고체 내 확산 경로 / Diffusion Paths in solid

일반적으로 고체에서의 확산 경로(diffusion path)는 크게 surface diffusion, boundary diffusion, volume (lattice) diffusion으로 구분한다. 같은 고체 내의 확산 속도는 surface, boundary, volume diffusion의 순서가 된다.  

 

High Diffusivity Paths (재료 내에 확산 속도가 빠른 경로)

Grain boundary diffusion

Dislocation & grain size effects

Diffusion along moving boundaries

Surface diffusion and shape change

Surface diffusion data and mechanisms

 

In solids, atoms can move from one location to another through diffusion. Diffusion occurs due to the random thermal motion of atoms, and the rate at which it occurs depends on several factors, including temperature, crystal structure, and the presence of defects in the material.

High diffusivity paths in solids refer to regions within a material where the diffusion rate is significantly higher than in other areas. These paths can be created by various mechanisms, such as grain boundaries, dislocations, or other types of defects in the crystal lattice.

At these high diffusivity paths, atoms can more easily move through the material and can diffuse over larger distances than in other regions of the material. This can have important implications for material properties and behavior. For example, high diffusivity paths in metals can lead to preferential corrosion or crack propagation along these paths.

Understanding the mechanisms that lead to the formation of high diffusivity paths in solids is essential for predicting material behavior and designing materials with specific properties.

 

Analysis of grain boundary diffusion

Grain boundary를 따라 일어나는 확산은 grain 내부를 지나가는 확산에 비해 속도가 일반적으로 빠르다. J.C. Fisher는 이러한 boundary와 volume 내에서의 확산 속도 차이가 크다는 것을 이용하여 확산이 일어나는 방향을 단순화 하여 boundary diffusion을 해석하였다.

 

Grain boundary diffusion에 대한 해석을 위한 모델의 모식도

 

Experimental observations on grain boundary diffusion

-. Grain boundary misorientation

-. Temperature dependence

-. Alloying effects: Alloying elements often segregate at grain boundaries or dislocations.

   This is especially true for elements whose solubility is low.

 

Dislocation & grain size effects

The effects are dependent on the relation between the diffusion distance in the lattice and the separation in the randomly oriented dislocations. An atom interacts with several dislocations in diffusing this far through the lattice. The dislocations increase the effective diffusivity of the solid.

 

Diffusion along moving boundaries

The transport from the surface into the lattice is aided by boundary diffusion. If the boundary moves as diffusion occurs, the mixing rate from the surface into the lattice is significantly increased. It has become clear that in samples containing strong concentration gradients, diffusion along existing grain boundaries can induce the boundaries to move and even induce nucleation and growth of new grains.

 

Surface diffusion and shape change

Many diffusion-controlled phenomena involve a change of shape and are driven by surface tension or applied stresses too low to move dislocations.

-. Creep at low stresses in polycrystalline materials can occur by diffusion through the lattice (Nabarro-Herring creep), or along grain boundaries (Coble creep).

-. The sintering of powders occurs by diffusion (lattice or boundary) from regions of sharp curvature to low curvature.

Analysis of surface smoothing

Surface diffusion data and mechanisms

Surface diffusion coefficient measurements are less accurate than volume diffusion, the most prominent reason being the difficulty of controlling the wide range of surface structures.

 

 

References

Paul Shewmon, Diffusion in Solid, 2nd edition