Materials Science
04/10/20
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Lecture 8
Today, we will answer the question of “how does all this atomic packing stuff actually affect material behavior anyway?”
Begin by looking at dislocation motion (slip) not parallel to a standard atomic plane.
The tighter the atomic packing along a direction or in a plane, the easier it is for dislocations to move in that direction or within that plane.
Slip Systems: Slip planes and Slip Directions
Slip Planes
Slip Directions
Example: Slip system and plane for a BCC metal

Materials Science
04/10/20
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Lecture 8
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FCC structures typically have 12 slip systems.
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BCC structures typically have 12 slip systems
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HCP structures typically have 3 slip systems
Question: Which one will be more brittle and which will be more ductile? Why?
Slip (dislocation movement) in a Crystals:
Brief Mohr’s Circle: The maximum shear stress resulting from a normal stress occurs at
45 degrees with respect to the normal stress.
Strengthening against deformation in Polycrystalline Materials:
Method 1: Grain size reduction:
How might grains affect slip (dislocation motion) in a material?
Hall-Petch Equation

Materials Science
04/10/20
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Lecture 8
Method 2: Alloying materials for strength (solid solution strengthening)

The existence of a dislocation produces local tensile and compressive lattice strains in its vicinity.

Atoms in solid solution within the crystal may alleviate those strains.
Method #3: Cold working (strain hardening)

As the material is plastically deformed, more dislocations develop (higher _________ density).

Dislocations (on average) repel one another, impeding their motion.

Impeded motion of dislocation (slipping) makes material harder to deform

But …
Percent cold work:

Materials Science
04/10/20
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Lecture 8
Basics of Heat Treating or Annealing
Cold working stores energy in the material in the form of lattice strains and increased numbers of grain boundaries.
Simple Heat treating (exposure to heat) can remove many of the effects of cold working.
The effects of heat treating vary with temperature and with time
Recovery
Recrystallization
Grain growth.