lec-8-dislocations and their role in plastic deformation ...modes of deformation slip twinning shear...
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PLASTIC DEFORMATION
Dislocations and their role in plastic deformation
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What are dislocations?
� Dislocations are line defects that exist in metals
� There are two types of dislocations: edge and screw
� The symbol for a dislocation is � The dislocation density in annealed
metals is normally ρ = 106/cm2
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Types of dislocations
Edge
Screw
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Dislocation motion⇔plastic deformation
Note: Dislocations normally move under a shear stress4
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How does a dislocation move?
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Stress field of a dislocation
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Analog to an electric charge
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Modes of deformation
� Slip
� Twinning
� Shear band formation
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Slip� Dislocations move on a certain
crystallographic plane: slip plane� Dislocations move in a certain
crystallographic direction: slip direction� The combination of slip direction and
slip plane is called a slip system
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Slip…..� Slip planes are normally close-packed planes� Slip directions are normally close-packed
directions
Recall for fcc close-packed planes are {111}Close-packed directions are <110>
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Slip systems
Crystalsystem
Slip plane Slipdirection
Totalnumber ofslipsystems
Activeslipsystems
fcc {111} <110> 12 5
hcp {0001} <2110> 3 2/3
bcc {110}{100}
<111> 48 2
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Dislocation interaction
⊥ ⊥ Repulsion
⊥
Attraction&
Annihilation
Positive Positive
Positive Negative
Note: More positive-positive interactions in reality12
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Positive-positive dislocation interaction
� Results in more stress to move dislocations (or cause plastic deformation):called work hardening
� This type of interaction also leads to dislocation multiplication which leads to more interactions and more work hardening
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Twinning
� Common in hcp and bcc structures
� Limited deformation but help in plastic deformation in hcp and bcc crystals
� Occurs on specific twinning planes and twinning directions
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Compare slip and twinning
SLIP TWINNING
HOMOGENEOUS LOCALIZED
COMMON IN FCC COMMON IN HCP &BCC
OCCURS UNDERSTATIC LOADING
OCCURS UNDERSHOCK LOADING
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Shear band formation� Limited non-homogeneous deformation
� Very large localized strain ε~1 or 100%
� Occurs especially under high strain rates
� Mechanism of deformation still unclear16
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Plastic deformation ⇔movement of dislocations
Strengthening methods
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Cold working
� Deformation at temperatures below 0.4 Tm
� Dislocation density increases from 106/cm2 to 1010-12/cm2
� High dislocation density results in a large number of dislocation interactions which results in high strength and hardness
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Solid solution strengthening� Interaction between stress fields of alloy
atoms and dislocations� This is the purpose of alloying
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Grain size refinement
� Small grains result in higher strength
� Small grains is equivalent to a large number of grain boundaries in the same volume
� Grain boundaries act as barriers to dislocation motion
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Mechanism
Strength is inversely proportional to grain sizeσ = σ0 + kyd-1/2
Hall-Petch equation
Smaller grains have more boundary area and hence morebarriers to dislocation motion
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Precipitation hardening
� Precipitates are second-phase particles
� Hard precipitates act as barriers to dislocation motion
� Applicable only to some alloy systems
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