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Bruce Mayer, PE Registered Electrical & Mechanical Engineer BMayer@ChabotCollege

Engineering 45. Dislocations & Strengthening (2). Bruce Mayer, PE Registered Electrical & Mechanical Engineer BMayer@ChabotCollege.edu. Learning Goals. Understand Why DISLOCATIONS observed primarily in METALS and ALLOYS Determine How are Strength and Dislocation Motion Related

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Bruce Mayer, PE Registered Electrical & Mechanical Engineer BMayer@ChabotCollege

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  1. Engineering 45 Dislocations &Strengthening (2) Bruce Mayer, PE Registered Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

  2. Learning Goals • Understand Why DISLOCATIONS observed primarily in METALS and ALLOYS • Determine How are Strength and Dislocation Motion Related • Techniques to Increase Strength • Understand How can HEATING and/or Cooling change strength and other properties Last Time

  3. 2-Phase Metal Strengthening • Last Time Studied SINGLE-Phase (ONE Xtal Structure) Strengthening • Grain Size Reduction • Solid Solution Alloying • Strain Hardening • This Time Examine TWO Phase (2) Strengthening by the Formation of Solid Precipitates • a.k.a., Precipitation Hardening

  4. Adjustment of Alloy Composition and/or Processing can Produce a (more or less) Uniform Distribution of Small 2nd Phase Particles (Precipitates) within the Base-Alloy matrix Strengthen-4 → Precipitates CuAl2 Precipitates Within an Al Matrix

  5. 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 PreCip Strength Mechanism • Empirical Relation: σy ~ 1/S

  6. Internal wing structure on Boeing 767 1.5mm App  PreCip Strengthening • Aluminum is strengthened with precipitates formed by alloying.

  7. View onto slip plane of Nimonic PE16 45Ni-31Fe-16.5Cr-3.5Mo-1.2Ti-1.2Al-1Co Precipitate volume fraction: 10% Average precipitate size: 64 b (b = 1 atomic slip distance) Simulation: Precip Hardening

  8. Tensile Tests on PolyXtal Iron 800 -200C 600 -100C Stress (MPa) 400 25C 200 0 0 0.1 0.2 0.3 0.4 0.5 Strain σ-εBehavior vs. Temperature • σy and TS DEcrease with INcreasing test temperature. • %EL INcreases with INcreasing test temperature. • Note Trends

  9. 800 -200C 600 -100C Stress (MPa) 400 25C 200 0 0 0.1 0.2 0.3 0.4 0.5 Strain 3 . disl. glides past obstacle 2. vacancies replace atoms on the obstacle disl. half 1. disl. trapped plane by obstacle σ-εBehavior vs. Temperature • Why thisBehavior? • IncreasedVacancyConcentration • Mechanism: • Vacancies help dislocationsmove past obstacles

  10. 1 hour treatment at Tanneal... Decreases σu, and Increases %El Post-ColdWork Heat Treatment • Heat Treating REVERSES the Effects of CW • Three Stages • Recovery • ReXtalization • Grain Growth 1 2 3

  11. Annihilation reduces dislocation density (ρd ↓) Recovery • Scenario1 • Hi-Temp Increases Diffusion • Scenario2 3 2 4 1

  12. New crystals are formed that have a much smaller dislocation density Are Small in Physical Size Relative to Existing Xtals Are More Energetically Favorable than CW Xtals ReCrystallization 0.6 mm 0.6 mm 33% cold worked brass New crystals nucleate after 3 sec. at 580C.

  13. All cold-worked crystals are consumed by the New Crystals ReCrystallization cont 0.6 mm 0.6 mm After 8 seconds After 4 seconds

  14. At Longer Times Some of the NEW Xtals consume OTHER NEW Xtals Grain Boundaries are Hi-Energy Regions, Thus Their Reduction is Thermodynamically Favored Grain Growth 0.6 mm 0.6 mm After 8 s, 580C After 15 min, 580C

  15. Many Metals Follow This Grain Growth Reln Grain Growth Quantified • Where • d  Grain Size (m) • t  Time (s) • d0  BaseLine Grain Size at t = 0 (m) • K  Slope Constant (mn/s) • n  Power Constant (unitless)

  16. Summary • Dislocations are Observed Primarily in Metals And Alloys • Metal/Alloy Strength Is Increased By Making Dislocation Motion Difficult • Techniques to increase strength • Decrease Grain Size • Solid Solution Lattice Straining • Cold Work to Increase Dislocation Density • Precipitates to Impede Dislocation Motion

  17. Summary cont. • Post-CW Heating (annealing) can reduce Dislocation density and increase grain size • Anneal Process Time-Phases • Recovery • ReCrystallization • Grain Growth

  18. Parallel(Elastic Recovery) σ-ε for C46400 (Naval) Brass WhiteBoard Work • Problem 7.27 ~0.16

  19. σ-ε Curve for Steel

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