The World of Atoms

1. The World of Atoms NCSU Instructor: Dr. Gerd Duscher http://www4.ncsu.edu/~gjdusche email: gerd_duscher@ncsu.edu Office:2156 Burlington Nuclear Lab. Office Hours:Tuesday: 10-12pm Host: Roman Slyth (919) 233 4593 Objective today: How material deforms ? What makes a metal hard ?

2. 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

3. What properties does that imply? • bond length, r • melting temperature, Tm Energy (r) F F r • bond energy, Eo r o r Energy (r) smaller T m unstretched length larger T m r o r E = Tm is larger if Eo is larger. o “bond energy”

4. Summary: Primary Bonds Ceramics large bond energy large Tm large E small a (Ionic & covalent bonding): Metals variable bond energy moderate Tm moderate E moderate a (Metallic bonding): Polymers directional Properties van der Waals bonding dominates small T small E large a (Covalent & Secondary): secondary bonding 18

5. Types of Imperfections • Vacancy atoms • Interstitial atoms • Substitutional atoms •Anti-site defects Point defects (0 dimensinal) Line defects (1 dimensional) Area defects (2dimensional) • Dislocations • Grain Boundaries 2

6. 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. How do materials deform?

7. 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

8. How Does it Look? b

9. Atomic Structure of an Edge Dislocation

10. Screw and Mixed Dislocations

11. Tensile Strength, Ts • Maximum possible engineering stress in tension. TS Adapted from Fig. 6.11, Callister 6e. stress engineering 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.

12. Tensile Strength: Comparison Graphite/ Metals/ Composites/ Ceramics/ Polymers Alloys fibers Semicond 5 000 C fibers Aramid fib E-glass fib 3 000 2 0 00 qt Steel (4140) (MPa) A FRE (|| fiber) Diamond 10 00 W (pure) GFRE (|| fiber) a Ti (5Al-2.5Sn) C FRE (|| fiber) a Steel (4140) cw Si nitride Cu (71500) hr Cu (71500) Al oxide Steel (1020) ag 3 00 Al (6061) a Room T values Ti (pure) 2 00 Ta (pure) a Al (6061) 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. Si crystal wood(|| fiber) 100 <100> Nylon 6,6 strength, TS Glass-soda PET PC PVC GFRE ( fiber) 4 0 Concrete PP C FRE ( fiber) 3 0 A FRE( fiber) H DPE Graphite 2 0 L DPE 1 0 Tensile wood ( fiber) 1

13. Ductility, %EL • Plastic tensile strain at failure: smaller %EL Engineering (brittle if %EL<5%) tensile A A f o s stress, L larger %EL o L f (ductile if %EL>5%) e Engineering tensile strain, • Another ductility measure: • Note: %AR and %EL are often comparable. --Reason: crystal slip does not change material volume. --%AR > %EL possible if internal voids form in neck. 19

14. Toughness • Energy to break a unit volume of material • Approximate by the area under the stress-strain curve. smaller toughness (ceramics) E ngineering tensile larg er toughness s stress, (metals, PMCs) smaller toughness- unreinforced polymers e Engineering tensile strain, 20

15. Hardness • Resistance to permanently indenting the surface. • Large hardness means: --resistance to plastic deformation or cracking in compression. --better wear properties. apply known force measure size e.g., (1 to 1000g) of indent after 10mm sphere removing load Smaller indents d D mean larger hardness. most brasses easy to machine cutting nitrided plastics Al alloys steels file hard tools steels diamond increasing hardness 21

16. 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

17. 4 Strategies For Strengthening: 1: Reduce Grain Size • Grain boundaries are barriers to slip. • Barrier "strength" increases with misorientation. • Smaller grain size: more barriers to slip. • Hall-Petch Equation: slip plane grain B grain A grain boundary 7

18. Dislocation Motion in Polycrystals • Slip planes & directions (l, f) change from one crystal to another. • tRwill vary from one crystal to another. • The crystal with the largest tR yields first. • Other (less favorably oriented) crystals yield later. 300 mm 6

19. Strengthening Strategy 2: Solid Solutions • Impurity atoms distort the lattice & generate stress. • Stress can produce a barrier to dislocation motion. • Smaller substitutional impurity • Larger substitutional impurity A C D B Impurity generates local shear at A and B that opposes disl motion to the right. Impurity generates local shear at C and D that opposes disl motion to the right. 11

20. 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 Side View to move dislocation toward precipitate and shear it. Uns lipped part of slip plane Dislocation Top View “advances” but precipitates act as “pinning” sites with S spacing S . S lipped part of slip plane • Result:

21. Simulation:Precipitation Strengthening • View onto slip plane of Nimonic PE16 • Precipitate volume fraction: 10% • Average precipitate size: 64 b (b = 1 atomic slip distance) 14

22. Application:Precipitation Strengthening 1.5mm • 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.

23. Strengthening Strategy 4: Cold Work (%Cw) die die • Room temperature deformation. • Common forming operations change the cross sectional area: force -Forging -Rolling roll die A d A A A o o d roll force -Drawing -Extrusion A o container A d die holder tensile force A o ram billet A extrusion d force die container

24. Dislocations During Cold Work • Ti alloy after cold working: • Dislocations entangle with one another during cold work. • Dislocation motion becomes more difficult.

25. 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.

26. Dislocation-dislocation Trapping • Dislocation generate stress. • This traps other dislocations. Red dislocation generates shear at A pts A and B that opposes motion of green disl. from B left to right.

27. Impact of Cold Work • Yield strength (s) increases. • Tensile strength (TS) increases. • Ductility (%EL or %AR) decreases. y

28. Cold Work Analysis 2 2 p - p r r o d = = % CW x 100 35 . 6 % 2 p r o • What is the tensile strength & ductility after cold working? Copper Cold work -----> D =15.2mm D =12.2mm o d tensile strength (MPa) yield strength (MPa) ductility (%EL) 6 0 8 00 7 00 4 0 6 00 5 00 Cu 300MPa Cu 3 00 2 0 4 00 340MPa Cu 7% 100 2 00 0 0 2 0 4 0 6 0 0 2 0 4 0 6 0 0 2 0 4 0 6 0 % Cold Work % Cold Work % Cold Work s TS=340MPa =300MPa %EL =7% y

29. s-e Behavior vs Temperature • Results for polycrystalline iron: 8 00 -200°C 6 00 -100°C 4 00 Stress (MPa) 25 °C 200 0 0 0.1 0.2 0.3 0.4 0.5 Strain • sy and TS decrease with increasing test temperature. • %EL increases with increasing test temperature. • Why? Vacancies help dislocations past obstacles. 3 . disl. glides past obstacle 2. vacancies replace atoms on the obstacle disl. half 1. disl. trapped plane by obstacle

30. Effect Of Heating After %Cw • 1 hour treatment at Tanneal... decreases TS and increases %EL. • Effects of cold work are reversed! Annealing Temperature (°C) 1 00 3 00 5 00 700 6 0 6 00 tensile strength 5 0 • 3 Annealing stages to discuss... 5 00 strength (MPa) 4 0 ductility (%EL) 4 00 3 0 ductility 2 0 tensile 300 Recovery Grain Growth Recrystallization

31. Recovery Annihilation reduces dislocation density. • Scenario 1 extra half-plane of atoms Disl. annhilate atoms and form diffuse a perfect to regions atomic of tension plane. extra half-plane of atoms • Scenario 2 t 3 . “Climbed” disl. can now R move on new slip plane 2 . grey atoms leave by 4 . opposite dislocations vacancy diffusion meet and annihilate allowing disl. to “climb” obstacle dislocation 1. dislocation blocked; can’t move to the right 25

32. 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

33. Further Recrystallization • All cold-worked crystals are consumed. 0.6 mm 0.6 mm After 8 seconds After 4 seconds

34. Low-Angle Grain Boundary

35. High-Angle Grain Boundary NiAl plane Ni plane Model 7

36. Z-Contrast Images of S5 Copper Grain Boundaries with and without Bismuth M.F. Chisholm

37. Cu in Al Grain Boudnary

38. a = 36° Characterization of Grain Boundaries

39. a b c b a c b a b c Stacking Fault

40. Result of Grain Boudnaries