Chapter 4 - Imperfections Flashcards
What are the three steps of the Solidification process of molten metal?
- Nuclei develop the solid phase form
- Crystals grow until their boundaries each other
- crystals touch, and become grains.
Is there such a thing as a perfect crystal?
No
What are the two types of grains, What makes them unique?
equiaxed → roughly the same dimension in all directions
Columnar → grains elongated in one direction
What usually creates Columnar grains?
slow cooling
What creates equiaxed grains?
rapid cooling
What is the difference between a high-angle grain boundary, and a small-angle grain boundary?
High angle: large angle of misalignment (greater than a couple of degrees)
Small angle: small angle of misalighnment
What are the types of imperfections in Solids?
Point defects → Vacancy atoms, Intersitital atoms, Substitutional atoms, Self-interstitial atoms
Line defects → Dislocations
Area Defects → Grain Boundaries, Surfaces
What are vacancy point defects?
Vacant atomic sites in a structure.
What are self-interstitial point defects?
An additional atom that puts itself between atomic sites.
What is this equation
𝑁𝑣 = 𝑁𝑒 [− 𝑄𝑣 /
(𝑘𝑇)]
do? What do the variables inside it represent?
Finds the number of defects.
𝑁𝑣 = # of defects
N = # of potential defect sites
𝑄𝑣 = Activation energy
𝑘 = Boltzmann’s constant (given in the formula sheet)
𝑇 = Temperature
Are pure metals consisting of only ONE type of atom possible?
NOPE
What are the three benifits of Alloying metals?
they can become more corrosive resistant, harder, and stronger.
what is a solid solution?
when solute atoms are added to the host material, the crystal structure is maintained, and no new structures are formed.
What are the two cases for when impurity atoms are added to a metal to create a solid solution?
Substitutional solid soln. → Where atoms around the same size take some of the host atoms’ places in unit cells
Interstitial solid soln. → a smaller atom fits within the void space of an atom.
What are the conditions for solid solutions for substitutional Impurity? (W. Hume – Rothery rule)
- The atomic radius must be within 15%
- They must have similar electronegativities
- Same crystal structure for pure metals
- Similar Valency
For substitutional Impurity, if all else is equal between 2 pure atoms, which one is more likely to dissolve?
If both valencies are the same distance apart, the one with the higher valency is more likely to dissolve.
For the crystalline structure, BCC give the Coordination #, # of sites, and # of equivalent interstitial atoms in the unit cell for both Tetrahedral and Octahedral sites.
Tetrahedral → coordination number 4, 24 sites, and 12 equivalent tetrahedral atoms in the cell
Octahedral → coordination number 6, 18 sites, and 6 equivalent tetrahedral atoms in the cell
For the crystalline structure, FCC give the Coordination #, # of sites, and # of equivalent interstitial atoms in the unit cell for both Tetrahedral and Octahedral sites.
Tetrahedral → coordination number 4, 8 sites, and 8 equivalent tetrahedral atoms in the cell
Octahedral → coordination number 6, 13 sites, and 4 equivalent tetrahedral atoms in the cell
Place the interstitial sites in both FCC, and BCC atoms
Did you do it?
In metallic materials is the concentration of interstitial impurity atoms high or low? why?
low because of the extremely high APF
For ceramic materials what type of point defects can occur?
Because of the polarity of the ions and medium APF all types of point defects can occur. (vacancies, interstitials, and subsitiutial defects) In addition different defects can happen with the different ions as they have different sizes and valencies.
For Ashby Maps, Rank the different types of compounds from low Young’s modulus (10^-3) and Density (0.1) to high (1000, and 12 respectively)
Polymers, metals, then ceramics (but ceramics and metals have same density, just different young’s modulus)
What does this formula do?
𝐶1 = 𝑚1 / (𝑚1 + 𝑚2) 𝑋100
Calculates the wieght percent of a unit cell.
Where m is the wieght of the atom.
What does 𝐶1′ = 𝑛𝑚1 / (𝑛𝑚1 + 𝑛𝑚2) 𝑋100 Do?
Calculates the atom percent of a unit cell.
where nm1 = number of moles of component
What does this formula do?
𝜌𝑎𝑣𝑒 = 100 / [(c1 / p1) + (c2 / p2)]
Calculates the average density of unit cell
What does this formula do?
A𝑎𝑣𝑒 = 100 / [(c1 / A1) + (c2 / A2)]
Calculates the average atomic weight of solid solution
What are Dislocations?
Are one-dimensional defects around which atoms are misaligned
What are edge dislocations?
An extra half-plane of atoms inserted in a crystal structure
What is a screw dislocation?
A spiral planar ramp resulting from shear deformation
The lattice plane shifts by one layer or more
What is burgers vector (b) facing in edge and screw dislocations
Edge dislocation = perpendicular line vector
Screw dislocation = parallel to line vector
What are Phase boundaries?
The junction of two crystals which differ in lattice structure and composition.
What are Twin boundaries?
one side of the crystal plane is positioned as a reflected version across a Twin plane
What are stacking faults?
The sequence ABCABC becomes ABCABABC
What are Dislocations? Give an example.
movement of a material when stress is applied
Permanent (plastic) deformation.
