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Prelims > General > Flashcards

General Flashcards

1
Q

4 Quantum Numbers

A

n (principal), l (subshell), m_l (spin moment), m_s (magnetic moment)

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2
Q

Pauli Exclusion Principle

A

Each electron state can hold no more than 2 electrons that must have opposite spin

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2
Q

How does bonding energy impact melting temperature and thermal expansion?

A

Higher bond energy means higher melting temperature and lower thermal expansion

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2
Q

In F vs. r curve, what does a steeper slope mean for mechanical properties?

A

Stiffer material

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3
Q

What are the three primary bonds?

A

Ionic, Covalent, Metallic

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4
Q

What are the Hume-Rothery Rules

A

1.) similar atomic size (size must differ by <15%)

2.) equal crystal structure in element pure state

3.) similar electronegativity

4.) better solubility with increasing atomic number

Tells us if mixed elements will form solid solution or separate phases

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5
Q

Fick’s first law

A

The diffusion flux is proportional to the concentration gradient. This relationship is used for steady-state diffusion situations.

J=-D(dC/dx)

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6
Q

Fick’s second law

A

The time rate of change of concentration is proportional to the second derivative of concentration. This relationship is used in nonsteady-state diffusion situations.

dC/dt = D(d2C/dx2)

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7
Q

Diffusivity, D equation

A

D = D_0 exp(-Q/RT)

Rate of diffusion influenced by diffusing species and tempreature. Q is activation energy for diffusion

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8
Q

Young’s Modulus

A

Stiffness of material, resistance to elastic deformation. 45GPa to 407 GPa for most metals and ceramics, lower for polymers

Measure of interatomic bonding forces

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9
Q

How does temperature affect Young’s Modulus?

A

Increase of temperature decreases Young’s Modulus

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10
Q

Poisson’s Ratio

A

For elastic deformation, the negative ratio of lateral and axial strains that result from an applied axial stress.

between .25-.5 usually, if isotropic (applies to polycrystalline materials)

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11
Q

Yield Strength

A

Stress level when plastic deformation occurs. Calculate with 0.002 offset intersection

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12
Q

Tensile Strength

A

Maximum stress on stress-strain curve. Corresponds to when sample starts necking

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13
Q

Ductility

A

% Elongation or % Reduction in Area sustained at fracture

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14
Q

How does temperature impact ductility?

A

decreasing temperature increases brittleness and decreases ductility

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15
Q

Resilience

A

Capacity to absorb energy when deformed elastically and have that energy recovered, area under elastic region

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16
Q

Toughness

A

Materials resistance to fracture when cracks are present, ability to plastically deform and absorb energy before fracturing. Area under entire curve

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17
Q

True Stress / Strain

A

Takes into account that the area after necking is smaller despite material actually getting stronger

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18
Q

How can we strengthen materials (3 ways)?

A

Decrease mobility of dislocations, but at the expensive of ductility and toughness
- Grain size reduction
- Solid-solution strengthening
- Strain hardening / work hardening / coldworking

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19
Q

What are the three stages of annealing treatment?

A
  1. Recovery - stored internal energy relieved by annihilation of dislocations (diffusion controlled)
  2. Recrystallization - growth of strain-free grains w/ low dislocation density
  3. Grain Growth - increased grain size leads to reduced boundary area and reduced total energy
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20
Q

What is Fracture

A

Crack formation and propagation to stress

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21
Q

Ductile Fracture

A

Plastic deformation with high energy absorbance before fracture - stable, preferred

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22
Q

Brittle Fracture

A

No deformation, rapid crack propagation perpendicular to applied tensile stress

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23
Q

Transgranular Fracture

A

Fracture crack passes through grains

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24
Q

Intergranular Fracture

A

Fracture crack passes along grains

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25
Q

Fracture Toughness

A

Resistance to brittle fracture when crack is present

26
Q

Fatigue

A

Failure is possible at lower loads if applied stress is dynamic or fluctuating. Brittle-like in nature even in ductile materials, very little plastic deformation is associated with failure

27
Q

What does an S-N Curve tell us?

A

Stress versus log(number of cycles to failure). Shows that a lower number of cycles need higher stresses to fail. There is sometimes a fatigue limit

28
Q

Two ways to prevent cracks and failure?

A

Case hardening (enhances surface and fatigue life by diffusing carbon to the outer layer) and Shoot-peening (plastically deform outer layer to improve fatigue behavior)

29
Q

Creep

A

High temperature and static mechanical load, time dependent

30
Q

What are the four stages of creep?

A
  1. Instantaneous elastic deformation
  2. Primary: strain hardening
  3. Secondary: stead-state, balancing strain hardening and recovery
  4. Tertiary: rupture, microstructural and metallurgical changes
31
Q

How does non-equilibrium cooling differ from equilibrium cooling?

A

Cored structure with slightly higher melting temp element in the core versus same composition in the entire grain

32
Q

Eutectoid

A

solid -> two solid phases

33
Q

Eutectic

A

liquid -> two solid phases

34
Q

Peritectic

A

solid + liquid -> solid

35
Q

Gibb’s Phase Rule

A

degrees of freedom (F = C+N-P)
C = number of components
N = number of noncompositional variables (T, V, or P)
P = number of phases present

36
Q

alpha-Ferrite

A

pure Iron, low solubility of carbon, BCC, room temperature

37
Q

Austenite

A

pure Iron, FCC, 912C

38
Q

Ferrite

A

pure Iron, BCC, 1394

39
Q

Cementite

A

Fe3C, iron carbide, very hard and brittle

40
Q

Pearlite

A

Lamellae between alpha-ferrite and cementite

41
Q

Bainite

A

Ultrafine, needle-like cementite in alpha-ferrite

42
Q

Spheroidite

A

alpha-ferrite matrix with Fe3C spheres

43
Q

Martensite

A

metastable iron-carbon phase with BCT crystal structure. Transformation is diffusionless

44
Q

Brass

A

Cu + Zn

45
Q

Bronze

A

Cu + tin, Al, Si, Ni

46
Q

What is the structure of silica glasses like?

A

Si tetrahedron as basic unit, otherwise disordered

47
Q

What do “network modifiers” and “intermediates” do in silica glasses?

A

Cations and other oxides that stabilize network, lower melting point, and lower viscosity by spreading out network

48
Q

Frankel Defect

A

Cation from normal place to interstitial place (vacancy + interstitial)

49
Q

Schottky Defect

A

One cation + one anion removed, placed externally

50
Q

Why is slip in ceramics difficult?

A

few slip system because of electrically charged ions that repel dislocation motion, complex dislocation structures

51
Q

soda-lime glass

A

70 wt% SiO2, Na2O (soda) + CaO (lime)

52
Q

What are the main types of bonding in polymers and where are they located?

A

Covalent bonding between monomers, van der Waals and hydrogen bonding between molecules

53
Q

What is the difference between low molecular weight and high molecular weight polymers?

A

Molecular weight - measure of how long on average a chain is

54
Q

Monodisperse vs. Polydisperse polymers

A

Monodisperse - each polymer about the same size
Polydisperse - each polymer different lengths

55
Q

Describe polymer properties depending on crystallinity, non-crystallinity, melting temp, and glass transition temperature

A

Crystallizable polymers:
- below melting temperature, means that there are semi-crystalline regions. If below glass transition temperature, they are brittle solids. If above glass transition temperature, they are tough solids
- above melting temperature, and are still solid, then it must be a cross-linked elastomer

Amorphous polymers:
- No melting temperature associated with them
- If above Tg and not cross-linked, then it is a viscoelastic liquid/solid
- If below Tg, then it is a polymer glass

56
Q

Difference between number average molecular weight and weight average molecular weight?

A

number average = sum of number fraction of polymer size * molecular weight (g/mol) per molecule. Sensitive to smaller molecules and a better indicator of Tm, mechanical properties. Polymer size that statistically appears more often

weight average = sum of weight fraction (weight of all polymer of certain size over total weight of all polymers) * molecular weight (g/mol) per molecule. Sensitive to larger molecules, better indicator of bulk properties (melt and solution viscosity)

57
Q

What is the polydispersity index?

A

Mw/Mn >=1

if PDI =1, all chains are the same MW

58
Q

Difference between chain growth and step growth?

A

Chain growth: use a initiator to create active sites that snakes around.

Step growth: functional groups react together

59
Q

Tacticity in polymers and how they impact properties?

A

Tacticity is the spatial arrangement of R groups in a polymer. Affects crystalline domains and thus the mechanical and thermal properties
Atactic - R groups random on chain. Amorphous, so no Tm, Tc
Isotactic - R groups same side
Syndiotactic - R groups alternating sides

Isotactic and Syndiotactic can have alternating R groups

60
Q

How do different structures of polymers impact crystallinity?

A

Favored by simple, linear polymers
Branched polymers are not highly crystalline
Networked/cross-linked polymers amorphous
Bulky / complex side groups also inhibit crystallization
Monomers that are ordered (block, alternating polymers) more likely to crystallize

61
Q

Impact of Tg and Tm on properties?

A

Below Tg: no thermal energy to activate movements of large chain segments, rigid and brittle
Above Tg: amorphous part of polymer starts to move, rubbery, easily deformable
Above Tm: crystalline part of polymer melts, loss of long range order

62
Q

Draw DSC curves for amorphouse and semi-crystalline polymers

A

DSC measures how temperature changes as input heat

Tm: positive heat flow - sample absorbs heat
Tc: negative heat flow - sample releases heat
Tg: sample absorbs heat

Amorphous: Tg: increase in heat, Tc, dip in heat as crystallizes, Tm: rise in heat as crystals melt

Semicrystalline: Tg: increase in heat, Tm: rise in heat as crystals melt

63
Q

Factors impacting Tg

A

Depends on presence of free volume - more free volume means higher mobility of chains, means less heat required to move around, means lower Tg

larger chain stiffness = larger Tg
hydrogen bonding groups = larger Tg
steric side groups = larger Tg (inhibits chain rotation)
cross linking = larger Tg
presence of crystals = larger Tg
larger molecular weight = larger Tg (increased chain entanglement)
immiscible blends (block copolymers) have 2 Tgs!
miscible blends / random copolymers have 1 intermediate Tg

64
Q
A

Prelims (5 decks)

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