Diffusion 

It is defined as, "The transportation of one atom from one state into other state".

• Used for hardening of surfaces like die parts, gear, bolt, machine element.
• Hardening can be done using Temperature-Heat treatment (diffusion couple).

There are two types of diffusion:

1. Inter-diffusion  ↔ in which atoms tend to migrate from region of high concentration to low concentration.
2. Self-diffusion  ↔  in which atoms migrate within a solid.

Diffusion Mechanism

There can be two conditions of moving atoms

1. Empty spaces between adjacent sites.
2. Atoms have sufficient energy to break bond with neighbors atoms which cause lattice distortion during displacement.

If we rise temperature, small fractions of atoms is capable of diffusive motion (which cause lattice distortion).

Types of Diffusion

1. Vacancy Diffusion

It is defined as, "interchange of an atom from a normal lattice position to the adjacent vacant lattice site".

• Extent of vacancy diffusion is a function of number of vacancy defect.

2. Interstitial Diffusion 

It is defined as, "atom migrate from an interstitial position to a neighboring one that is empty".

• Mostly hydrogen, carbon, nitrogen, oxygen are used to fit into interstices.
• In metal alloys, Interstices Diffusion  > Vacancy Diffusion.
• Probability of interstitial atomic movement is greater than for vacancy diffusion.

Steady-State Diffusion

It is defined as, "the quantity of an element that is transported within another as a function of time".

• Diffusion Flux (J)  ↔  The rate at which diffusion occurs (mass transfer) perpendicular to a unit cross-sectional area of solid.

Fick's First Law relates concentration gradient and diffusion flux, which is given by:

Non-Steady State Diffusion

It is defined as, "variation of diffusion flux and concentration gradient at some particular point in a solid with time".

• Concentration of diffusion mass is a function of time C = C(x, t).

Fick's Second Law deals with non-steady diffusion by introducing partial differential equation for concentration profiles which is given by:

Factors Affecting Diffusion

1. Diffusion Species  ↔  Deals with diffusion flux which tells us the rate at which atoms are diffused over a surface.
2. Temperature  ↔  Diffusion coefficient increases with temperature and dependence of diffusion coefficients is:

References:

• Material from Class Lectures + Book named Materials Science and Engineering: An Introduction by Callister and Rethwick + my knowledge. 
• Pics and GIF from Google Images.  
• Videos from YouTube.