Quiz 3 Flashcards
atoms tend to migrate from regions of…
high concentration to low concentration
Vavancy diffusion
atoms and vacancies exchange positions
-occur above 0 K
-applies to host and substitutional impurity atoms
-diffusion rate depends on 1. number of vacancies and 2. activation energy to exchange
self diffusion
migration of host atoms in pure metals
interstitial diffusion
small interstitials move from one interstitial position to another
-more rapid than vacancy diffusion b/c space already available
doping
diffusion of very small concentrations of atoms of an impurity (e.g. P) into semiconductor silicon
steady state diffusion
-rate of diffusion (flux) indep. of time
-Flux (J) proportional to concentration gradient
-Fick’s first law
non-steady state diffusion
-concentration is a function of time and position (depends on time and position)
-Fick’s second Law
Diffusion coefficient D is dependent on..
temperature
-D has exponential dependence on T
elastic deformation
nonpermanent, reversible (material will bounce back)
-generally valid at small deformations
-linear stress strain curve
-young’s modulus/modulus of elasticity, E
-hooke’s law
Plastic deformation
permanent and nonrecoverable
-stress strain curve is nonlinear
yield strength
stress at which noticeable plastic deformation has occured
tensile strength (TS)
max stress on engineering stress-strain curve (highest stress value)
ductility
amount of plastic deformation at failure
resilience
ability of a material to absorb energy during elastic deformation
-modulus of resilience, U_r
-U_r is area under stress strain curve until yielding
toughness
-amount of energy absorbed before fracture
- total area under stress strain curve
hardness
measure of resistance to surface plastic deformation, dents, or scratches
-large hardness means high resistance to compressive deformation, better wear properties
-small indents mean large hardness
stiffness
material’s resistance to elastic deformation
strength
a material’s resistance to plastic deformation
-yield and tensile strengths
slip
plastic deformation by dislocation motion (edge, screw, mixed)
-atomic bonds broken and reformed along slip plane as dislocation moves
-caterpillar analogy
edge dislocation motion
moves in direction of shear stress tau/parallel
screw dislocation motion
moves perpendicular to shear stress tau
dislocation characteristics for metals
-dislocation motion relatively easy due to non-directional metallic bonding
-occurs on close packed planes in packing direction
dislocation characteristics for ceramics (covalently bonded)
-relatively difficult dislocation motion due to strong covalent bonding directions
-ex. silicon, diamond
dislocation characteristics for ceramics (ionic bonding)
-relatively difficult dislocation motion
-few slip systems because motion of nearby ions of like charge (+, -) restricted by electrostatic repulsive forces
-ex. NaCl, MgO
slip system
-combination of slip plane and slip direction
slip plane
-plane with high planar density
-Close packed plane!
slip direction
-direction with high linear density
-most atoms touching
-packing direction
FCC Slip System
-slip system for FCC is {111} <110>
-dislocation motion on {111} planes
-dislocation motion in <110> directions
-total of 12 independent slip systems for FCC
When does slip occur for single crystals?
when resolved shear stress > critical resolved shear stress
single crystal slip
parallel slip steps form on surface of single crystal
-steps result from motion of large numbers of dislocations on same slip plane
-slip steps on single crystals sometimes appear as “slip lines”
-parallel slip planes
slip in polycrystallines materials
-many grains with random directions
-slip plane orientation and slip directions (lamda, phi) vary from grain to grain
-slip occurs in each grain on the most favorable slip system (largest resolved shear stress, or when resolved shear stress> Tcrss)
before rolling polycrystalline materials
-grains equiaxed and randomly oriented
-properties isotropic (independent of orientation)
-after rolling polycrystalline materials
-grains elongated in rolling direction
-properties become somewhat anisotropic (dependent on orientation)
strengthening mechanisms for metals
- reduce grain size
- solid-soln strengthening
- strain hardening
reducing grain size
-increases grain boundary area (barrier to dislocation motion)
-more barriers to dislocation motion
-increases yield strength, tensile strength, and hardness
solid soln strengthening (introduce small substitutional impurities in compressive region)
-small substitutional impurities introduce tensile strains
-when locate above slip line for edge dislocation, partial cancellation of tensile and compressive strains
solid soln strengthening (introduce large substitutional impurities in tensile region)
-when located below slip line, there is partial cancellation of compressive and tensile stresses
strain hardening (cold working)
-plastically deforming metals at room temp makes them harder and stronger
-deformation: often reduction in cross-sectional area
-as %CW increases, so do yield strength, tensile strength. ductility decreases
-CW increases dislocation density. dislocation motion hindered by presence of more dislocations
heat treating/softening/annealing
-heat treating cold-worked metals
-decreases TS, increases ductility (%EL)
3 stages
1. Recovery
2. Recrystallization
3. Grain growth
recovery
-reduction in dislocation density
-annihilation of dislocations
recrystallization temperature
temp at which deformed grains are replaced by new grains
-depends on %CW: T_r decreases with increasing %Cw
-depends on purity of metal: T_r decreases with increasing purity
recrystallization
new grains form that…
-have low dislocation densities
-are small in size
-consume and replace cold-worked grains
cold vs hot working
-hot working: deformation above T_r
-cold working: deformation beclow T_r
grain growth
-avg. grain size increases
- small grains shrink and ultimately disappear
-large grains continue to grow
metals with small grains
strong and tough at low temps
metals with large grains
good creep resistance at high temps
(creep=tendency of metal to undergo slow deformation when under persistent mechanical stresses)
