Transcript Pt/SrTiO3 Interface - Nc State University

The World of Atoms
Instructor:
Office:
Office Hours:
Dr. Gerd Duscher
http://www4.ncsu.edu/~gjdusche
email: [email protected]
2156 Burlington Nuclear Lab.
Tuesday: 10-12pm
Host:
Roman Slyth (919) 233 4593
Objective today: How material deforms ?
What makes a metal hard ?
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Literature
The New Science of Strong Materials or
Why You Don't Fall Through the Floor
by James Edward Gordon
Paperback: 288 pages ; Dimensions (in inches): 0.78 x 7.98 x 5.09
Publisher: Princeton University Press; Reissue edition (May 1, 1984)
ISBN: 0691023808
Structures: Or
Why Things Don’t Fall Down
by J. E. Gordon
Paperback: 424 pages ; Dimensions (in inches): 1.13 x 8.40 x 5.82
Publisher: Da Capo Press; Reprint edition (July 8, 2003)
ISBN: 0306812835
List Prices: about $20
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What properties does that imply?
• bond length, r
F
• melting temperature, Tm
Energy (r)
F
r
• bond energy, Eo
ro
r
Energy (r)
smaller T m
unstretched length
ro
r
Eo =
“bond energy”
larger T m
Tm is larger if Eo is larger.
Summary: Primary Bonds
Ceramics
(Ionic & covalent bonding):
Metals
(Metallic bonding):
Polymers
(Covalent & Secondary):
large bond energy
large Tm
large E
small a
variable bond energy
moderate Tm
moderate E
moderate a
directional Properties
van der Waals bonding dominates
small T
small E
large a
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Types of Imperfections
• Vacancy atoms
• Interstitial atoms
• Substitutional atoms
• Anti-site defects
Point defects
(0 dimensinal)
• Dislocations
Line defects
(1 dimensional)
• Grain Boundaries
Area defects
(2dimensional)
2
How do materials deform?
That is what happens when pulling wires.
• before deformation
• after tensile elongation
slip steps
Dislocation move, more dislocation get generated and
entangle (interact) with themselfs, and other defects.
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Incremental Slip
• Dislocations slip planes incrementally...
• The dislocation line (the moving red dot)...
...separates slipped material on the left
from unslipped material on the right.
push
Simulation of dislocation
motion from left to right
as a crystal is sheared.
fixed
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How Does it Look?
b
Atomic Structure of an Edge Dislocation
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Screw and Mixed Dislocations
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Tensile Strength, Ts
• Maximum possible engineering stress in tension.
TS
Adapted from Fig. 6.11,
engineering
stress
Callister 6e.
Typical response of a metal
strain
• Metals: occurs when noticeable necking starts.
• Ceramics: occurs when crack propagation starts.
• Polymers: occurs when polymer backbones are
aligned and about to break.
Tensile Strength: Comparison
Metals/
Alloys
Tensile strength, TS (MPa)
5000
3000
2000
1000
300
200
100
40
30
20
Graphite/
Ceramics/ Polymers
Semicond
Composites/
fibers
C fibers
Aramid fib
E-glass fib
Steel (4140) qt
Diamond
W (pure)
Ti (5Al-2.5Sn)aa
Steel (4140)
Si nitride
Cu (71500) cw
Cu (71500) hr Al oxide
Steel (1020)
Al (6061) ag
Ti (pure)a
Ta (pure)
Al (6061) a
Si crystal
<100>
Glass-soda
Concrete
Graphite
AFRE(|| fiber)
GFRE(|| fiber)
CFRE(|| fiber)
Nylon 6,6
PC PET
PVC
PP
HDPE
wood(|| fiber)
GFRE( fiber)
CFRE( fiber)
AFRE( fiber)
LDPE
10
wood( fiber)
1
TS(ceram)
~TS(met)
~ TS(comp)
>> TS(poly)
Room T values
Based on data in Table B4,
Callister 6e.
a = annealed
hr = hot rolled
ag = aged
cd = cold drawn
cw = cold worked
qt = quenched & tempered
AFRE, GFRE, & CFRE =
aramid, glass, & carbon
fiber-reinforced epoxy
composites, with 60 vol%
fibers.
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Ductility, %EL
L f  Lo
x100
• Plastic tensile strain at failure: %EL 
Lo
Engineering
tensile
stress, s
smaller %EL
(brittle if %EL<5%)
larger%EL
(ductile if
%EL>5%)
Lo
Ao
Af
Lf
e
Engineering tensile strain,
Ao  A f
• Another ductility measure: %AR 
x100
Ao
• Note: %AR and %EL are often comparable.
--Reason: crystal slip does not change material volume.
--%AR > %EL possible if internal voids form in neck.
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Toughness
• Energy to break a unit volume of material
• Approximate by the area under the stress-strain
curve.
Engineering
tensile
stress, s
smaller toughness (ceramics)
larger toughness
(metals, PMCs)
smaller toughnessunreinforced
polymers
Engineering tensile strain, e
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Hardness
• Resistance to permanently indenting the surface.
• Large hardness means:
--resistance to plastic deformation or cracking in
compression.
--better wear properties.
e.g.,
10mm sphere
apply known force
(1 to 1000g)
D
most
plastics
brasses
Al alloys
measure size
of indent after
removing load
Smaller indents
mean larger
hardness.
d
easy to machine
steels
file hard
cutting
tools
nitrided
steels
diamond
increasing hardness
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Bond Breaking And Remaking
• Dislocation motion requires the successive bumping
of a half plane of atoms (from left to right here).
• Bonds across the slipping planes are broken and
remade in succession.
push
Atomic view of edge
dislocation motion from
left to right as a crystal
is sheared.
fixed
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4 Strategies For Strengthening: 1:
Reduce Grain Size
• Grain boundaries are
barriers to slip.
• Barrier "strength"
increases with
misorientation.
• Smaller grain size:
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slip plane
grain A
more barriers to slip.
• Hall-Petch Equation:
s yield  so  k y d 1/ 2
7
Dislocation Motion in Polycrystals
• Slip planes & directions
(l, f) change from one
crystal to another.
• tR will vary from one
crystal to another.
• The crystal with the
largest tR yields first.
• Other (less favorably
oriented) crystals
yield later.
300 mm
6
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Strengthening Strategy 2:
Solid Solutions
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• Impurity atoms distort the lattice & generate stress.
• Stress can produce a barrier to dislocation motion.
• Smaller substitutional
impurity
• Larger substitutional
impurity
A
C
B
Impurity generates local shear at
A and B that opposes disl motion
to the right.
D
Impurity generates local shear at
C and D that opposes disl motion
to the right.
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Strengthening Strategy 3:
Precipitation Strengthening
• Hard precipitates are difficult to shear.
Ex: Ceramics in metals (SiC in Iron or Aluminum).
precipitate
Large shear stress needed
to move dislocation toward
precipitate and shear it.
Side View
Top View
Unslipped part of slip plane
S
Slipped part of slip plane
1
• Result: s y ~
S
Dislocation
“advances” but
precipitates act as
“pinning” sites with
spacing S.
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Simulation:
Precipitation Strengthening
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• View onto slip plane of Nimonic PE16
• Precipitate volume fraction: 10%
• Average precipitate size: 64 b (b = 1 atomic slip distance)
14
Application:
Precipitation Strengthening
• Internal wing structure on Boeing 767
Adapted from Fig.
11.0, Callister 5e.
(Fig. 11.0 is
courtesy of G.H.
Narayanan and A.G.
Miller, Boeing
Commercial
Airplane Company.)
• Aluminum is strengthened with precipitates formed
by alloying.
1.5mm
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Strengthening Strategy 4:
Cold Work (%Cw)
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• Room temperature deformation.
• Common forming operations change the cross
sectional area:
-Forging
force
-Rolling
die
Ao
roll
Ao
Ad
Ad
roll
-Drawing
die
Ao
die
force
Ad
-Extrusion
Ao
tensile
force
force
container
ram
billet
container
Ao  Ad
%CW 
x100
Ao
die holder
extrusion
die
Ad
Dislocations During Cold Work
• Ti alloy after cold working:
• Dislocations entangle
with one another
during cold work.
• Dislocation motion
becomes more difficult.
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Simulation: Dislocation
Motion/Generation
• Tensile loading (horizontal dir.) of a FCC metal with
notches in the top and bottom surface.
• Over 1 billion atoms modeled in 3D block.
• Note the large increase in disl. density.
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Dislocation-dislocation Trapping
• Dislocation generate stress.
• This traps other dislocations.
Red dislocation
generates shear at
pts A and B that
opposes motion of
green disl. from
left to right.
A
B
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Impact of Cold Work
• Yield strength (sy ) increases.
• Tensile strength (TS) increases.
• Ductility (%EL or %AR) decreases.
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Cold Work Analysis
• What is the tensile strength &
ductility after cold working?
pro2  pr d2
%CW 
x100  35.6%
pro2
yield strength (MPa)
Copper
Cold
work
----->
Do=15.2mm
tensile strength (MPa)
60
700
800
500
600
40
400 340MPa
20
300
300MPa
100
0
20
Cu
40
% Cold Work
sy=300MPa
60
200
0
20
Cu
40
% Cold Work
60
TS=340MPa
Dd=12.2mm
ductility (%EL)
Cu
7%
00
20
40
60
% Cold Work
%EL=7%
s-e Behavior vs Temperature
800
Stress (MPa)
• Results for
polycrystalline iron:
600
-200°C
-100°C
400
25°C
200
0
0
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0.1
0.2
0.3
Strain
0.4
• sy and TS decrease with increasing test temperature.
• %EL increases with increasing test temperature.
3. disl. glides past obstacle
• Why? Vacancies
2. vacancies
help dislocations
replace
atoms on the
past obstacles.
obstacle
disl. half
plane
1. disl. trapped
by obstacle
0.5
Effect Of Heating After %Cw
• 1 hour treatment at Tanneal...
decreases TS and increases %EL.
Annealing Temperature (°C)
100
300
500
700
60
600
tensile strength
50
500
40
400
30
ductility
300
20
ductility (%EL)
tensile strength (MPa)
• Effects of cold work are reversed!
• 3 Annealing
stages to
discuss...
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Recovery
Annihilation reduces dislocation density.
• Scenario 1
• Scenario 2
extra half-plane
of atoms
atoms
diffuse
to regions
of tension
extra half-plane
of atoms
3. “Climbed” disl. can now
move on new slip plane
2. grey atoms leave by
vacancy diffusion
allowing disl. to “climb”
1. dislocation blocked;
can’t move to the right
Disl.
annhilate
and form
a perfect
atomic
plane.
tR
4. opposite dislocations
meet and annihilate
obstacle dislocation
25
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RECRYSTALLIZATION
• New crystals are formed that:
--have a small disl. density
--are small
--consume cold-worked crystals.
0.6 mm
0.6 mm
Adapted from
Fig. 7.19 (a),(b),
Callister 6e.
(Fig. 7.19 (a),(b)
are courtesy of
J.E. Burke,
General
Electric
Company.)
33% cold
worked
brass
New crystals
nucleate after
3 sec. at 580C.
26
Further Recrystallization
• All cold-worked crystals are consumed.
0.6 mm
After 4
seconds
0.6 mm
After 8
seconds
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Low-Angle Grain Boundary
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High-Angle Grain Boundary
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NiAl plane
Ni plane
Model 7
Z-Contrast Images of S5 Copper Grain
Boundaries with and without Bismuth
M.F. Chisholm
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Cu in Al Grain Boudnary
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Characterization of Grain
Boundaries
a = 36°
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a
b
c
b a c b
Stacking Fault
a
b
c
Result of Grain Boudnaries
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