MAE 3324 Ch4 Flashcards
Define “crystalline defect.”
Refers to a lattice irregularity having one or more of its dimensions on the order of an atomic diameter. Classification of crystalline imperfections is frequently made according to the geometry of the defect.
What is a point defect?
A defect associated with one or two atomic positions.
Is a linear defect a one or two-dimensional defect?
One dimensional
What is the simplest point defect? Describe it.
Vacancy
A vacant lattice site. It’s normally occupied but in this case, an atom is missing.
T or F
All crystalline solids contain vacancies and it is impossible to create materials without them to some extent?
True
Vacancies are due to what thermodynamic principle?
Entropy. Vacancies increase the entropy of the crystal.
The number of vacancies in a crystal increases or decreases with what? Give the equation proving this.
Temperature Nv = number of vacancies N = total number of atomic sites [#/m^3] Qv = Energy required to for the formation of a vacancy [J/mol or eV/atom) T = absolute temp [K] k = boltsman constant [1.38E-23 J/atom*k
N_v = Nexp(-Qv/(kt))
The number of vacancies increases _______ with temperature?
Exponentially.
Define self-interstitial
An atom from the crystal that is crowded into an interstitial site which is a small void space that, under ordinary circumstances, is not occupied.
What is more likely; self-interstitial or vacancy
vacancy.
Define “solid solution.”
A new metal that is formed by adding impurity atoms to a metal.
Extra Notes: The addition of impurity atoms to a metal results in the formation a solid solution and/or a new second phase depending on the kinds of impurity their concentrations and the temperature of the alloy.
Define “solvent” wr/ to solid solutions.
The solvent is the element or compound that is present in the greatest amount.
Define “solute” wr/ to solid solutions.
The solute is the element or compound in a minor concentration.
“As far as white people go at UTA, you’re the solute.”
What impurity defects are found in solid solutions?
Substitutional and interstitial.
A solid solution is formed when?
solute atoms are added to the host material, and the crystalline structure is maintained and no new structures are formed.
Describe a solid solution substitutional impurity.
Solute or impurity atoms replace or substitute for the host atoms, creating a new material.
Extra Notes: Several features of the solute and solvent atoms determine the degree to which the former dissolves in the latter. These are expressed as the four Hume-Rothery rules.
Describe the first Hume-Rothery rule.
Atomic size factor: Appreciable quantities of a solute may be accommodated in this type of solid solution only when the difference in atomic radii between the two atom types is less than about +/- 15%. Otherwise, the solute atoms create substantial lattice distortions and new phase forms.
Describe the second Hume-Rothery rule
Crystal Structure: For appreciable solid solubility, the crystal structures for metals of both atoms types must be the same.
Describe the third Hume-Rothery rule
Electronegativity factor: The more electropositive one element and the more electronegative the other, the greater the likelihood that they will form an intermetallic compound instead of a substitutional solid solution.
Describe the fourth Hume-Rothery rule
Valances: Other factors being equal, a metal has more of a tendency to dissolve another metal of higher valency than to dissolve one of a lower valency.
For both FCC and BCC crystal structure, there are how many types of interstitial sites?
2.
Tetrahedral
octahedral
These are distinguished by the number of nearest neighbor host atoms (coordination number).
Tetrahedral interstitial sites have a coordination number of?
Four
Octahedral interstitial sites have a coordination number of?
Six
T or F
In metallic materials, the relatively high atomic packing factor means that interstitial positions are relatively large?
False
Interstitial positions are very small in metals.
What are the two most common ways to express the composition of alloys?
Weight percent and atom percent.
What is the formula for the weight percent for a two element alloy?
For two atoms c1 an c2, the concentration of c1 is
C1 = m1/(m1+m2)*100
mi = weight of individual element.
Atom percent is based upon?
moles of one element vs total moles in the alloy.
The number of moles in some specified mass of a hypothetical element 1 may be computed as?
n_m1 = m1’/A1
where
m1’ = mass in grams
A1 = atomic weight
Computation of atom percent in a two-element alloy.
C1’ = n_m1 / (n_m1 + n_m2) * 100
Define a “dislocation.”
A linear or one-dimensional defect around which some of the atoms are misaligned.
What is an edge dislocation?
When an additional plane of atoms terminates inside a crystal. See figure 4.4 in the book.
What is a screw dislocation and how is it formed?
A dislocation that is due to shear stress. Induces a twisting or helical structure. See figure 4.5 in the book.
What is the burgers vector?
The magnitude and direction of the lattice distortion associated with a dislocation.
For and edge dislocation, the Burgers vector, and the dislocation line are?
perpendicular.
For a screw dislocation, the Burgers vector and the dislocation line are
parallel
How might dislocations be introduced into a material?
- Solidification
- Plastic deformation
- quenching
Define “interfacial defects.”
Boundaries that have two dimensions and normally separate regions of the material that have different crystal structures and/or crystallographic orientations.
What do interfacial defects include?
- External surfaces
- Grain boundaries
- Phase boundaries
- Twin boundaries
- Stacking faults
“Every Goat Poops Twice on Saturday.”
Describe “external surfaces” wr/ to interfacial defects.
At the boundary of the material, surface atoms are not bonded to the max number of nearest neighbors and are therefore in a higher energy state than the atoms inside the material. The bond of these surface atoms that are not satisfied gives rise to a surface energy expressed in units of energy per unit area. To reduce this energy, materials tend to minimize if at all possible the total surface area.
Describe “grain boundaries” wr/ to interfacial defects.
The boundary separating two small grains or crystals having different crystallographic orientations in a polycrystalline structure.
Describe “phase boundaries” wr/ to interfacial defects.
Exist in multi-phase materials in which a different phase exists on each side of the boundary. Has a large role in mechanical characteristics.
Describe “twin boundaries” wr/ to interfacial defects.
A special type of grain boundary in which there is a specific mirror lattice symmetry; that is, atoms on one side of the boundary are located in mirror image positions to those of the atoms on the other side.
Twins result from atomic displacement due to shear forces (mechanical twins) or from annealing heat treatments.
T or F
Atoms are always vibrating in their lattice though this doesn’t mean they all possess the same amplitude and frequency?
True
T or F
With transmission electron microscopy the beam of electrons passes through the specimen?
True
T or F
SEM relies on electron backscattering?
True
T or F
SPM does not produce a 3-D topographical map of a substance?
False
Grain size may be specified by?
Average grain diameter.
Give some examples of structure insensitive properties.
- Elastic constants
- melting points
- Density
- specific heat
- Coefficient of thermal expansion
Give some examples of structure-sensitive properties.
- Electrical conductivity.
- Semi-conductive properties.
- Yield strength
- Fracture strength
- Creep strength
Point defects include…
- Vacancy atoms
- Interstitial atoms
- Substitutional atoms
“Very Interesting Snakes.”
Area defects include…
- Grain boundaries
2. Stacking faults
Line Defects include…
Dislocations.
Volume defects include…
- Pores
2. Cracks
Low density in grain boundaries results in what three properties?
- High mobility
- High diffusivity
- High chemical reactivity
“H cubed MDC”
Describe the difference between a vacancy and a self-interstitial.
A vacancy is an empty atomic site in a structure. An interstitial is the structural opposite. An extra atom is positioned between atomic sites.
T or F
Graphene tends to loosely pack?
True
T or F
Self-interstitials are much more likely in metals.
False, since atoms are very closely packed in metals it takes a lot of energy to squeeze an atom into space that’s already occupied.
The presence of vacancies increases the ___ of the material.
Entropy.
T or F
A solid solution is compositionally homogeneous, and impurity atoms are randomly and uniformly scatter throughout the solid.
True
What two impurity defects are found in solid solutions?
- Substitutional
2. Interstitial
Describe a substitutional impurity defect with respect to solid solutions.
For the substitutional type, solute or impurity atoms replace or substitute for the host atoms.
Extra Notes: Several features of the solute and solvent atoms determine the degree to which the former dissolves in the latter. These are expressed as four Hume–Rothery rules.
Mixed Dislocations are a combination of what?
Both edge and screw dislocations.
