“ “ PLASTIC DEFORMATION ””

GUIDED BY:

ARIF SIR

BRANCH : M.E. (PRODUCTION ENGINEERING) SUBJECT : ADVANCE MATERIAL TECHNOLOGY

S. N. PATEL INSTITUTE OF TECHNOLOGY &

RESEARCH CENTRE Vidyabharti Campus, Umrakh, Bardoli, Surat – 394345.

PREPARED BY:HITENKUMAR J MISTRY

PEN NUMBER :140490728003

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 ρ = 106/cm2

Types of dislocations

Edge

Screw

Dislocation motion⇔plastic deformation

Note: Dislocations normally move under a shear stress

How does a dislocation move?

Stress field of a dislocation

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 ε~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 sizeσ = σ0 + 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