Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,

DiffusionDiffusion• 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

Predicting Predicting DiffusionDiffusion

• Fick’s first LawJ = -D (J = -D (C/C/xx))Where:

J = Flux

D = Diffusion Coefficient

C/x = Concentration Gradient

• Diffusion CoefficientD = DD = D00 exp (-Q/(RT)) 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

““Imperfections” in the Crystal LatticeImperfections” in the Crystal Lattice

Mechanisms of DiffusionMechanisms of Diffusion• Self Diffusion (pure Self Diffusion (pure

metals)metals)• Vacancy DiffusionVacancy Diffusion• Interstitial DiffusionInterstitial Diffusion

Types of DiffusionTypes of Diffusion• Volume DiffusionVolume Diffusion

• Grain Boundary Grain Boundary DiffusionDiffusion

• Surface DiffusionSurface Diffusion

Volume Diffusion

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

Grain Boundary Diffusion

• Diffusion along the GRAIN BOUNDARY

• More room

• More defects

• FASTER!

Surface diffusion

• Diffusion along a material SURFACE

• Lots of room

• Lots of defects

• FASTEST!!!

Types of DiffusionTypes of Diffusion

• Volume DiffusionVolume Diffusion

• Grain Boundary Grain Boundary DiffusionDiffusion

• Surface DiffusionSurface Diffusion

Factors Affecting DiffusionFactors Affecting Diffusion

• Diffusion Mechanism

• Type of Diffusion

• Crystal Structure

• Bonding

• Temperature

• Ionic Materials

• Polymers

Compare and Contrast

Slip

• Movement of DISLOCATIONS through a crystalline material

• Responsible for plastic deformation

• Affected by:– Crystal structure

– Bonding

– Temperature (since it affects bonding)

Diffusion

• 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

• Tensile specimen is subjected to constant load at elevated temp;

• Specimen will elongate continuously until failure

• Applied stress below yield strength of that material

Creep

Str

ain,

in/i

n

Time, hours

de/dt

I IIIIIFracture

eo

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

Diffusion and Materials ProcessingDiffusion and Materials Processing• Surface Treating

• Grain Growth

• Diffusion Bonding

• Sintering