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

Predicting Diffusion Fick’s first Law J = -D (  C/  x) Where: J = Flux D = Diffusion Coefficient  C/  x = Concentration Gradient Diffusion Coefficient D = D 0 exp (-Q/(RT)) Where: D = diffusion coefficient Q = Activation Energy R = Gas Constant (1.987 cal/mol.K) T = Absolute Temp, K (C +273) D 0 = Constant for diffusion system

“Imperfections” in the Crystal Lattice

Mechanisms of Diffusion Self Diffusion (pure metals)Self Diffusion (pure metals) Vacancy DiffusionVacancy Diffusion Interstitial DiffusionInterstitial Diffusion

Types of Diffusion Volume DiffusionVolume Diffusion Grain Boundary DiffusionGrain Boundary Diffusion 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 Diffusion Volume DiffusionVolume Diffusion Grain Boundary DiffusionGrain Boundary Diffusion Surface DiffusionSurface Diffusion

Factors 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 Strain, in/in Time, hours de/dt IIIIII Fract ure eoeo

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 Processing Surface Treating Grain Growth Diffusion Bonding Sintering