Chapter 4 Flashcards
What are the types of imperfections?
- 0 Dimensional (point)
- 1 Dimensional (linear)
- 2 Dimensional (Planar/interfacial)
- 3 Dimensional (volume)
Give 3 examples of point defects
- Vacancy Atoms
- Interstitial atoms
- Substitutional atoms
Give an example of a linear defect
Dislocations
Give 3 examples of planar/interfacial Defects
- Grain Boundaries
- Twin Boundaries
- Stacking Faults
Give 3 examples of volume defects
- Impurity inclusions
- Pores
- Cracks
What is a vacancy?
- A point defect where there are vacant atomic sites in the structure
What is an interstitual atom?
A point defect where an atom is added into the structure
Extra atoms are added between atomic sites.
What is a substitutional atom?
A point defect where one atom is replaced with another different atom not orignally in the structure
What is the equilibrium concentration for point defects formula?
For Point Defects in Alloys, what are two outcomes if an impurity (B) is added to a host (A)
- Solid solution of B in A (i.e. random dist. of point defects)
- Solid solution of B in A plus particles of a new phase (usually for a larger amount of B)
What are the weight percent and atom percent formulas?
What are dislocations?
- They are line defects that produce permanent plastic deformation
- Are 1D defects around which atoms are misaligned
- Slip between crystal planes result when dislocations move
What are the 2 Types of Linear dislocations?
- Edge dislocation
- Screw Dislocation
What is an edge dislocation?
- Dislocation where an extra portion of a plane of atoms, or half-plane has been inserted into the structure.
- The edge terminates within the crystal
- The burgers vector is perpendicular to the dislocation line
What is an screw dislocation?
- Spiral planar ramp resulting from shear deformation
- Burgers Vector is parallel to dislocation line
What is a Burgers vector?
- A vector that denotes the magnitude and direction of lattice distortion associated with a dislocation
Determine the Burgers vector in the following picture:
- To determine the vector, count around the edge of the structure.
- The number of times counted along one edge must equal the number of times on the edge opposite.
- The vector drawn from the final position to the initial is the burgers vector.
Compare the crystal Structures of FCC, BCC AND HCP
Close packed planes and directions are preferred
FCC- Many close packed planes/directions
HCP: only one plane, 3 directions
BCC: none.
What is a twin boundary?
A planar defect that is a reflection of atom positions across the twin plane
What are Stacking faults?
- Planar defects where there is an error in the packing sequence
e. g. ABCABABC - There is no C in the second ABC
e. g. ABCABCBABC - There is a B which has been inserted.
What is the relationship between twins and SFs?
If Stacking faults occur periodically and remove every second letter, it appears as if twinning has occured.
e.g
ABCABCABCABCABCABCABC
ABCABCABC | BACBAC
Name some other interfacial defects
- External surfaces
- Grain boundaries
- Anti-phase domain boundaries
- Phase boundaries
Which defect is the most important in determining the mechanical properties of a structure?
Line Defects
What are mixed Dislocations?
- A dislocation that has both edge and screw components
What are impurity iclusions in volume defects?
Impurities are precipitated into the structure
What controls Vacancy Concetrations in point defects?
Temperature
Increasing T causes surface island of atoms to grow.
Why? The equil. vacancy conc. increases via atom motion from the crystal to the surface, where they join the island.
What is the dislocation line?
- The line that extends along the end of the extra half-plane of atoms for an edge dislocation and along the center of the spiral of a screw dislocation.
