5 LOOKING INSIDE MATERIALS Determining atomic and molecular dimensions oExplain how an STM, AFM and SEM work oDetermine resolution, magnification and atomic dimensions from microscope data oEstimate molecular size from experimental data

Cotton wool SEM 150x Space shuttle tile SEM 2000x

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 spiderWill he catch the fruit fly?

SEM of ant

SEM of snowflake

Fracture behaviour oLearn how to calculate fracture energy oDistinguish between strength and toughness in terms of fracture behaviour of materials oExplain why metals are tough

Energy stored in stretched material Energy stored = area under graph = ½ x F x e = ½ x (k x e) x e = ½ x k x e 2

Intensive measurement of stored energy Energy stored per unit volume in elastic region E vol = ½ x stress x strain Generally: E vol = area under stress strain graph

Fracture surfaces in metals Which shows ductile fracture, and which shows brittle fracture?

Fracture of CFRP in a tennis racquet

Bone mechanical properties Density 1500 kg m -3 Young’s modulus17 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 large plastic 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.

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 of this term. What affects the size of crystal grains in a polycrystalline material? Research, illustrate and explain the effect of the following microstructural features: GRAIN BOUNDARIES DISLOCATIONS VACANCIES INTERSTITIALS SUBSTITUTIONAL 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. ALLOYING WORK HARDENING ANNEALING TEMPERING 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

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?

Comparisons of materials of different classes (metals, ceramics, polymers) See p 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, but only 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

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