1
ISSUES TO ADDRESS...
• Why are the number of dislocations present
greatest in metals?
• How are strength and dislocation motion related?
• Why does heating alter strength and other properties?
Lec 11: Deformation & Strengthening
Mechanisms
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Material Sciencerbrnttntymtymty,y, Lecture notes of Material Science and Technology
Deformation elastic sacgwecbwyvwjvwjbaj,j,h,yu,y,y,y,y,uy,
Typology: Lecture notes
2011/2012
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ISSUES TO ADDRESS...
- Why are the number of dislocations present greatest in metals?
- How are strength and dislocation motion related?
- Why does heating alter strength and other properties? Lec 11: Deformation & Strengthening Mechanisms
2
Review: what is a dislocation?
Linear defects (Dislocations) are one-dimensional defects that cause misalignment of nearby atoms. Linear defects are associated primarily with mechanical deformation. Types of dislocations: edge, screw, mixed. Burger’s vector, b : measure of lattice distortion Edge dislocation: ▪ extra half-plane of atoms inserted in a crystal structure; the edge of the plane terminates within the crystal. ▪ Around the dislocation line there is some localized distortion. ▪ b perpendicular (⊥) to dislocation line
Where is the dislocation?
Where is the dislocation?
Screw dislocation:
- Named for the spiral stacking of crystal planes around the dislocation line; results from shear deformation
- b parallel ( ) to dislocation line
Plastic Deformation by Dislocation Motion
- Atomic bonds broken and reformed along slip plane as dislocation (extra half plane) moves.
- Plastic deformation occurs by motion of dislocations (edge, screw, mixed) – process called slip
- Applied shear stress can cause extra half-plane of atoms [and edge dislocation line ( )] to move as follows: (^) Fig. 7.1, Callister & Rethwisch 10e.
Motion of Edge and Screw Dislocations
- Direction of edge disl. line ( ) motion—in direction of
applied shear stress.
Edge dislocation Screw dislocation Fig. 7.2, Callister & Rethwisch 10e.
- Direction of screw disl. line ( ) motion—perpendicular to direction of applied shear stress.
Dislocation Characteristics Metals
- Metals:
- Examples: copper, aluminum, iron
- Dislocation motion—relatively easy
- Metallic bonding—non-directional
- Close-packed planes and directions for slip electron cloud ion cores + +
- +^ +^ +^ +^ +
Visualize dislocation using bubble raft Bragg and coworkers invented bubble rafts around 1940, and ever since then scientists have used bubble rafts to help model defects in crystals, especially dislocations.
https://www.youtube.com/watch?v=1r-UqxmuYCw In this video, a man thought to be W.L. Bragg visually explains the nature and the effects of dislocations in crystals using soap bubbles in water. http://homepages.cae.wisc.edu/~stone/bubble%20raft%20movie s.htm#machining More bubble raft videos
Slip System—Combination of slip plane and slip direction
- Slip Plane
- Crystallographic plane on which slip occurs most easily
- Plane with high planar density
- Slip Direction
- Crystallographic direction along which slip occurs most easily
- Direction with high linear density Slip Systems