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MSE 2001 Exam 3

Terms in this set (189)

At low temp and high stress, deformation of crystals occurs
through slip of lattice planes
Slip planes occur in the
Most densely packed planes
Slip directions occur in
the most densely packed lattice directions
Shear stresses have a maximum value at
planes 45 degrees to the tensile axis
main types of dislocations
screw dislocations
edge dislocations
mixed dislocation
loop dislocations
Burgers vector
A vector that denotes the magnitude and direction of lattice distortion associated with a dislocation.

The slip vector which characterizes a dislocation.

mismatched atom # lengths forming a closed circuit around the dislocation
Screw dislocations are identified by
dislocation line parallel to burger's vector

shear stress is in same direction as burger's vector
what type of dislocation is this?
screw
what type of dislocation is this?
edge
edge dislocations are identified by
the dislocation line perpendicular to burger's vector

shear stress is in the same direction as burger's vector
the picture shows what type of dislocation?
screw
the burgers vector shows what type of dislocation?
edge, noted by the dissimilar # of atoms along parallel vectors
the picture shows what type of dislocation?
screw
the picture shows what type of dislocation?
edge
the picture shows what type of dislocation?
mixed
the dislocation closest to the viewer is a screw dislocation, while the dis. on the side is a edge dislocation
dislocation density
lenght of dislocation line / unit are
OR
#of intersection points /unit are
mean dislocation spacing
d=1/sqrt(disloc.dens)
units of dislocation density
1/M^2
characteristic interaction stress
sig~= Gibs*sqrt(disloc.dens)
characteristics of the atoms in the vicinity of an edge location
region of tension (gap)

region of compression (region above the gap)
consequence of dislocations
internal stresses cause distortions in the lattice resulting in stress strain fields

5% of deformation work is stored as elastic energy of the stress field

only dislocations with the smallest burgers vectors occur

the stress fields decay according to sig~GIBS/r where r is the vertical distance from the dislocation
dislocation loops are formed by
atoms removed from plane and cause a 'loop' to form where the missing atoms were
dislocation climb
dislocations can only move perpendicular to its glide plane if matter is added or removed: 'climb motion'

'climb is possible if diffusion processes lead to transport of matter

climbing allows dislocations to move past obstacles
dislocation climb is usually accomplished through what type of diffusion?
Vacancy
rate of climb depends on:
concentration and diffusivity of lattice vacancies, which are proportional to the 'self-diffusion' rate
equation for climb velocity
v=v0(tow/Gib)^Nc (e^-(Esd/KT)

where Nc is the stress exponent of the climb, Esd is the activation energy for the self diffusion(formation+migration energy of cacancies)
microcracks form by
gliding dislocations which accumulate at the intersection of planes
dislocations in ionic crystals occur because of
insertion of a (single) plane which violates alternating charge requirement and therefore leads to an edge dislocation

insertion plane must be odd # for dislocation to occur because of charge
intrinsic stacking fault
plane missing atom(s)
extrinsic stacking fault
extraneous atom(s) entered between planes
grain boundaries are
boundaries in a material where the orientation of the crystal is no longer uniform; crystals grow until they contact another crystal - forming the grain boundary
tilt boundaries are formed in what type of dislocation
edge dislocations - through the ordered arrangement
twist boundaries are formed in what type of dislocations
screw dislocations
Twin Boundaries
mirror image at boundary
flow stress
stress required to move dislocations

increases with the amount of defects (impurities, solute atoms, etc) because it becomes harder to move dislocations through the crystal
Solute drag
dislocations traped at core of solute atoms, which are therefore dificulte to 'drag' out from material, thereby increasing its strength
strain hardening
Towflow=TOW0+Ksqrt(dis.dens)

Hardening that occurs as a result of deforming a metal. During strain hardening, dislocations are generated and the high dislocation density makes it difficult for other dislocations to move.
dislocations with paralle burgers vectors
repel each other
dislocations with antiparalle burgers vectors
attract ant annhilate each other
workhardening of a material is caused by
interaction of dislocations of grains
Hall-Petch equation
sig0 = sigs + BDg^-1/2

sig0 = yield strength of poly crystaline
sigs = yield S of single crystal
Dg=grain diameter
B is constant
'Glass' is
a material that lacks long range order and is below the glass transition temperature

under impact - breaks
'rubber' is
amorphous solid that is ABOVE THE GLASS TRANSITION.
a portion of molecules behave elastically

under impact - absorbs elastic energy and stretches
glass transition temp
critical temperature that separates glassy behavior from rubbery behavior
what is the phase to the left of the slope, right of slope, in the large wedge, in small wedge
liquid
solid
super-cool liq
glass
volumetric (bulk) therm expansion coefficient
av=(1/v)(dV/dT)
same as coeff. of therm exp. such that av=3ath
viscosity is
n=n0e(Q/RT)
inverse of fluidity (fluidity=n^-1)
vis : A liquid's resistance to flowing
as temperature increases, _____ decreases and _____ increases
viscosity

fluidity
Viscoelastic for polymers
solid like at short time, low temp, high freq, high deform
liq like at long time, high temp, low freq, low deform
degree of polymerization
number of repeting mers linked in covalent bond
only thermoplastic polymers
average molecular weight , types
* number average: Mn= sum(#iWi)/#tot
*sum of # of molecu. of type i multiplied by weight of i, divided by sum of # of molecu. of type i

*weight average molecular wei.: Mw = sum(#iWi^2)/(#i*Wi)
**sum of # of molecu. of type i multiplied by weight of i multiplied by the weight of i again; divided by the sum of the #of i multiplied by the weight of i
weight average VS number average permits
longer and heavier chains to contribute more to the result of calc
polydispersity
pd=Mw/Mn

=weight avg/num weight avg

measure of 'non-uniformity' of a material
polydispersity of 1 means
all molecules have same molecular weight
Tacticity types of polymers
isotactic = single side symetry *best for crystalinity
syndiotactic = mirror symetry *will crystal, not the best
atactic = no symetry *no crystal
the figure displays what type of tacticity
isotactic
the figure displays what type of tacticity
syndiotactic
the figure displays what type of tacticity
atactic
factors affecting polymer crystalinity
size of side groups = bulky groups cannot be tightly packed

extent of chain branching =longer more complex repeat units are less likely to crystalize

tacticity = isotactic is best for crystalinty, atactic will not crystalize

degree of secondary bonding=istactic polar mols can for better crystalline regions than isotactic polymers that do not contribute to secondary bonding
temperature influence on elastic modulus of crystal
increase temp=increase percent crystalinity of material as well as increas in cross-link density
Rubber elasticity
stretching coil by force increases the rate of crystalization. expressed by:
Lextended=ML0sin(theta/2)
where L0 is the distance between polymer link ends
Zachariasen's Rules for Oxide Glass Formation
1. oxide glass networks must be made of an oxide polyhedron
2. CN of EACH oxygen atom in the network MUST be 2
3. the CN of the center metal atom in the polyhedron must be 3 or 4
4. polyhedrons can ONLY share CORNERS
5. EACH polyhedron MUST share 4 CORNERS
does the pictured oxide represent a Zachariasen oxide network?
Yes!
network forming oxides:
usually made by mixing groups 3->6 with S,Se,Te

Oxides satisfying Zakariasen's rules
network modifiers
oxides incapable of forming a glass, but modify propeties of material for different applications(requires less temp, more echonomical to produce). break up 3d primary bonds, cannot form extended primary bond networks,
polymers that are good glass formers are
those with large unit cells, large monomers, or random co-polymers
Tg, the glass trans temp denotes
the temp at which large-scale molecular motion is prohibited, but it is NOT a thermodynamic variable, and is affected by cooling rate
viscosity is used to describe
mechanical behavior above Tg (glass trans temp)
rubbers can only be formed by
polymers because the long chains of polymers give the unique spring properties
Elastomers (rubbers)
Lightly crosslinked polymers with elastic properties which can be stretched many times their original length, and can be recovered from this process completely. the uncoiling of the chains of the meres facilitate the elastic properties of rubber
a single phase is identified by
a chemically and structurally homogeneous region of of a material
a single phase material will have
same composition and structure at EVERY point
a component is
a chemically distinct element/indivisible agent in a subtance
pure fcc Al is an example of
single component single phase
mix of ice and water is an example of
single component 2 phase
a solid solution of Cu and Ni is an example of
2component 1 phase
Extensive variables
variables dependent on the size or amount of the system
intensive variables
independent of the size of the system, (temp, pressure)
Gibbs Phase Rule
F = C - P + 2;
F = dof
C= # components
P= # phases @ equlib

*if pressure is kept constant:
F = C - P + 1
dof for 1 component 2 phases
dof=1-2+2
1,
what is the dof for the phase diagram of water
dof=1-3+2
dof=0
atomic percentage fraction (weight fraction) eq.
at%A=((wt%A)/(atwtA))/sumTOT(wt%i/at.wtAi)
mole fraction
Na=na/(na+nb+...)
such that Na+Nb+...=1
isomorphous systems
only occurs in substitutional alloys

complete solubility of one component into other according to Hume-rothery rules (size must be <15%, similar EN and valence, same crystal struct
the lower line on the graph is the:
the upper is the:
the middle region is:
solidus
liquidus
liquid+solid
leveler rule
by extending a line through the state points and parallel to the compsition axis (x axis) until line intersects the phase boundary

composition of the two phases are given by the horizontal coordinates of the 'tie-line' intersection with the phase boundary
phase fraction
the phase fraction of each phase can be calculated by drawing a line (L) intersecting the temp of interest, the tie line, and a percent composition,
The frac of each is then:
fracA=XB-L/lengthTie
fracB=XA-L/lengthTie
the x axis of a phase diagram represents ____ and the y axis is the
composition
temp
in an ideal situation for two component diagram
clustering occurs when Eab is
ordering occurs when Eab is
Eab=(Eaa+Ebb)/2

greater than avg of Eaa and Ebb

less than avg of Eaa and Ebb
in the figure, what is Xs1, Xl2?
Xs1 is the 1st solid to form
Xl2 is the second liquid
the red dotted line is the
eutectic line
in the Eutectic Phase Diagram, the left 'leaf' closest to alpha contains
L+alpha
for a eutectic phase diagram with 2 different agents (at constant p), what is the dof in the solvus region
dof=2-2+1 = 1
the line outlining the region at top of the eutectic phase diagram is the _____
the lines outlining the side sections are the_____
the area bordering the lower portion of the tie line is the_____
liquidus
solidus
solvus
the figure shows
microstructure at eutectic point
the greater the deviation of a material from hume-rothery rules
will decrease the max solubility of a solid in the primary solution
diffusion occurs along
the liquid/solid interface
the graph is a representation of what type of diagram
eutectic
the graph is a representation of what type of diagram
peritectic
a phase reaction which contains an upper region liquid and 2 side regions of solids

L<-->a+b
eutectic
a phase reaction which contains an upper region solid and 2 side regions of solids

S<-->a+b
eutectoid
a phase reaction which contains an upper region liquid, a mid upper region of liquid + alphaS, and 2 side regions of solids

L+b<-->a
peritectic
a phase reaction which contains an upper region of 2 solids and 2 side regions of solids solidA+mix and solidB+mix

a+b<-->S
peritectoid
driving force for phase change
change in gibbs free energy of system
change in entropy (randomness)
change in enthalpy(internal e)
systems tend to equlibriate towards
lower gibbs free energy
nucleation types
homogeneous = the probability of nucleation occurring at point in the parent phase is same throughout the parent phase

Heterogenous = some areas prefered for nucleation in parent phase
solidification is:
nucleation followed by growth
heterogeneous nucleation
cos(theta)= Elm-Ems/Els

Elm=liquid/mold interface
Ems=liquid/solid interface
Els=mold/solid interface
Coherent interface
An interface separating two phases where there is a one-to-one match of lattice planes across the boundary.
Nucleation process= homogeneous
Interfacial energy=lowest

location of precipitates=throughout
partially coherent interface
coherent interface with periodic dislocations in the interface
Nucleation process = heterogeneous
Interfacial energy=moderate

location of precipitates=@ dislocations
incoherent interface
An interface separating two phases that do not have similar atomic arrangements across the interface.
Nucleation process = Heterogeneous

Interfacial energy=high

location of precipitates=grain boundary
Interfacial energy
the energy associated with the boundary between two grains (phase)

Increases from: coherent< partially coherent<incoherent
growth characteristics
must overcome diffusion barriers
needs incubation time up to nucleation
slow initial growth, rapid proliferation, slow final growth (sigmoidal curve)
graph of growth shows sigmoidal - 3stage growth over time
X=1-e^(-kt^n)
TTT diagram
Time-Temperature-Transformation or Isothermal transformation
y axis=temp,x=time
Hypoeutectoid
Steels whose compositions are to the left of the eutectoid are referred to as hypoeutectoid alloys.
Hypereutectoid
An alloy whose composition is to the right of the eutectoid composition.
proeutectoid ferrite
Ferrite that forms prior to the eutectoid ferrite
proeutectoid cementite
Cementite that forms prior to the eutectoid cementite
In the Fe-cementite graph shown, area 29 is
25 is
39 is
Peritectic
Eutectic
Eutectiod
the large middle region and large lower region of the graph are
Eutectic=austenite+cementite
perlite=ferrite+cementite
figure shows
Austenite (fcc)
figure shows
ferrite (bcc)
figure shows _____ type of steel phase
pearlite from a eutectoid transformation
bainite is
An unstable carbon transformation caused on a supersaturated carbon plate.

Can take two different routs:

one where carbon diffuses into austenite and then the carbide precipitates from austenite into upper bainite (high temp)

or carbon diffuses into austenite and then some carbide precipitates into ferrite, the more carbide precipitates from austenite into lower bainite (low temp)
Martensite
transformation occurs without composition change, and occurs by shear without need for diffusion, through slight movements

shear changes the shape of the transforming region -rusulting in plate-like shape of high shear energy
the amount of martensite formed in a material is a function of
the temp at which the sample was quenched, not time
the hardness of martensite is a function of
carbon content in sample, (but hardness will also contribute to brittleness)
Tempering of Martensite is done to
A heat treatment applied to martensitic steel to reduce brittleness, increase toughness, and increase ductility
austenite crystal structure
FCC iron
Martensite crystal structure
BCT
cooling of athermal martensite
as temp decreases, the fraction of transformed martensite increases
The effect of tempering (for martensites)
tempering temp changes shape of martensite's.

Increase temp, increase size of resulting phase
driving force for transformation is the reduction in surface free energy
in the picture, A is
B is
edge
screw
in the figure, the red solid line is a
dislocation line
in the figure, line A is a
line B is a
Screw
edge
Point defects increase strength through
Defects can migrate to edge dislocations and cause jogs, which makes them denser
Cold work results in an increase in strength because
cold work increases the number of dislocations
the following is an example of
dislocation climb
the following is an example of
tilt boundary
what is the significance of the Frank Read mechanism
It is how dislocations multiply and increase in size
The hall-petch equation is
and is used to describe
and variables are
sig0= sigs + BDg^-1/2

describes the effect of grain size on yield strength

B=strengthening coefficient (constant)
sigS=initial resistance to disl. motion, yield strength of single crystal
sig0=yield strength of material
Dg=grain diameter
as temp increases
Viscosity decreases
what is the degree of polymerization of
4
which of the following is not a type of tacticity?
Orthotactic
Atactic
Syndiotactic
Isotactic
Orthotactic
the C, P, for aluminum is
1 C, 1 P
the C, P, for NaCl (FCC)
C=1, P=1
the C, P, for LaMnO3 is
C=1 P=1
the C, P, for Carbon Steel (Fe + C) (dissolved)
C=2, P=1
for the temp press. phase diagram shown, what is the dof of
region 2
curve 2
point A
2
1
0
what is the dof for the low region
middle region
top line
3
2
3
what is the difference between y-FeC3
and alphaFeC3 in the graph?
they have different crystal structures
Isomorphous phase diagram occurs in
substitutional alloys
What is this point represented
congruent melting point
the arrow points to the

the line by alpha, below the blue dotted line, is the

the line by alpha, above the blue dotted line, is the

the bottom of the line by alpha
eutectic point
solvus
solidus
max solid solubility of B in A
the figure is a represents
hypoeutectic
What are the factors affecting phase transformations
strain
diffusion
interfacial energy
For the isothermal transformation curve, what is on the x axis, y axis
time
temp
what is the structure of the alpha phase (ferrite) of FeC3?
the gamma phase (ferrite)?
BCC
FCC
the following microstructure is
alpha ferrite
the following microstructure is
pearlite
the following microstructure is
Martensite
Strength may increase as a result of
decreasing grain size
adding impurities (lower e of system)
adding precipitates (blocks dislocation motion)
coldworking (increase # dislocations)
which is not a method of strengthening a metal
A. heat & hold @ high temp -then cool quick
B. heat & hold @ high temp -then cool slow
C. hammer or draw the metal while cool
D. add alloying substance to metal
B. heat & hold @ high temp -then cool slow
T/F in edge dislocations, dislocation line is parallel to the motion of the dislocation
False
T/F in screw dislocations, Berger's vector is in same direction as the shear stress
True
T/F in screw dislocations, dislocation line is parallel to the motion of the dislocation
False
flow stress is proportional to
SQRT(dislocationDensity)
why does pure iron become harder when quenched
smaller grains are formed by the rapid cooling causing increased hardness
a paper clip that is repeatedly bent becomes brittle and eventually breaks. What explains this behavior
strain hardening
the diagram represents the phase transition of
crystalline materials
the C, P of a Al-2%SiC solid solution is
C=2
P=1
the C, P of a mix of BCC iron and FCC iron
C=1
P=2
in a 2 C mix with a dof of 1, the max P that can co-exist at constant pressure is
P=2
The Gibbs phase rule for a 2 component system with variable pressure and temp is
dof=C-P+2
==dof+p=2C+2

dof+p=4
what is the dof above the liquidous boundary in a eutectic phase diagram at 1 atm
2
dof=2-1+1
During what reaction upon cooling, does a solid phase turn into a solid plus another solid phase?
a. Peritectoid b. Eutectic c. Eutectoid d. Peritectic
eutectoid
thee solvus boundary separates
a single phase solid with two-phase solid regions
what is the maximum solubility of Ag in Cu in the following figure
7.9%
the eutectoid mixture of steel is
the mixture of cementite and ferrite
which phase of steel is the least strong
Ferrite
according to the leveler rule, what is the fraction of the liquid phase
Xo-Xs/Xl-Xs
medium Carbon steel is
.4-.77%
the 2 metastable phases
martensite
bainite
in BCC iron, as carbon content in the material increases, the alloy becomes more
brittle
microstructure I (green circle)
just above the eutectic temp
Microstructure II (blue and red)
just below the eutectic temp
an athermal, or diffusionaless transformation takes place when austenite is rapidly quenched to 200 degrees. the product above the quenching is
martensite
what is the proper definition of cementite
iron carbide phase, Fe3C
the following is a representative of
brittle fracture
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