Diffusion

1
Diffusion

• Movement of atoms in a material
• Thermal Energy Atom Movement
• Eliminates concentration differences
• Important for material processing (heat treating,
  solidification, etc.)
• Arrhenius Relationship used to predict rate

2
Predicting Diffusion

• Ficks first Law
• J -D (DC/Dx)
• Where
• J Flux
• D Diffusion Coefficient
• DC/Dx Concentration Gradient

• Diffusion Coefficient
• D D0 exp (-Q/(RT))
• Where
• D diffusion coefficient
• Q Activation Energy
• R Gas Constant (1.987 cal/mol.K)
• T Absolute Temp, K (C 273)
• D0 Constant for diffusion system

3
Imperfections in the Crystal Lattice
4
Mechanisms of Diffusion

• Self Diffusion (pure metals)
• Vacancy Diffusion
• Interstitial Diffusion

5
Types of Diffusion

• Volume Diffusion
• Grain Boundary Diffusion
• Surface Diffusion

6
Volume Diffusion

• Diffusion through VOLUME of crystal
• Highest packing eff.
• Least amount of defects
• SLOWEST!!

7
Grain Boundary Diffusion

• Diffusion along the GRAIN BOUNDARY
• More room
• More defects
• FASTER!

8
Surface diffusion

• Diffusion along a material SURFACE
• Lots of room
• Lots of defects
• FASTEST!!!

9
Types of Diffusion

• Volume Diffusion
• Grain Boundary Diffusion
• Surface Diffusion

10
Factors Affecting Diffusion

• Diffusion Mechanism
• Type of Diffusion
• Crystal Structure
• Bonding
• Temperature
• Ionic Materials
• Polymers

11
Compare and Contrast

• Slip

• Diffusion

• Movement of DISLOCATIONS through a crystalline
  material
• Responsible for plastic deformation
• Affected by
• Crystal structure
• Bonding
• Temperature (since it affects bonding)

• Movement of ATOMS in a material (can be
  crystalline or amorphous)
• Eliminates concentration differences
• Affected by
• Mechanism and type
• Temperature
• Bonding
• Material structure (crystal or amorphous)

12
Creep

• Tensile specimen is subjected to constant load
  at elevated temp
• Specimen will elongate continuously until failure
• Applied stress below yield strength of that
  material

13
Creep and Dislocation Climb

• Movement of dislocation perpendicular to its slip
  plane by diffusion of atoms to or from the
  dislocation line
• Vacancies must move to or from dislocations to
  cause plastic strain
• Dislocations escape from lattice imperfections,
  continue to slip and causes additional
  deformation of specimen even at low applied
  stress
• Diffusion controlled phenomenon
• Arrhenius Relationship
• creep rate K s n exp (Q c / R T)
• R gas constant
• T temp, K
• c, K, n material constants
• Q Activation energy related to self diffusion
  when dislocation climb is important

14
Diffusion and Materials Processing

• Surface Treating
• Grain Growth
• Diffusion Bonding
• Sintering