Introduction to Materials Science

1
Dislocation And Strengthening Mechanisms
t
Ideal shear stress
FCC
a
t G d/a
106 psi (calc.)
d
10103 psi (measured !)
This is due to other mechanisms dislocation
movement or fracture propagation
Plastic deformation through dislocation Slip
2
Dislocation Movement
Take much less energy !
3
Lattice strains in dislocation
4
Slip systems
Slip direction closest distance or highest
linear atomic density
Slip plane planes with a highest atomic density
Easier deformation (higher ductility)
Brittle !
5
A combination of close-packed planes and
close-packed directions on those planes where
slip occurs.
Slip systems
Slip distance
FCC
Slip plane
HCP
Slip distance
6
(No Transcript)
7
Burgers vectors and slip systems in FCC
8
Slip in ionic materials
NiO
V3 is favorable, since it is the shortest vector
connecting crystallographically equivalent potions
(Lowest-energy Burgers vector)
9
Critical Resolve Shear Stress (CRSS)
Polycrystalline Cu
Zn single crystal
CRSS min stress required for slip
Or
Yield strength
10
Plastic Deformation Of Polycrystalline Materials
Equiaxed
After deformation
Before deformation
Slip band
11
Deformation By Twining
BCC and HCP
BCC (112) 111 for twining
12
Strengthening Mechanisms
Grain size reduction
GB??Slip movement
Hall-Petch Eq
Solid solution hardening
13
Strain Hardening
Cold work
14
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.
15
Recovery
Recrystallization
Grain growth
Annealing temperature
Tensile strength
Ductility
Grain size
16
Grain growth Grain growth to reduce the
interfacial energy