszék, Előadó terem
-------- Debrecen, Bem tér 18/b.

PROGRAM:
------- Reactive diffusion and stresses in spherical geometry

ERDÉLYI Zoltán

Debreceni Egyetem
Szilárdtest Fizika Tanszék


We have developed a complete set of analytical equations to describe 
reactive diffusion in spherical core shell nanostructures. [Acta Mater 
2012,60:2817] The model takes into account elastic stress, its plastic 
relaxation, as well as possible non-equilibrium vacancy densities. 
Furthermore, thermodynamic driving forces are included to model 
formation of intermetallic product phases in intermediate composition 
range. The complex model can be considered as Stephenson’s model [[Acta 
Metal 1988;36:2663] on spherical geometry. Using our derivation, 
however, even the original equations for the planar case may be deduced 
in an easier way than in the original work by Stephenson.

We apply the model to the reaction in spherical triple layers A/B/A and 
B/A/B, for which Schmitz et al. [Acta Mater 2009;57:2673] observed by 
atom probe tomography that growth rate depends on the stacking order. 
The inhomogeneous stress field induces an inhomogeneous distribution of 
vacancies. The resulting vacancy gradient provides an additional driving 
force that hinders the outward flux of vacancies. As a consequence, 
diffusion of the faster component towards the center can only proceed by 
slow Nernst–Planck interdiffusion instead of conventional fast Darken 
transport.

The model calculations provide evidence that the experimentally observed 
growth asymmetry can only appear if vacancy sinks and sources are 
present at the incoherent interphase boundaries but not within the 
formed intermetallic product layer.

The developed model will presumably also be useful in clarifying the 
role of stress in today’s popular core shell nanostructures.

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