materials exam 2

Diffusion

Mass transport by atomic motion

Solids Diffusion (vacancy diffusion)

• substitutional

• self diffusion: atomic migration in a pure metal

Solids (interstitial diffusion)

• impurity diffusion

• diffusion of atoms of one material into another material

Diffusion Mechanism 1: Vacancy diffusion

• substitutional

• atoms and vacancies exchange positions

• applies to host and subs impurity atoms

• diffusion rate depends on # of vacancies & activation energy to exchange

Diffusion Mechanism 2: Interstitial Diffusion

• small atoms move from one interstitial position to an adjacent one

• more rapid than vacancy diffusion

Self- Diffusion

• migration of host atoms in pure metals

• labeled atoms moving to another position

Interdiffusion (impurity)

• atoms tend to migrate from high concentration to low concentration

Case Hardening

• outer surface selectively hardened by diffusing carbon atoms into surface

• carbon atoms makes iron harder

• improves wear resistance of gear & fatigue failure

Rate of diffusion (J)

• usually time dependent

• rate of mass transfer

• Measure mass of diffusing species (M) that pass through the sheet over time period (t)

Steady State Diffusion

• flux is independent of time

• J flux proportional to concentration gradient

Effect of Temp on Diffusion 1

Diffusion coefficient increases with T

Non-steady diffusion

• the concentration of diffusing species is a function of time and position (C= c(x,t))

• the eqn assumes D is independent of concentration

chp 7: Metals Dislocation Characteristics

• quite deformable

• dislocation motion: relatively easy

• metallic bonding- non-directional

• close-packed planes & directions for slip

Ceramics Dislocation characteristics (Covalent Bonding)

• fragile and much less deformable

• Dislocation motion difficult

• covalent bonding- directional

Ceramics Dislocation characteristics (Ionic Bonding)

• dislocation motion: relatively difficult

• few slip systems

Plastic Deformation by Dislocation motion

• called slip too

• applied shear stress can cause extra half-plane of atoms

• atomic bonds broken and reformed along slip plane as dislocation moves

Dislocation motion

movement of extra plane of atoms by breaking/reforming interatomic bonds

Motion of Dislocations

• Edge dislocation line parallel to direction of applied shear stress

• screw dislocation line perpendicular to direction of applied shear stress

Single Crystal Slip

• parallel slip steps form on surface of crystal

• result from motion of large #s of dislocations on same slip plane

Critical Resolved Shear Stress

• crystal doesn’t start deforming until shear stress on a specific plane reaches ciritcal value

• the applied stress needed depends on orientation

• slip occurs when it reaches the cirtical value → plastic deformation

• in a single crystal: multiple slips/ variety orientations

Polycrystalline Materials- Slip

• many grains, random crystallographic orientations

• slip orientation: vary from grain to grain