5 looking inside materials determining atomic and molecular dimensions

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


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

precipitatesarrays of dislocations

alloying element

dislocation

atoms and electronsSource: 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


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 image of gold 111

AFM of rhodium screw dislocations

Say hallo to “carbon monoxide man” (STM image)

AFM of DNA strand

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 featuresMetals 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'

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

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


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%


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 800temperature / 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) a tom 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 peratu res, m ore atom s becom e ion ised . Theconductiv ity increases because there are m ore charge carriers free to m ove.E ffects o f extra la ttice v ibra tions 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

5 looking inside materials determining atomic and molecular dimensions

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