General Materials Science

Question 1

What is Bragg’s Law, and how can you prove it?

Answer 1

n(lambda)=2dsin(theta)
Draw two layers of atoms in a crystal and calculate the angle to form a coherent diffracted beam

Question 2

What is the symbol (like parenthesis) for crystallographic planes, plan families, directions, and direction families?

Answer 2

( Plane )
{ Plane Family }
[ Direction ]
< Direction Family >

Question 3

What is the difference between saturated and unsaturated polymers?

Answer 3

Saturated polymers: only single bonds, a bonded atom must be removed to add a new one
Unsaturated polymers: includes double or triple bonds, means that it can be attached to a larger unsaturated molecule by rearranging the bonds

Question 4

Explain the different patterns in polymers: cis/trans, and stereoisomerism

Answer 4

Cis polymers have functional groups on the same side, while trans polymers have functional groups on opposite sides of the carbon chain.
Isotactic polymers have functional group pattern: R-H-R-H-R-H
Syndiotactic polymers have functional group pattern: R-H-H-R-R-H-H-R (like syncopated music)
Atactic polymers have a random pattern

Question 5

What are the two kinds of point defects in ionic materials (like ceramics)?

Answer 5

Schottky Defects: one missing cation and one missing anion
Frenkel Defects: one cation/anion (typically the smaller one) moves from its correct interstitial site to an adjacent one (doubling up)

Question 6

What are the rules for substitutional solubility?

Answer 6

1. No more than 15% difference in atomic radii
2. should have the same crystsal structure
3. should have similar electronegativity
4. Same valency preffered; an element of higher valency is more likely to be dissolved by an element of lower valency; too great a difference and intermetallic compounds are more likely

Question 7

What are the Hume-Rothery rules for interstitial solubility?

Answer 7

1. Solute atoms should have a radius less than 59% of the solvent atoms
2. They should have similar electronegativity
3. They should have the same valence

Question 8

What are the two main types of diffusion movement?

Answer 8

Vacancy diffusion and interstitial diffusion

Question 9

Identify the important parts of a tensile stress strain curve

Answer 9

Elastic region: linear part at the beginning, used to find Young’s modulus
Plastic region: everything else
Yield strength: local maxima right after elastic regime
Ultimate strength: absolute maxima
Strain hardening: after yield strength before ultimate strength
Necking: after ultimate strength

Question 10

What are the three main kinds of polymeric stress strain curves?

Answer 10

Brittle: fairly linear, large slope
Plastic: after yield strength has a wide dip, then failure
Elastic: small slope, gradually increases until failure

Question 11

What are the two types of dislocations and how to identify them with their Burger’s vector?

Answer 11

Edge dislocation: Burger’s vector is perpendicular to dislocation line vector, a row of atoms was added/removed,
Screw dislocation: Burger’s vector is parallel to dislocation line vector, part of a plane of atoms is pushed down

Question 12

What are the different kinds of strengthening mechanisms? What are the sections of annealing process?

Answer 12

Solid solution strengthening, precipitation hardening, strain hardening, and grain size reduction
Recovery, recrystallization, and grain growth

Question 13

How can you strengthen a polymer?

Answer 13

Drawing, orienting the crystals of a semicrystalline polymer along the tensile axis

Question 14

What is the difference between ductile and brittle fracture?

Answer 14

Brittle fracture has cleavage, ductile fracture has necking, and in between likely has cupping

Question 15

What are the different regions within a ceramic fracture site?

Answer 15

In order of increasing radii from origin:
Mirror region: flat, no noticeable texture
Mist region: small dust/mist like imperfections
Hackle region: larger cracks that propagate through the material

Question 16

What does metastable mean?

Answer 16

A metastable material occupies a local minima in an energy curve, but is not the absolute minima. Often times describes materials that are not thermodynamically stable but are prevented from transitioning by kinetics

Question 17

If a molten alloy is cooled quickly, but not quenched, what kind of pattern is formed once cooled?

Answer 17

When crystals begin nucleating, it will have a higher composition of the component with a higher melting point (meaning that it is easier to solidify), the composition will change as it cools and solidifies, have a higher fraction of the component with a lower melting temperature, creating a composition gradient

Question 18

What is an intermetallic compound?

Answer 18

Intermetallic compounds are intermediate compounds with distinct formulas, often found in alloys with large valence difference, and occupy the middle portion of the phase diagram

Question 19

What are the three kinds of transformations upon cooling?

Answer 19

Eutectic: L -> a + b
Eutectoid: a -> b + c
Peritectic: a + L -> b

Question 20

What are the different phases of steel, and their characteristics?

Answer 20

Ferrite: pure iron, room temperature, α-iron, BCC
Austenite: pure iron, 912-1394 °C, γ-iron, FCC
Cementite: 6.70 wt% C, Fe3C, orthorhombic
Pearlite: ferrite and cementite lamellae, eutectoid (.76 wt%)
Bainite: ferrite and cementite like pearlite, but forms very fine needles or plates instead of lamellae, formed at a lower temperature than pearlite
Spheroidite: formed if pearlite or bainite are heated just below eutectoid temperature for a long time, ferrite and cementite precipitates
Martensite: BCT, metastable phase formed when austenite is quenched
Tempered martensite: separated from a single phase into ferrite and cementite, like spheroidite but oblong

Proeutectoid: means that it was formed above eutectoid temperature, visually is the non-banded portion next to pearlite

Question 21

In polymers, what happens at the glass transition temperature and melting temperature?
What are the three aspects of polymers that influence chain flexibility and Tg?

Answer 21

Polymer goes from liquid to rubber to rigid solid
Chain flexibility goes down and Tg increases with
1. Bulky side groups
2. Polar groups
3. Double bonds and aromatic groups

Question 22

What are the different kinds of ceramic materials?

Answer 22

Glass-ceramic materials: heat treating an amorphous ceramic to transform it into fine grained crystals
Structural clay products: bricks and tiles
Whiteware: porcelain, china, toilets etc
Refractories: ceramics that are extremely thermally and chemically stable
Abrasive Ceramics: used in things like polishing
Cement: clay and lime plus calcined clinker

Question 23

What are some different fabrication techniques for metals?

Answer 23

Working: forging, rolling, extruding, and drawing
- Can be hot or cold working depending if it is above or below recrystallization temperature
- Hot working allows for large deformations, retention of ductility, but surface oxidation leads to material loss and poor finish
Cold working: strain hardening and loss of ductility, good finish and great dimensional control
Casting: molten metal is poured into a type of mold and allowed to cool; most beneficial for complicated shapes and extremely low ductility alloys
Powder Metallurgy: Lasers melt certain parts of a powder layer to develop a nonporous 3D structure, good for metals with low ductility, often leaves sample with internal stress, can be annealed

Question 24

What are the different kinds of casting?

Answer 24

Sand Casting: two piece sand mold is packed around desired shape
Die Casting: liquid metal is forced into the die at high pressure
Investment / Lost Wax Casting: Wax is formed in the desired shape, then set plaster around it, wax is melted out and filled with metal
Lost Foam / Expendable Pattern Casting: Polystyrene beads or other plastics/foams are formed into the desired shape, and sand is packed around it; the metal is poured directly on the foam which vaporizes
Continuous / Strand Casting: Metal is poured into a continuous strand of the desired cross section and solidified in a water cooled mold

Question 25

How does crystal structure influence mechanical properties?

Answer 25

Having more close packed planes like FCC and HCP makes it easier for dislocations to move, this is because the Burger’s vector is smallest in a CPP

Question 26

What is the stacking sequence for FCC and HCP materials?

Answer 26

FCC: ABC
HCP: ABA

Question 27

What is the crystal structure of Gold, Aluminum, Chromium, Tungsten, Titanium, Silicon, and Steel?

Answer 27

Gold: FCC
Aluminum: FCC
Chromium: BCC
Tungsten: BCC
Titanium: HCP
Silicon: Diamond cubic
- Steel
Ferrite: BCC
Austenite: FCC
Cementite: Orthorhombic

Question 28

What are the seven crystal systems?

Answer 28

α = β = γ =90
- a = b = c: Cubic
- a = b ≠ c: Tetragonal
- a ≠ b ≠ c: Orthorhombic
Other
- a = b = c & α = β = γ ≠90: Rhombohedral/Tetragonal
- a ≠ b ≠ c & α = β = 90 ≠ γ: Monoclinic
- a ≠ b ≠ c & α ≠ β ≠ γ ≠ 90: Triclinic
- a=b≠c & α=β=90 γ=120: Hexagonal

Question 29

What are the other unit cells?

Answer 29

Rock Salt (AX): FCC with another FCC structure shifted any direction by 1/2 edge length
Cesium Chloride (AX): SC with a different atom in the middle
Zinc Blend (AX) : FCC with a different atom at tetragonal sites
Fluorite (AX2): Rock Salt except all of one atom, with another atom at tetragonal sites like Zinc Blend
Perovskite (ABX3): A as SC, B in the center, and X at the centers of the faces
Diamond: like Zinc Blend but all one atom
Graphite: Layers of graphite, the center of each ring becomes the intersection of three rings in the layer below it

Question 30

Derive Gibb’s phase rule

Answer 30

F = C - P + 2
Variables = P(C-1)+2
Relations = C(P-1)

V-R –> GPR