CHAPTER 8: DEFORMATION AND STRENGTHENING MECHANISMS

1. CHAPTER 8: DEFORMATION AND STRENGTHENING MECHANISMS ISSUES TO ADDRESS... • Why are dislocations observed primarily in metals and alloys? • How are strength and dislocation motion related? • How do we increase strength? • How can heating change strength and other properties? 1

2. DISLOCATION MOTION Plastically stretched zinc single crystal. • Produces plastic deformation, • Depends on incrementally breaking bonds. • If dislocations don't move, deformation doesn't happen! 3

3. DISLOCATIONS & MATERIALS CLASSES • Metals: Disl. motion easier. -non-directional bonding -close-packed directions for slip. electron cloud ion cores • Covalent Ceramics (Si, diamond): Motion hard. -directional (angular) bonding • Ionic Ceramics (NaCl): Motion hard. -need to avoid ++ and -- neighbors. 2

4. DISLOCATIONS & CRYSTAL STRUCTURE • Structure: close-packed planes & directions are preferred. view onto two close-packed planes. • Comparison among crystal structures: FCC: many close-packed planes/directions; HCP: only one plane, 3 directions; BCC: none • Results of tensile testing. Mg (HCP) tensile direction Al (FCC) 6

5. STRESS AND DISLOCATION MOTION • Crystals slip due to a resolved shear stress, tR. • Applied tension can produce such a stress. slip plane normal, ns slip direction slip direction slip direction 7

6. CRITICAL RESOLVED SHEAR STRESS • Condition for dislocation motion: • Crystal orientation can make it easy or hard to move disl. 8

7. DISL. 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 9

8. 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: 10

9. GRAIN SIZE STRENGTHENING: AN EXAMPLE • 70wt%Cu-30wt%Zn brass alloy • Data: 0.75mm 11

10. ANISOTROPY INsyield • Can be induced by rolling a polycrystalline metal -after rolling -before rolling rolling direction 235 mm -isotropic since grains are approx. spherical & randomly oriented. -anisotropic since rolling affects grain orientation and shape. 12

11. 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 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. 14

12. EX: SOLID SOLUTIONSTRENGTHENING IN COPPER • Tensile strength & yield strength increase w/wt% Ni. • Empirical relation: • Alloying increases sy and TS. 15

13. STRENGTHENING STRATEGY : COLD WORK (%CW) • Room temperature deformation. • Common forming operations change the cross sectional area: -Forging -Rolling -Drawing -Extrusion 16

14. DISLOCATIONS DURING COLD WORK • Ti alloy after cold working: • Dislocations entangle with one another during cold work. • Dislocation motion becomes more difficult. 17

15. RESULT OF COLD WORK • Dislocation density (rd) goes up: Carefully prepared sample: rd ~ 103 mm/mm3 Heavily deformed sample: rd ~ 1010 mm/mm3 • Ways of measuring dislocation density: 40mm OR • Yield stress increases as rd increases: 18

16. DISLOCATION-DISLOCATION TRAPPING • Dislocation generate stress. • This traps other dislocations. 20

17. IMPACT OF COLD WORK • Yield strength (s ) increases. • Tensile strength (TS) increases. • Ductility (%EL or %AR) decreases. y 21

18. COLD WORK ANALYSIS • What is the tensile strength & ductility after cold working? 22

19. s-e BEHAVIOR VS TEMPERTURE • Results for polycrystalline iron: • sy and TS decrease with increasing test temperature. • %EL increases with increasing test temperature. • Why? Vacancies help dislocations past obstacles. 23

20. EFFECT OF HEATING AFTER %CW • 1 hour treatment at Tanneal... decreases TS and increases %EL. • Effects of cold work are reversed! • 3 Annealing stages to discuss... 24

21. RECOVERY Annihilation reduces dislocation density. • Scenario 1 • Scenario 2 25

22. RECRYSTALLIZATION • New crystals are formed that: --have a small disl. density --are small --consume cold-worked crystals. 0.6 mm 0.6 mm 33% cold worked brass New crystals nucleate after 3 sec. at 580C. 26

23. FURTHER RECRYSTALLIZATION • All cold-worked crystals are consumed. 0.6 mm 0.6 mm After 8 seconds After 4 seconds 27

24. GRAIN GROWTH • At longer times, larger grains consume smaller ones. • Why? Grain boundary area (and therefore energy) is reduced. 0.6 mm 0.6 mm After 8 s, 580C After 15 min, 580C coefficient dependent on material and T. • Empirical Relation: exponent typ. ~ 2 elapsed time grain diam. at time t. 28

27. Example Using the diagram, compute the time for the average grain size to increase form 0.01-mm to 0.1-mm at (a) 500 oC and (b) 600 oC

28. TENSILE RESPONSE: BRITTLE & PLASTIC 29

29. PREDEFORMATION BY DRAWING • Drawing... --stretches the polymer prior to use --aligns chains to the stretching direction • Results of drawing: --increases the elastic modulus (E) in the stretching dir. --increases the tensile strength (TS) in the stretching dir. --decreases ductility (%EL) • Annealing after drawing... --decreases alignment --reverses effects of drawing. • Compare to cold working in metals! Adapted from Fig. 15.12, Callister 6e. (Fig. 15.12 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.) 30

30. THERMOPLASTICS VS THERMOSETS • Thermoplastics: --little cross linking --ductile --soften w/heating --polyethylene (#2) polypropylene (#5) polycarbonate polystyrene (#6) • Thermosets: --large cross linking (10 to 50% of mers) --hard and brittle --do NOT soften w/heating --vulcanized rubber, epoxies, polyester resin, phenolic resin 31

31. TENSILE RESPONSE: ELASTOMER CASE • Compare to responses of other polymers: --brittle response (aligned, cross linked & networked case) --plastic response (semi-crystalline case) 32

32. SUMMARY • Dislocations are observed primarily in metals and alloys. • Here, strength is increased by making dislocation motion difficult. • Particular ways to increase strength are to: --decrease grain size --solid solution strengthening --precipitate strengthening --cold work • Heating (annealing) can reduce dislocation density and increase grain size. 33