Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration...
-
Upload
reanna-spoor -
Category
Documents
-
view
219 -
download
5
Transcript of Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration...
![Page 1: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/1.jpg)
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
![Page 2: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/2.jpg)
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
![Page 3: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/3.jpg)
““Imperfections” in the Crystal LatticeImperfections” in the Crystal Lattice
![Page 4: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/4.jpg)
Mechanisms of DiffusionMechanisms of Diffusion• Self Diffusion (pure Self Diffusion (pure
metals)metals)• Vacancy DiffusionVacancy Diffusion• Interstitial DiffusionInterstitial Diffusion
![Page 5: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/5.jpg)
Types of DiffusionTypes of Diffusion• Volume DiffusionVolume Diffusion
• Grain Boundary Grain Boundary DiffusionDiffusion
• Surface DiffusionSurface Diffusion
![Page 6: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/6.jpg)
Volume Diffusion
• Diffusion through VOLUME of crystal– Highest packing eff.– Least amount of defects– SLOWEST!!
![Page 7: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/7.jpg)
Grain Boundary Diffusion
• Diffusion along the GRAIN BOUNDARY
• More room
• More defects
• FASTER!
![Page 8: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/8.jpg)
Surface diffusion
• Diffusion along a material SURFACE
• Lots of room
• Lots of defects
• FASTEST!!!
![Page 9: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/9.jpg)
Types of DiffusionTypes of Diffusion
• Volume DiffusionVolume Diffusion
• Grain Boundary Grain Boundary DiffusionDiffusion
• Surface DiffusionSurface Diffusion
![Page 10: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/10.jpg)
Factors Affecting DiffusionFactors Affecting Diffusion
• Diffusion Mechanism
• Type of Diffusion
• Crystal Structure
• Bonding
• Temperature
• Ionic Materials
• Polymers
![Page 11: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/11.jpg)
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)
![Page 12: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/12.jpg)
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
![Page 13: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/13.jpg)
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
![Page 14: Diffusion Movement of atoms in a material Thermal Energy = Atom Movement Eliminates concentration differences Important for material processing (heat treating,](https://reader035.fdocuments.us/reader035/viewer/2022062712/56649c995503460f9495510f/html5/thumbnails/14.jpg)
Diffusion and Materials ProcessingDiffusion and Materials Processing• Surface Treating
• Grain Growth
• Diffusion Bonding
• Sintering