PLASTIC DEFORMATION

Dislocations and their role in plastic deformation

What are dislocations?s

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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 dislocationsScrew

Edge

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 deformations

Slip Twinning Shear band formation

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Slips s

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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..s s

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

Slip systemsCrystal system fcc hcp bcc Slip plane Slip direction {111} {0001} {110} {100} Total number of slip systems 12 3 48 Active slip systems 5 2/3 2

Dislocation interactionPositive Positive

Repulsion

Positive

Negative Attraction & Annihilation

Note: More positive-positive interactions in reality

Positive-positive dislocation interactions

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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

Twinnings

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 twinningSLIPHOMOGENEOUS COMMON IN FCC OCCURS UNDER STATIC LOADING

TWINNINGLOCALIZED COMMON IN HCP & BCC OCCURS UNDER SHOCK LOADING

Shear band formations

Limited non-homogeneous deformation Very large localized strain ~1 or 100% Occurs especially under high strain rates Mechanism of deformation still unclear

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Plastic deformation movement of dislocations

Strengthening methods

Cold workings

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 strengthenings s

Interaction between stress fields of alloy atoms and dislocations This is the purpose of alloying

Grain size refinements

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 size = 0 + kyd-1/2 Hall-Petch equation Smaller grains have more boundary area and hence more barriers to dislocation motion

Precipitation hardenings

Precipitates are second-phase particles Hard precipitates act as barriers to dislocation motion Applicable only to some alloy systems

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