Dislocation And Strengthening Mechanisms Plastic deformation through dislocation: Slip Ideal shear...

Dislocation And Strengthening Mechanisms

Plastic deformation through dislocation: SlipPlastic deformation through dislocation: Slip

Ideal shear stress

da

~ G d/a ~ G d/a ~ 106 psi (calc.)

~ 10~103 psi (measured !)

FCC

This is due to other mechanisms ~ dislocation movement or fracture propagation

Dislocation Movement

Take much less energy !

Lattice strains in dislocation

Slip systems

Slip direction : closest distance or highest linear atomic density

Slip plane : planes with a highest atomic density

Easier deformation(higher ductility)

Brittle !

Slip systemsSlip systems

Slip distance

Slip planeSlip plane

Slip distance

FCC

HCP

A combination of close-packed planes and close-packed directions on those planes where slip occurs.

A combination of close-packed planes and close-packed directions on those planes where slip occurs.

Burgers vectors and slip systems in FCCBurgers vectors and slip systems in FCC

]101)[2/( 0

ab ]101)[2/( 0

ab

Slip in ionic materialsSlip in ionic materialsNiO

V3 is favorable, sinceit is the shortest vector connecting crystallographically equivalent potions

(Lowest-energy Burgers vector)(Lowest-energy Burgers vector)

Critical Resolve Shear Stress (CRSS)

max)coscos((max) RCrss max)coscos((max) RCrss

max)coscos( Crss

y max)coscos(

Crssy Or

Yield strength

coscosR coscosR

Znsinglecrystal

CRSS: min stress required for slip

PolycrystallineCu

PolycrystallineCu

Plastic Deformation Of Polycrystalline Materials

After deformationEquiaxed

Before deformation

Slip band

Deformation By Twining

BCC and HCP

BCC: (112) [111] for twining

Strengthening Mechanisms

Grain size reduction GB阻斷 Slip movement

Solid solution hardening

2/10

dk yy 2/10

dk yy Hall-Petch Eq:

Strain Hardening

100%0

0

A

AACW d

Cold work

Recovery, Recrystallization, Grain Growth

Recovery: The stored energy is relieved by dislocation motion at the elevated temperature. Some physcail properties, such as electrical conductivity, are improved.

Recrystalliztaion: After recovery, the residual strain is further reduced by the formation of strain-free ann equiaxed grains.

Tensile strength

Ductility

Grain sizeGrain size

Annealing temperature

RecoveryRecrystallization

Grain growth

Grain growth: Grain growth to reduce the interfacial energy

Ktdd nn 0Ktdd nn 0