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