Diffusion

DIFFUSION: 

DIFFUSION

Introduction: 

Introduction If two pieces of different metals are joined together as shown here - for example, copper and nickel, and they are then heated for a long time (but below their melting points), the atoms from the metals migrate, or diffuse into the other.

Mechanisms of Diffusion: 

Mechanisms of Diffusion At an atomic level, atoms are arranged in a lattice pattern, e.g. as shown simply in the diagram. Diffusion is just the stepwise migration of atoms from lattice site to lattice site.

Mechanisms of Diffusion: 

Mechanisms of Diffusion A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry. Patterns are located upon the points of a lattice , which is an array of points repeating periodically in three dimensions.

Mechanisms of Diffusion: 

Mechanisms of Diffusion Substitutional or Vacancy Diffusion

Mechanisms of Diffusion: 

Mechanisms of Diffusion Substitutional or Vacancy Diffusion

Mechanisms of Diffusion: 

Mechanisms of Diffusion Interstitial Diffusion

Mechanisms of Diffusion: 

Mechanisms of Diffusion Interstitial Diffusion

Steady-State Diffusion: 

Steady-State Diffusion Diffusion is a time - dependent process, and often it is necessary to know how fast it occurs, or the rate of mass transfer. This rate is known as the diffusion flux , J, and is defined as the mass, M, diffusing through a unit cross - sectional area of solid, per unit of time.

Steady-State Diffusion: 

Steady-State Diffusion Where A is the area across which diffusion is occuring , and t is the elapsed diffusion time. If the diffusion flux does not change with time, a steady state condition exists, and this is called steady – state diffusion .

Non – Steady State Diffusion: 

Non – Steady State Diffusion This is the case when the diffusion flux depends on time, which means that a type of atoms accumulates in a region or that it is depleted from a region (which may cause them to accumulate in another region).

Factors That Influence Diffusion: 

Factors That Influence Diffusion There is a barrier to diffusion created by neighboring atoms that need to move to let the diffusing atom pass. Thus, atomic vibrations created by temperature assist diffusion. Also, smaller atoms diffuse more readily than big ones, and diffusion is faster in open lattices or in open directions. Similar to the case of vacancy formation, the effect of temperature in diffusion is given by a Boltzmann factor: D = D 0 × exp(– Q d / kT ).

Other Diffusion Paths: 

Other Diffusion Paths Diffusion occurs more easily along surfaces, and voids in the material (short circuits like dislocations and grain boundaries) because less atoms need to move to let the diffusing atom pass. Short circuits are often unimportant because they constitute a negligible part of the total area of the material normal to the diffusion flux.