Topic 3- Structure of Crystalline Solids Flashcards
Define crystalline.
Crystalline materials have atoms situated in an ordered, periodic array over large atomic distances.
What materials are crystalline?
Metals and most ceramics.
Give two examples of properties that depend on the crystal structure of a material.
Density and ductility.
Define amorphous/non-crystalline.
Amorphous materials have no long range order. They are a random arrangement of atoms.
What materials are amorphous?
Glasses (silica), plastics and rapidly cooled metals.
How fast is rapid cooling for a metal?
1x10^5 degrees C/s.
When do amorphous structures usually occur?
When cooling happens too quickly for nucleation to occur.
What is a region in a crystalline structure called?
Grain.
What are the lines in crystalline regions called?
Twins.
Define polycrystalline.
Polycrystalline materials are made up of lots of crystalline regions and form like a lake freezing over.
What is the name of the line where two crystalline regions meet?
Grain boundary.
Describe the formation of a polycrystalline structure.
‘Nuclei’ form during solidification, each of which grows into a crystal. The crystals grow and meet. These crystals are separated by an amorphous grain boundary which influences properties.
How does packing affect energy?
Dense, regular-packed structures tend to have lower energy.
Are dense, regularly-packed atoms stable or unstable?
Stable - this is the preferred atomic arrangement.
For a dense, regularly-packed atoms will separation distance be slightly greater or slightly less than equilibrium position?
Slightly greater.
Why are non-dense, randomly packed atoms less stable?
A range of bond lengths, means average bond length is greater so there is a higher energy state.
What is one negative impact of non-dense, random packing?
Resistance to corrosion would be worse.
What is a lattice?
A 3D array of regularly spaced points.
What is hard sphere representation?
Atoms denoted by hard, touching spheres.
What is reduced sphere representation?
Atoms denoted by small circles allowing position in 3D to be seen.
What is a unit cell?
Basic building block that repeats in space to create the crystal structure, usually a parallelpiped or prizm.
Roughly how many different types of unit cell are there?
14.
What does FCC stand for?
Face Centered Cubic.
What does BCC stand for?
Body Centered Cubic.
What does HCP stand for?
Hexagonal Close Packed.
What is the atomic packing factor?
Volume of atoms in unit cell / volume of unit cell.
What to things is ductility (ease of plastic deformation) linked to?
Crystal structure and close packed planes.
Where does slip occur?
On specific atomic planes and in specific crystallographic slip directions. I.e. slip systems.
What are slip planes/directions?
The most densely packed planes, and in that plane the closely packed direction.
What is the coordination number (CN)?
Number of nearest neighbours or touching atoms.
What is CN for simple cubic structure (SC)?
6
What is APF for SC?
0.52
Which element has simple cubic structure?
Polonium (Po)
Where are atoms located in an FCC structure?
At each corner and the centre of the cube faces.
What is CN for FCC?
12
What is APF for FCC?
0.74
How many slip planes in FCC?
4 (face diagonals)
How many slip directions in FCC?
3
How many slip systems in FCC?
4x3 = 12
Which is the most efficient way to pack? E.g. supermarket fruit
FCC
What materials have a FCC structure? Give four examples.
Ductile metals as there are many opportunities for planes to slide over each other.
Copper, aluminium, silver, gold.
Where are atoms located in BCC structure?
At each corner and the cube centre.
What is CN for BCC?
8
What is APF for BCC?
0.68
How many slip planes does BCC have?
6 (body diagonal)
How many slip directions does BCC have?
2
How many slip systems does BCC have?
6x2 = 12
What do BCC materials experience?
A ductile-brittle transition.
What do BCC materials feature? Give 3 examples.
A fatigue limit.
Chromium, Ferrite Iron (alpha), Tungsten.
What is the arrangement of a HCP unit cell?
Hexagonal unit cell where top and bottom consists of 6 atoms with another atom in the centre, and the middle plane consists of 3 atoms.
What is CN for HCP?
12
What is APF for HCP?
0.74 (like FCC)
How many slip planes does HCP have?
1 (hex. side)
How many slip directions does HCP have?
3
How many slip systems does HCP have?
1x3 = 3
Which metals have HCP structure? Give four examples.
Least ductile.
Cadmium, magnesium, titanium, zinc.
What is the difference between HCP and FCC structure?
HCP is ABA, FCC is ABC. FCC has more symmetry.
What are single crystal materials?
When the periodic and repeating arrangement of atoms is perfect and extends through the entirety of the specimen.
Give 3 examples of extreme technology single crystal materials are used for.
Electronic and optical material (Si wafers).
High performance turbine blades.
Abrasive materials (synthetic diamond).
What does anisotropic mean?
Properties vary with direction.
What impacts the production of silicon wafers?
Silicon in elemental abundance but high embodied energy and cost of production.
What are single crystal turbine blades made form?
Nickel-based superalloys incorporating chromium, cobalt and rhenium.
What crystal structure do most engineering materials have?
Polycrystalline.
Polycrystalline materials often have isotropic properties, what does this mean?
Same properties in all directions.
When do polycrystalline materials have anisotropic properties?
When the grains are textured.
When do amorphous materials occur?
For complex structures and rapid cooling.
What are many important material properties due to?
Presence of imperfections.
What are the three types of point defects (0D)?
Vacancy atoms.
Interstitial atoms.
Substitutional atoms.
What are line defects (1D)?
Dislocations.
What are area defects (2D)?
Grain boundaries.
What are the three types of volume defects (3D)?
Precipitates.
Cracks.
Porosity.
What are vacancies?
Vacant atomic sites in a structure.
How does diffusion in a solid occur?
Interchange of a vacancy and a neighbouring atom.
What are self-interstitials?
“Extra” atoms positioned between atomic sites.
Why does the equilibrium concentration of point defects vary with temperature?
Greater temperature means greater energy, therefore atoms move around more so there are more defects.
What are intrinsic point defects?
Defects due imperfections without an impurity.
What are extrinsic point defects?
Defects due to an impurity being added to a host (alloy).
What are substitutional point defects?
When an impurity atom replaces a host atom.
What are interstitials?
When a smaller impurity atom positions between host atomic sites.
What occurs when your have larger amounts of an impurity?
Second phase particle which has a different composition and often a different structure.
What are the four conditions for substitutional solid solution point defects?
- Difference in atomic radii <15%.
- Proximity in periodic table. I.e. similar electronegatives.
- Same crystal structure for pure metals.
- Valency - all else being equal, a metal will have greater tendency to dissolve a metal of higher valency than lower valency.
What is an edge dislocation?
Extra plane of atoms inserted in a crystal structure.
What is a screw dislocation?
Spiral planar ramp resulting from shear deformation.
What is the Burger’s Vector?
A measure of lattice distortion.
What causes dislocations to move?
Applied stress.
How do dislocations move?
Incrementally breaking/re-making bonds.
How does plastic deformation occur in metals and alloys?
Large numbers of dislocation move (or will just fracture like a ceramic).
Why is dislocation motion easier in metals?
Non-directional bonding.
Close-packed directions for slip.
Why is dislocation motion hard in covalent ceramics? E.g. Si, diamond.
Directional (angular) bonding.
Why is dislocation motion hard in ionic ceramics? E.g. NaCl
Need to avoid ++ and – neighbours.
What typically determines the strength of a material?
Dislocation density.
What are the two definitions of dislocation density?
- Total dislocation length per unit volume of material.
2. Number of dislocations that intersect a unit are of a random section.
Typical dislocation density for carefully solidified metals?
10^3 / mm^2
Dislocation density for heavily deformed metals?
10^9 - 10^10 / mm^2
Dislocation density for heat treated metals?
10^5 - 10^6 / mm^2
Dislocation density for ceramics?
10^2 - 10^4 / mm^2
Dislocation density for single crystal silicon for ICS?
0.1 - 1 / mm^2
What does the low density in grain boundaries allow for?
High mobility, high diffusivity, high chemical reactivity.
How does the angle of a grain boundary influence mechanical properties?
The angle influences the way dislocations move past which influences properties.
What happens everytime a dislocation meets a grain boundary?
There is an increase in force.
Where do impurities collect?
Grain boundaries. = weakness.
How does grain size affect yield stress?
A reduction in grain size leads to an increase in yield stress.
How can we view grain boundaries?
Using electron backscatter diffraction (EBSD).
How do crystallites (grains) vary in size?
Large - single crystal of quartz, grains on a Al lamppost.
Small - mm or less, have to use a microscope to observe.
What is an example of a deliberate volume defect? Why?
Precipitates in Al alloys. Precipitates act as barriers to dislocations and increase strength.
What volume defects affect brittle materials?
Cracks and damage.
What volume defect occurs in ceramics and metals?
Porosity
