PLASTIC DEFORMATION Dislocations and their role in plastic deformation.
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Transcript of PLASTIC DEFORMATION Dislocations and their role in plastic deformation.
PLASTIC DEFORMATION
Dislocations and their role in plastic deformation
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 = 106/cm2
Types of dislocations
Edge
Screw
Dislocation motionplastic deformation
Note: Dislocations normally move under a shear stress
How does a dislocation move?
Stress field of a dislocation
Analog to an electric charge
Modes of deformation
Slip
Twinning
Shear band formation
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
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>
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
Dislocation interaction
Repulsion
Attraction&
Annihilation
Positive Positive
Positive Negative
Note: More positive-positive interactions in reality
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
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
Compare slip and twinning
SLIP TWINNING
HOMOGENEOUS LOCALIZED
COMMON IN FCC COMMON IN HCP &BCC
OCCURS UNDERSTATIC LOADING
OCCURS UNDERSHOCK LOADING
Shear band formation Limited non-homogeneous deformation
Very large localized strain ~1 or 100%
Occurs especially under high strain rates
Mechanism of deformation still unclear
Plastic deformation movement of dislocations
Strengthening methods
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
Solid solution strengthening Interaction between stress fields of alloy
atoms and dislocations This is the purpose of alloying
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
Mechanism
Strength is inversely proportional to grain size = 0 + kyd-1/2
Hall-Petch equation
Smaller grains have more boundary area and hence morebarriers to dislocation motion
Precipitation hardening
Precipitates are second-phase particles
Hard precipitates act as barriers to dislocation motion
Applicable only to some alloy systems