1. CHAPTER 5 : DISLOCATION & METAL STRENGTHENING MECHANISMS

2. TOPIC 1) INTRODUCTION 2) ELASTIC & PLASTIC DEFORMATION
3) SLIP & TWINNING
4) STRAIN HARDENING
Beginning course details and/or books/materials needed for a class/project.

3. INTRODUCTION Deformation :
Changing of shape of metal under the action of force
Give desired shape
Deformed metals – more superior than cast metal
Operation of deformation – rolling, forging, spinning.
Materials may experience 2 kinds of deformation :
Elastic deformation
Plastic deformation
Objectives for instruction and expected results and/or skills developed from learning.

4. ELASTIC & PLASTIC DEFORMATION
Elastic Deformation
Plastic Deformation
Defined as process of deformation which appears & disappears with the application & removal of stress
Defined as process of permanent deformation &exist after removal of stress
At the beginning of the deformation progress
Takes place after elastic deformation has stopped
a) Original state b) Elastic deformation with vertical force applied c) Elastic deformation with diagonal force applied d) The beginning of plastic deformation upon a slip plane e) Deformation Proceeding upon further slip plane.

5. Modes of Plastic Deformation
There are 2 basic modes :
1) slip
2) twinning
Slip
Twinning
Due to sliding of atomic planes over the others
Due to change of orientation of a crystal with other
Crystal orientation above & below slip plans is the same after deformation
Twinned portion is the mirror image of the original lattice
Atomic movements are over large distance.
Atomic movements are over a fraction of atomic space.
Require lower stress for atomic movement
Require higher stress for atomic movements
Occurs on widely spaced planes
Occurs on every atomic planes involved in twinned region

6. STRAIN HARDENING
Defined as – process deforming ductile metal to become stronger & harder.
Also called ‘work hardening’ or ‘cold working’.
Deformation takes place at temperature that is ‘cold’ relative to melting temperature.
Below its recrystallization temperature.
During cold working :
Certain amount of work done is stored internally in form of strain energy
The energy produces internal stress
Can lead to cracking
Relieve metal from internal stress by annealing – metal is heated (below melting temperature)
Metal loses its stored energy – come back to strain free condition

7. - metal losses its stored energy in 3 stages :
Recovery
Recrystallization
Grain growth

8. Remove internal stress
Recovery
Defined as – process of removing internal stress in metal by heating it to a relatively low temperature (below melting temperature)
Remove internal stress
Does not affect – grain structure
- hardness & strength
Increase ductility
During cold working :
Dislocation pile up at grain boundary
During recovery :
Dislocation number starts reducing & rearrange themselves

9. Recrystalization
Defined as :
Process of forming a new set of strain free grains in metal by heating it to a temperature known as recrystallization temperature.
The new strain free & equiaxed grains:
Have low dislocation density
Characteristics of precold-worked condition
Driving force :
The difference in internal energy between strained & unstrained material
New grains form as very small nuclei & grow to replace parent material
Involved short range diffusion
Take place through 3 process :
nucleation
Primary grain growth
Secondary grain growth

10. Recrystallization produces :
Grain Growth
Defined as – process of forming strain-free grains larger in size by heating a metal to a temperature above recrystallization temperature.
Recrystallization produces :
Strain-free new grain smaller in size but of equal shape
Grain will continue to grow if temperature is increased
Grain growth does not need to be preceded by recovery & recrystallization
May occur in all polycrystalline material (metal& ceramic)
Growth rate –rapid & becomes slow when temp increased
During growth , combination of individual grains takes place.

11. Reducing grain boundary area
Reduction in total energy
Grains becomes stable
Driving force for grain growth