Quiz 3

Question 1

atoms tend to migrate from regions of…

Answer 1

high concentration to low concentration

Question 2

Vavancy diffusion

Answer 2

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

Question 2

self diffusion

Answer 2

migration of host atoms in pure metals

Question 3

interstitial diffusion

Answer 3

small interstitials move from one interstitial position to another
-more rapid than vacancy diffusion b/c space already available

Question 4

doping

Answer 4

diffusion of very small concentrations of atoms of an impurity (e.g. P) into semiconductor silicon

Question 5

steady state diffusion

Answer 5

-rate of diffusion (flux) indep. of time
-Flux (J) proportional to concentration gradient
-Fick’s first law

Question 6

non-steady state diffusion

Answer 6

-concentration is a function of time and position (depends on time and position)
-Fick’s second Law

Question 7

Diffusion coefficient D is dependent on..

Answer 7

temperature
-D has exponential dependence on T

Question 8

elastic deformation

Answer 8

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

Question 9

Plastic deformation

Answer 9

permanent and nonrecoverable
-stress strain curve is nonlinear

Question 10

yield strength

Answer 10

stress at which noticeable plastic deformation has occured

Question 11

tensile strength (TS)

Answer 11

max stress on engineering stress-strain curve (highest stress value)

Question 12

ductility

Answer 12

amount of plastic deformation at failure

Question 13

resilience

Answer 13

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

Question 14

toughness

Answer 14

-amount of energy absorbed before fracture
- total area under stress strain curve

Question 15

hardness

Answer 15

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

Question 16

stiffness

Answer 16

material’s resistance to elastic deformation

Question 17

strength

Answer 17

a material’s resistance to plastic deformation
-yield and tensile strengths

Question 18

slip

Answer 18

plastic deformation by dislocation motion (edge, screw, mixed)
-atomic bonds broken and reformed along slip plane as dislocation moves
-caterpillar analogy

Question 19

edge dislocation motion

Answer 19

moves in direction of shear stress tau/parallel

Question 20

screw dislocation motion

Answer 20

moves perpendicular to shear stress tau

Question 21

dislocation characteristics for metals

Answer 21

-dislocation motion relatively easy due to non-directional metallic bonding
-occurs on close packed planes in packing direction

Question 22

dislocation characteristics for ceramics (covalently bonded)

Answer 22

-relatively difficult dislocation motion due to strong covalent bonding directions
-ex. silicon, diamond

Question 23

dislocation characteristics for ceramics (ionic bonding)

Answer 23

-relatively difficult dislocation motion
-few slip systems because motion of nearby ions of like charge (+, -) restricted by electrostatic repulsive forces
-ex. NaCl, MgO

Question 24

slip system

Answer 24

-combination of slip plane and slip direction

Question 25

slip plane

Answer 25

-plane with high planar density
-Close packed plane!

Question 26

slip direction

Answer 26

-direction with high linear density
-most atoms touching
-packing direction

Question 27

FCC Slip System

Answer 27

-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

Question 28

When does slip occur for single crystals?

Answer 28

when resolved shear stress > critical resolved shear stress

Question 29

single crystal slip

Answer 29

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

Question 30

slip in polycrystallines materials

Answer 30

-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)

Question 31

before rolling polycrystalline materials

Answer 31

-grains equiaxed and randomly oriented
-properties isotropic (independent of orientation)

Question 32

-after rolling polycrystalline materials

Answer 32

-grains elongated in rolling direction
-properties become somewhat anisotropic (dependent on orientation)

Question 33

strengthening mechanisms for metals

Answer 33

1. reduce grain size
2. solid-soln strengthening
3. strain hardening

Question 34

reducing grain size

Answer 34

-increases grain boundary area (barrier to dislocation motion)
-more barriers to dislocation motion
-increases yield strength, tensile strength, and hardness

Question 35

solid soln strengthening (introduce small substitutional impurities in compressive region)

Answer 35

-small substitutional impurities introduce tensile strains
-when locate above slip line for edge dislocation, partial cancellation of tensile and compressive strains

Question 36

solid soln strengthening (introduce large substitutional impurities in tensile region)

Answer 36

-when located below slip line, there is partial cancellation of compressive and tensile stresses

Question 37

strain hardening (cold working)

Answer 37

-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

Question 38

heat treating/softening/annealing

Answer 38

-heat treating cold-worked metals
-decreases TS, increases ductility (%EL)
3 stages
1. Recovery
2. Recrystallization
3. Grain growth

Question 39

recovery

Answer 39

-reduction in dislocation density
-annihilation of dislocations

Question 40

recrystallization temperature

Answer 40

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

Question 41

recrystallization

Answer 41

new grains form that…
-have low dislocation densities
-are small in size
-consume and replace cold-worked grains

Question 42

cold vs hot working

Answer 42

-hot working: deformation above T_r
-cold working: deformation beclow T_r

Question 43

grain growth

Answer 43

-avg. grain size increases
- small grains shrink and ultimately disappear
-large grains continue to grow

Question 43

metals with small grains

Answer 43

strong and tough at low temps

Question 44

metals with large grains

Answer 44

good creep resistance at high temps
(creep=tendency of metal to undergo slow deformation when under persistent mechanical stresses)