Post on 14-Dec-2015
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)
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• 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
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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