Imperfections and dislocations Flashcards
What are the types of point defects in crystals?
-Vacancies
-Interstitial defects
What is the relationship between temperatures and the number of vacancies per unit volume?
As the temperature increases, the number of vacancies per unit volume increases.
What phenomena is diffusion important in?
- Creep
- Recrystallization
- Grain Growth
- Phase transformations
What does diffusion allow atoms to do?
It allows atoms to re-arrange
themselves to obtain lower
energy configurations
What are dislocations?
Linear crystalline
defects whose motion gives rise to
plastic deformation in crystalline
materials
What causes plastic deformation in crystalline solids?
Propagation (movement) of dislocations.
When are dislocations introduced?
During solidification, plastic deformation,
and from thermal stresses.
When were dislocations first experimentally observed?
They were observed experimentally by transmission
electron microscopy (TEM) in 1950s.
What is slip?
The process by which plastic deformation is produced by
dislocation movement
What happens at the atomic level during slip?
Severing and re-forming atomic bonds
What is slip a result of?
Applied shear stress
Define slip plane
The plane along which the dislocation moves
Where can dislocations end and where can they NOT end?
Dislocations cannot end within a crystal or grain, but only at grain boundaries or at a free surface
What is a slip system?
Describes the preferred plane (slip plane) and direction (slip direction) for dislocation movement.
What does the slip system depend on?
Depends on the crystal structure
Where does slip occur for a given crystal structure?
- Along the plane with the densest atomic
packing (slip plane) - Along the direction with the highest linear
density (slip direction)
What condition is required for each grain to have deformation occur?
For each grain, deformation occurs
along the slip system that has the
most favourable orientation.
What are slip systems in a single crystal?
Slip systems in a single crystal refer to combinations of slip planes and slip directions.
For a single crystal with multiple slip system, how many can be oriented favourably and what is unique about one of the slip systems?
One is usually oriented favorably and has
greatest resolved shear stress
What is the critical resolved shear stress?
A material property that indicates when yielding
occurs
Why does the direction of slip vary in polycrystalline materials (grains)?
Due to the random crystallographic orientations of the grains.
Compare the strength between polycrystalline metals and their single crystal equivalent and explain your reasoning.
Polycrystalline metals are stronger than
their single crystal equivalent because
deformation cannot occur unless
dislocations must propagate along
adjacent, less favorably oriented grains
What is required to break atomic bonds during slip simultaneously on a slip plane
A very high stress
Are all bonds on a slip plane broken simultaneously? If not, what does happen?
Because not all bonds on a slip
plane are severed
simultaneously; instead it
involves breaking atomic
bonds in one plane at a time and reforming bonds.
What types of fields do dislocations cause?
The presence of a dislocation
causes elastic stress and strain
fields in the surrounding region.
What is the difference between edge dislocations and screw dislocations?
- Edge dislocations introduce tensile
stresses and
compressive stresses - Screw dislocations introduce shear
stress fields around them
When do dislocations repel each other?
When the compressive & tensile stresses for both dislocations are in the same location (either above or below the slip plane).
When are dislocations attracted to each other? What happens?
Two dislocations of opposite sign moving on the same slip plane exert an attractive force on each other, which causes them to attract each other and annihilate (perfect crystal).
What is dislocation density?
dislocation density = total length of dislocation line / volume
When do shear components of an applied stress NOT exist?
At parallel or perpendicular
alignments to stress direction
What are resolved shear stresses?
Shear components of an applied stress.
What do resolved shear stresses depend on?
Orientation of the slip plane and direction.
What happens when a polished metal surface is plastically deformed? What does it indicate?
“Slip lines” appear on the surface. This indicates that atoms are shearing across each other.
What is minimized in a slip system?
Atomic distortion is
minimized with dislocation movement
When does slip occur along the slip system in a single crystal?
Slip occurs along this slip system when the
critical resolved shear stress is reached.
