Imperfections in crystals Flashcards
What is the process of solidification? (4)
- As material cools down, small crystallites appear out of molten material and act as nuclei sights of growth.
- Particular orientation with respect to each other
- As cooling proceeds the crystals get bigger and bigger, as thermal energy drops they can grow
- As they solidify it becomes grains as they grow they start to touch and random angles
What types of defects arise in solids?
?
Can the number and type of defects be varied
and controlled?
?
How do defects effect material properties?
?
Are defects undesirable?
?
Why can diffusion easily occur in grain boundaries?
Since bonds are not at maximum strength they are easily broken, therefore diffusion can occur easily
Why is the chemical reactivity high in grain boundaries?
As it’ll oxidise around the grain boundaries before the inner parts due to the bonds not being strong and there being gaps
What is the weak point in polycrystalline materials?
the high-angle grain boundary
What shape can grains become? (2)
equiaxed (roughly same size in all directions)
columnar (elongated grains)
How do columnar grains occur in solidification?
in areas with less undercooling
How do equiaxed grains occur in solidification?
shell of equiaxed grains due to rapid cooling (greater ∆T) near wall
Which direction to the grains follow?
They grow in the direction of the heat flow
What is grain refiner? (2)
- added to make more smaller, more uniform, equiaxed grains when they meet
- you can get more nuclei per unit volume
What are characteristics of zone melting? (4)
- only a small part of the charge is molten
- material is added to molten region
- molten zone is advanced by moving the charge or the gradient
- axial temperature gradient is imposed along the crucible
Why are imperfections sometimes bad?
as they can cause material failure such as metal fatigue which arises from the growth of cracks
Why are imperfections sometimes good?
some imperfection can be used to stop cracks from growing eg. carbon impurities are put into iron to make steel, this strengthens the material by stopping the growth of cracks
Why are imperfections sometimes good?
some imperfection can be used to stop cracks from growing eg. carbon impurities are put into iron to make steel, this strengthens the material by stopping the growth of cracks
or putting in crystallites
Where does metal fatigue occur usually?
structures affected by waves and subject to a lot of pressure and decompression. Causing cracks to join up and meet up
what are three types of defects?
point defects
line defects
area defects
What are point defects?
one atom is out of place
What are three types of point defects from atoms?
Vacancy atoms
interstitial atoms
substitutional atoms
What are vacancy atoms?
an atom is missing
What are interstitial atoms?
atoms not sitting on the right spot and actually are in between lattice sides
What are substitutional atoms?
wrong atom in there
What do vacancy atoms cause? (2)
a distortion of planes
and helps diffusion go more quickly
What do interstitial atoms cause?
distortion of planes
How does exponential dependence occur in a graph with defect concentration against temperature?
as we increase the temperature, then the number of vacancies will start to increase.
Like melting , before it becomes liquid It’ll have a lot of vacancies as we’ve given atoms enough energy to move out of the minimum energy configuration through the thermal energy applied
What are the two outcomes if impurity B added to host A ?
Solid solution of B in A, creates random dist. of point defect
solid solution of B in A plus particles of a new phase (usually for a larger amount of B)
What are two types of point defects in alloys?
substitutional solid solution
interstitial solid solution
Give an example of what substitutional atoms do in alloys.
Copper replaces nickel atoms, copper atoms are smaller atoms than nickel
Give an example of what interstitial atoms do in alloys.
Random distribution of carbon in Fe.
Copper is much more smaller than Fe and fits into the gaps between Fe atoms
What is the new phase when a solid solution of B is added into A?
different composition, often different structure
What is the new phase when a solid solution of B is added into A?
different composition, often different structure such as columns of atoms
Why does increasing temperature cause surface island of atoms to grow?
the equilibrium vacancy concentration increases via atom motion from the crystal to the surface where they join the island.
The island grows/shrinks to maintain equilibrium vacancy concentration in the bulk
What are 4 conditions for substitutional solid solution (rough indication of what alloys go together)?
- ∆r (atomic radius) < 15%
- Proximity in periodic table
• i.e., similar electronegativities - Same crystal structure for pure metals
- Valency
• All else being equal, a metal will have a greater tendency
to dissolve a metal of higher valency than one of lower
valency
What are dislocations in line defects? (3)
- one dimensional defects, a line of point defects creating misalignment
- slip between crystal planes result when dislocations move
- produces permanent (plastic deformation
How are dislocations caused?
after tensile elongation pulling it apart creates slip steps
What are two types of dislocations?
edge dislocation and screw dislocation
What is edge dislocation?
extra half-plane of atoms inserted in a crystal structure, creaintg an extra step
b is perpendicular to dislocation line
What is burger’s vector, b?
measure of direction and magnitude of lattice distortion
How does edge dislocation occur?
requires successive bumping of a half plane of atoms.
bonds across the slipping planes are broken and remade in succession, creating a new line
What is screw dislocation?
spiral planar ramp resulting from shear deformation
b is parallel to dislocation line
How do dislocations effect the electrical properties of solids?
Can affect electrical properties as if electricity hits an atom out of place it can scatter, if there are a lot of dislocations atoms will scatter a lot.
What are area defects?
crystallites are 3D and surface area is the grain boundaries
What are the directions and planes among crystal structures (FCC, HCP, BCC)?
FCC: many close-packed planes/directions
HCP: only one plane, 3 directions;
BCC: none
What are planar defects in solids?
one case is a twin boundary (plane), essentially a reflect of atoms positions across the twin plane, making it weak
plane is split in half and one is almost perpendicular to the other
