Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 1 Chapter 7 Continued Cover Eqs. 7.1 through 7.6

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 2 Solid-Solution Strengthening (III)

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 3 Strengthening by increase of dislocation density (Strain Hardening = Work Hardening = Cold Working) Ductile metals strengthen when deformed plastically at temperatures well below melting point. Reason  increased dislocation density. Average distance between dislocations decreases; dislocations start blocking each others motion. Percent cold work (%CW)  degree of plastic deformation: %CW another measure of degree of plastic deformation, like strain. where A 0 is the original cross-section area, A d is the area after deformation.

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 4 Strain Hardening (II) New yield strength  yi higher than initial yield strength,  y0. Reason  strain hardening.

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 5 Strain Hardening (III) Yield strength + hardness increased due to strain hardening, but ductility decreased (material becomes more brittle).

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 6 Strain Hardening (IV)

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 7 Recovery, Recrystallization, and Grain Growth  Plastic deformation increases dislocation density + changes grain size distribution  Therefore, stored strain energy (dislocation strain fields + grain distortions)  External stress removed: most dislocations, grain distortions and associated strain energy retained.  Restoration to state before cold-work by heat-treatment:  Recovery and Recrystallization, followed by grain growth.

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 8 Recovery Heating  increased diffusion  enhanced dislocation motion  decrease in dislocation density by annihilation, formation of low-energy dislocation configurations  relieves internal strain energy Some of the mechanisms of dislocation annihilation: vacancies slip plane Edge dislocation

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 9 Recrystallization (I)  After recovery grains can still be strained. Strained grains replaced upon heating by strain-free grains with low density of dislocations.  Recrystallization: nucleation and growth of new grains  Driving force: difference in internal energy between strained and unstrained  Grain growth  short-range diffusion Extent of recrystallization depends on temperature and time.  Recrystallization is slower in alloys

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 10 Recrystallization (II) Recrystallization temperature: temperature at which process is complete in one hour. Typically 1/3 to 1/2 of melting temperature (can be as high as 0.7 T m in some alloys). Recrystallization decreases as %CW is increased. Below "critical deformation", recrystallization does not occur.

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 11 Recrystallization (III)

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 12 Grain Growth  Deformed polycrystalline material maintained at annealing temperature  following recrystallization further grain growth occurs  Driving force: reduction of grain boundary area and energy: Big grains grow at the expense of small ones  Grain growth during annealing occurs in all polycrystalline materials (i.e. they do not have to be deformed first).  Boundary motion occurs by short range diffusion of atoms across the grain boundary  strong temperature dependence of the grain growth.

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 13 Summary  Cold working  Critical resolved shear stress  Dislocation density  Grain growth  Lattice strain  Recovery  Recrystallization  Recrystallization temperature  Resolved shear stress  Slip  Slip system  Strain hardening  Solid-solution strengthening Make sure you understand language and concepts:

Introduction to Materials Science, Chapter 7, Dislocations and strengthening mechanisms University of Virginia, Dept. of Materials Science and Engineering 14 Reading for next class: Chapter 8: Failure  Mechanisms of brittle vs. ductile fracture  Impact fracture testing  Fatigue (cyclic stresses)  Crack initiation and propagation  Creep (time dependent deformation) Optional reading (Parts that are not covered / not tested): Parts of 8.5 Principles of fracture mechanics 8.10 Crack propagation rate 8.16 Data extrapolation methods