5 looking inside materials determining atomic and molecular dimensions oexplain how an stm, afm and...
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5 LOOKING INSIDE MATERIALSDetermining atomic and molecular dimensions
o Explain how an STM, AFM and SEM work
o Determine resolution, magnification and atomic dimensions from microscope data
o Estimate molecular size from experimental data
Cotton wool SEM 150x Space shuttle tile SEM 2000x
Looking inside glasses
cracks
human scale
fracture strength
flow
stiffnessthermal conduction
semiconductionoptical
Si Si
1 m
1 mm
1 m
1 nm
1 pm
random atomic packing
atoms
Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.
cell wall fibre composite
human scale
Looking inside woods
thermal
strengthstiffness
strengthstiffnessthermal
strengthstiffnessthermal
1 m
1 mm
1 m
1 nm
1 pm
cellular structure
cellulose fibres
Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.
cellulose molecules
human scale
Looking inside metals and ceramics
stiffnesselectricalopticalthermal
yield strength(metals)
yield strength(metals)
fracturestrength(ceramics)
XXXX
crack
1 m
1 mm
1 m
1 nm
1 pm
precipitates
arrays of dislocations
alloying element
dislocation
atoms and electrons
Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.
human scale
Looking inside polymers
electrical
stiffnessthermaloptical
flow
strength
XXX X
H
C
H
H
C H
C
H
H
C
H
H
C
H
H
C
C
H
H
C
H
H
C
H
H
C
H
C
C
C
1 m
1 mm
1 m
1 nm
1 pm
craze
tangled molecules
molecules
atoms
Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.
World’s smallest advertisement: STM of xenon atoms
STM of iron on copper
STM of iron on copper: “The atomic corral”
Making the corral
STM of metal surface showing instrumentally-induced distortion of atom shapes
AFM
AFM image of gold 111
AFM of rhodium screw dislocations
Say hallo to “carbon monoxide man” (STM image)
AFM of DNA strand
SEM
SEM of fruit fly head. Be afraid........... Be very afraid..........
SEM of solar spider Will he catch the fruit fly?
SEM of ant
SEM of snowflake
Fracture behaviour
o Learn how to calculate fracture energy
o Distinguish between strength and toughness in terms of fracture behaviour of materials
o Explain why metals are tough
Energy stored in stretched materialEnergy stored = area under graph
= ½ x F x e
= ½ x (k x e) x e
= ½ x k x e2
Intensive measurement of stored energy
Energy stored per unit volume in elastic region
Evol = ½ x stress x strain
Generally:
Evol = area under stress strain graph
Fracture energy and tensile strength
tensileforce
tensileforce
cross-sectionalarea
energy of increasedlattice vibration
energy offlying fragments
energy to createvoids in material
energy to createnew surface area
energy of soundin material
energy to move atomsaround (e.g. slip)
Large fracture energy = tough Large tensile strength = strong
fracture energy =total energy used to fracture
specimen cross-sectional area
tensile strength =breaking force
specimen cross-sectional area
strongbrittle
strongtough
weakbrittle
weaktough
glass bonenylonwood
1 10 100 1000 10000 100000
WEAK
fracture energy / J m–2
STRONG
TOUGH
10
1
100
1000
BRITTLE
brick stone
high tensilesteel
mildsteel
epoxy resinpolyester
Strong and tough
Cracks and stress
Material is bent,upper surfacestretchedlower surfacecompressed.
h
Crack deflectstensile stress.Stressconcentratedbelow crack.
with a crack
Contours ofequal tensilestress. Largeststress nearsurface.
no cracks
Under tensile stress, cracks propagate through materials.
very small areaat tip of crack:stress verylarge here
bond between atomsat tip highly stressed bond breaks
next bond is stressed
Fracture surfaces in metals
Which shows ductile fracture, and which shows brittle fracture?
Fracture of CFRP in a tennis racquet
ductile metal flows, crack blunted
Stopping cracks propagating
Metals resist cracking because they are ductile. Cracks are broadened and blunted. They do not propagate.
Metals are tough because they are ductile
Metals
stress stress stress stress
high stress stress reduced
Bone mechanical properties
• Density 1500 kg m-3
• Young’s modulus 17 GPa
• Strength (compressive) 180 MPa(tensile) 150 MPa
Composite materials
• Know the meaning of the term composite material
• For a range of composite materials (ferroconcrete, bone, CFRP etc.), explain how creating a composite can improve on the properties of the individual components
Starter: Give 2 reasons why metals have a largeplastic region and undergo ductile fracture.
Now give 2 reasons why glasses undergo brittle fracture with no plastic region.
Starter answers
Metals undergo ductile fracture because:1. Regular structure allows
planes of atoms to slip over each other (and allow dislocations , which we shall meet later, to move)
2. Non-directional metallic bonding allows metal to change shape in the region of highest stress, without fracturing.
Glasses undergo brittle fracture because:1. The bonding is highly
directional between ions, and can only respond to stresses by bond-breaking
2. The amorphous (random) nature of the glass’s structure does not allow planes of atoms to slip over each other, as there are no definable, ordered planes of atoms as in a metal.
Fibre-reinforced materials use a matrix to share stress amongst many strong fibres. The matrix also protects the fibresfrom cracks forming.
Fibre-reinforcement
Fibre-reinforced materials are tough because cracks can't propagate through the soft matrix
one fibre breaks, stress takenup by other fibres
strong fibre
stress stress stress stress
soft matrixsticks to fibres
Composite materials• Investigate properties of composite materials based on ice
Starter: Q1. Write 2 column headings, STEEL and CONCRETE.
Q2. Assign each of the following properties to the correct material. Some may be used for both, some not at all.TOUGH STIFF STRONG IN COMPRESSION HARD BRITTLE DENSE STRONG IN TENSION SOFT HIGH FRACTURE ENERGY LOW FRACTURE ENERGY
Q3. Show, on a 2-D strength against toughness plot, where concrete and steel would lie.
Q4. Explain why concrete might be unsuitable for the beams of a road bridge.Q5. Explain why steel on its own might be unsuitable for the same application.Q6. How might you exploit the properties of both materials to solve the problem?
Metal microstructures
• Research and illustrate the various atomic-scale features of metals
• Explain their effect on the properties of metals
Starter: Brainstorm all of the properties of a typical metal. How does the atomic structure and bonding in a metal account for these properties?
Metal microstructural features
Metals are normally polycrystalline. Research the meaning ofthis term. What affects the size of crystal grains in a polycrystalline material?
Research, illustrate and explain the effect of the following microstructural features:GRAIN BOUNDARIESDISLOCATIONSVACANCIESINTERSTITIALSSUBSTITUTIONAL IMPURITIES
Modifying properties of metals
Research each of the following methods of treating metals.
• Describe what is involved in the treatment process.• State how the mechanical properties of the metal are
altered.• Explain in terms of the metal microstructure why the
properties are altered.
ALLOYINGWORK HARDENINGANNEALINGTEMPERING AND QUENCHING
Grain boundaries
A dislocation: an incomplete row of atoms
Vacancies, interstitials and substitutional impurities
Metal microstructures
• Explain the effects that micro structural features have on the properties of metals
Shaping and slippingAtoms in gold are in a regular array: a crystal lattice. To shape the metal, onelayer must be made to slide over another.
To slip, layer ofatoms mustmove as a whole
Layer hasmoved oneatomic spacing
Atoms canmove oneby one
All atoms move:layer moves
Dislocationreachesedge ofcrystal
atommoves
dislocationmoves
One atommoves:dislocationmoves
in both a layer has slipped by one atomic spacing
dislocation
dislocation free to move: slip occurs easily
Pure crystal Alloy
alloy atom pins dislocation: slip is more difficult
dislocation pinneddislocation
Alloys are generally less ductile than pure metals
Questions on modifying the properties of metals
1. Draw diagrams to illustrate the following:(a) the pinning of a dislocation by a foreign atom(b) a large substitutional impurity atom in a crystal(c) an interstitial atom 2. What common effect(s) on the metal’s properties do all of the
modifications described in Q1 have? 3. How can excessive work hardening of a metal be reduced? 4. A metal contains large crystal grains. How could you change the
crystal grain size to create smaller grains? 5. Now try Questions 70X from Folio Views
Heat treatment of steel
• Investigate and explain how various heat treatments of steel can affect its properties
Stiffness and elasticity
• Explain stiffness and elasticity in metals, ceramics and polymers
Starter: The stress-strain graph for rubber is shown on the right. Rubber shows very elastic behaviour. Explain how you can tell this from the graph.
What would the stress-strain graph for a typical metal look like if you stressed it until you were in the plastic region, then took the load away?
Ceramics versus metals
Ceramics have rigid structures
Covalent structures
example: silica (also diamond, carborundum)
Covalent bonds share electrons between neighbouratoms. These bonds are directional: they lock atomsin place, like scaffolding.
The bonds are strong: silica is stiff
Atoms are linked in a rigid giant structure
oxygen atom
joins to others
silicon atom
The atoms cannot slip: silica is hard and brittle
Metals have non-directional bonds
Metallic structures
example: gold
Atoms in metals are ionised. The free electronsmove between the ions. The negative charge of theelectrons 'glues' the ions together. But the ions caneasily change places.
The bonds are strong: metals are stiff
The ions can slip: metals are ductile and tough
Ions are held together, but can move
negativeelectron 'glue'
gold ion
++ + + + + +
+ + + + + + +
+ + + + + + +
+ + + + + + +
Ionic structures
example: common salt
chlorine ion
sodium ion
Ionic bonds pass electrons from one atom to another.Because like charges repel and unlike charges attract,the charged ions hold each other in place.
The bonds are strong: salt crystals are stiff
The ions cannot slip: salt crystals are hard and brittle
Ions are linked in a rigid giant structure
+
+
+
+–
– –
– –
Summary
Ceramics covalent or ionic
Metals
strong, rigid scaffolding,stiff, hard, brittle
mobile strong electron glue,stiff, ductile, tough
Explaining stiffness and elasticity
Metals
++ + + + + +
+ + + + + + +
+ + + + + + +
+ + + + + + +
a metal is an array ofpositive ions bondedby negative electron'glue'
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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stretching has to pull bonds apart
Elastic extensibility ~ 0.1% Young modulus
~1011 — 1012 Pa
Stretching a metal stretches bonds — but not much.
gaps open up a little
Explaining stiffness and elasticity
Polythene
polythene is along flexiblechain moleculewhich folds up
bondrotates
stretching can rotate some bonds,making the folded chain longer
Young modulus
~108 — 109 Pa
chains arefolded
bondrotates
Stretching polythene rotates bonds
Elastic extensibility ~ 1%
Explaining stiffness and elasticity
Stiffer polymers
Young modulus
~1010 Pa
Young modulus
~109 — 1010 Pa
Polystyrene hasbenzene ringssticking out sideways.They make chainrotations difficult.
Bakelite hasmassively cross-linked chains. Thecross-links stopthe chains fromunfolding.
Polystyrene Bakelite – a thermoset
Explaining plasticity
polythene strip 10 mm 100 mm thin crystalline strip ‘pulled outof’ wider region
Polythene is semi-crystalline. Think of polytheneas like cooked spaghetti. In amorphous regionsthe chains fold randomly. In crystalline regionsthe chains line up.
When stretched plastically, the chains slip pasteach other. More of the material has lined-upchains. More of it is crystalline.
Plastic extensibility > 100%
Polythene
‘neck’
crystalline amorphous new crystallineregion
Rubber
Rubber stretches and contracts by chains uncoiling and coiling up again. Rubber is elastic, not plastic.
In stretched rubber, the chain bonds rotate, and chainsfollow straighter paths between cross-links. When let go,the chains fold up again and the rubber contracts.
In unstretched rubber, chains meander randomlybetween sulphur cross-links.
sulphur cross-linkssulphur cross-links
Elastic extensibility > 100%
Comparisons of materials of different classes(metals, ceramics, polymers) See p112-114 and the summary table on p118. Q1. Give an example of a material with (a) giant covalent structure; (b) an ionic structure; (c) metallic structure Q2. Explain why ceramics, salts and metals are all stiff, butonly metals are ductile and tough Q3. Why are polymers generally much less stiff than metals? Q4. How can some polymers be made stiffer? Q5. Why does rubber get stiffer the more it is stretched?
Electrical conductivity
• Investigate and explain the temperature dependence of the conductivity in metals, semiconductors and insulators
• copper.swf• nichrome.swf
temperature / K
200 400 600 800
logarithmicscale
silicon
108
alloy
Conduction by metals and semiconductors
104
1
10–4
10–8
pure m eta l
Metal and alloy
200 400 600 800
temperature / K
10
5
0
linearscale
Observation Metals conduct very well.
Explanation All the atoms in the metal are ionised. The ‘spare’ electrons arefree to move.
Explanation No more electrons become free to move. Moving electrons scatterfrom the vibrating lattice – so move a little less freely as the temperature risesand lattice vibrations increase (impurities and defects also reduce the conductivity).
Observation The conductivity of a metal decreases a little as it gets warmer.
‘soup’ of freeelectrons
all atoms ionised
++ + + + + +
+ + + + + + +
+ + + + + + +
+ + + + + + +
alloy
pure m eta l
temperature / K
200 400 600 800
logarithmicscale
silicon
108
alloy
104
1
10–4
10–3
pure metal
temperature / K
silicon
300 350
10
5
0
Silicon(part of temperature range only)
Observation Seminconductors conduct much less well than metals, muchmore than insulators.
Explanation O nly a few (1 in 1012) atom s are ion ised. There are only these fewelectrons free to m ove.
Observation The conductivity of a pure semiconductor increases dramaticallyas it gets warmer.
Explanation A t h igher tem peratures, m ore atom s becom e ionised. Theconductiv ity increases because there are m ore charge carriers free to m ove.E ffects of extra la ttice v ibrations are m uch sm aller.
linearscale
rare freeelectrons
+occasionalatoms ionised
+
temperature / K
200 400 600 800
logarithmicscale
silicon
108
alloy
104
1
10–4
10–8
pure metal
Chapter 5 consolidation
Q1. Sketch stress-strain graphs for low-carbon and high-carbon steels on the same set of axes.
Q2. Describe in words the differences between low- and high-carbon steels referring to stiffness, strength, ductility etc.
Q3. Explain the differences in terms of how the added carbon atoms are incorporated into the structure and the effects this has.
Q3. Complete the glossary exercise on material properties