5 LOOKING INSIDE MATERIALS Determining atomic and molecular dimensions

1. 5 LOOKING INSIDE MATERIALSDetermining atomic and molecular dimensions • Explain how an STM, AFM and SEM work • Determine resolution, magnification and atomic dimensions from microscope data • Estimate molecular size from experimental data

2. Space shuttle tile SEM 2000x Cotton wool SEM 150x

6. World’s smallest advertisement: STM of xenon atoms

7. STM of iron on copper

8. STM of iron on copper: “The atomic corral”

9. Making the corral

10. STM of metal surface showing instrumentally-induced distortion of atom shapes

11. AFM

13. AFM image of gold 111

14. AFM of rhodium screw dislocations

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

16. AFM of DNA strand

17. SEM

18. SEM of fruit fly head. Be afraid........... Be very afraid..........

19. SEM of solar spider Will he catch the fruit fly?

20. SEM of ant

21. SEM of snowflake

22. Fracture behaviour • Learn how to calculate fracture energy • Distinguish between strength and toughness in terms of fracture behaviour of materials • Explain why metals are tough

23. 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 e2

24. 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

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

32. Fracture of CFRP in a tennis racquet

34. Bone mechanical properties • Density 1500 kg m-3 • Young’s modulus 17 GPa • Strength (compressive) 180 MPa (tensile) 150 MPa

35. 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.

36. Starter answers Metals undergo ductile fracture because: • Regular structure allows planes of atoms to slip over each other (and allow dislocations , which we shall meet later, to move) • Non-directional metallic bonding allows metal to change shape in the region of highest stress, without fracturing. Glasses undergo brittle fracture because: • The bonding is highly directional between ions, and can only respond to stresses by bond-breaking • 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.

38. 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?

40. 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?

41. 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

42. 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

44. Grain boundaries

45. A dislocation: an incomplete row of atoms

46. Vacancies, interstitials and substitutional impurities

47. Metal microstructures • Explain the effects that micro structural features have on the properties of metals

50. 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