General Flashcards
4 Quantum Numbers
n (principal), l (subshell), m_l (spin moment), m_s (magnetic moment)
Pauli Exclusion Principle
Each electron state can hold no more than 2 electrons that must have opposite spin
How does bonding energy impact melting temperature and thermal expansion?
Higher bond energy means higher melting temperature and lower thermal expansion
In F vs. r curve, what does a steeper slope mean for mechanical properties?
Stiffer material
What are the three primary bonds?
Ionic, Covalent, Metallic
What are the Hume-Rothery Rules
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
Fick’s first law
The diffusion flux is proportional to the concentration gradient. This relationship is used for steady-state diffusion situations.
J=-D(dC/dx)
Fick’s second law
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)
Diffusivity, D equation
D = D_0 exp(-Q/RT)
Rate of diffusion influenced by diffusing species and tempreature. Q is activation energy for diffusion
Young’s Modulus
Stiffness of material, resistance to elastic deformation. 45GPa to 407 GPa for most metals and ceramics, lower for polymers
Measure of interatomic bonding forces
How does temperature affect Young’s Modulus?
Increase of temperature decreases Young’s Modulus
Poisson’s Ratio
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)
Yield Strength
Stress level when plastic deformation occurs. Calculate with 0.002 offset intersection
Tensile Strength
Maximum stress on stress-strain curve. Corresponds to when sample starts necking
Ductility
% Elongation or % Reduction in Area sustained at fracture
How does temperature impact ductility?
decreasing temperature increases brittleness and decreases ductility
Resilience
Capacity to absorb energy when deformed elastically and have that energy recovered, area under elastic region
Toughness
Materials resistance to fracture when cracks are present, ability to plastically deform and absorb energy before fracturing. Area under entire curve
True Stress / Strain
Takes into account that the area after necking is smaller despite material actually getting stronger
How can we strengthen materials (3 ways)?
Decrease mobility of dislocations, but at the expensive of ductility and toughness
- Grain size reduction
- Solid-solution strengthening
- Strain hardening / work hardening / coldworking
What are the three stages of annealing treatment?
- Recovery - stored internal energy relieved by annihilation of dislocations (diffusion controlled)
- Recrystallization - growth of strain-free grains w/ low dislocation density
- Grain Growth - increased grain size leads to reduced boundary area and reduced total energy
What is Fracture
Crack formation and propagation to stress
Ductile Fracture
Plastic deformation with high energy absorbance before fracture - stable, preferred
Brittle Fracture
No deformation, rapid crack propagation perpendicular to applied tensile stress
Transgranular Fracture
Fracture crack passes through grains
Intergranular Fracture
Fracture crack passes along grains
Fracture Toughness
Resistance to brittle fracture when crack is present
Fatigue
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
What does an S-N Curve tell us?
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
Two ways to prevent cracks and failure?
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)
Creep
High temperature and static mechanical load, time dependent
What are the four stages of creep?
- Instantaneous elastic deformation
- Primary: strain hardening
- Secondary: stead-state, balancing strain hardening and recovery
- Tertiary: rupture, microstructural and metallurgical changes
How does non-equilibrium cooling differ from equilibrium cooling?
Cored structure with slightly higher melting temp element in the core versus same composition in the entire grain
Eutectoid
solid -> two solid phases
Eutectic
liquid -> two solid phases
Peritectic
solid + liquid -> solid
Gibb’s Phase Rule
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
alpha-Ferrite
pure Iron, low solubility of carbon, BCC, room temperature
Austenite
pure Iron, FCC, 912C
Ferrite
pure Iron, BCC, 1394
Cementite
Fe3C, iron carbide, very hard and brittle
Pearlite
Lamellae between alpha-ferrite and cementite
Bainite
Ultrafine, needle-like cementite in alpha-ferrite
Spheroidite
alpha-ferrite matrix with Fe3C spheres
Martensite
metastable iron-carbon phase with BCT crystal structure. Transformation is diffusionless
Brass
Cu + Zn
Bronze
Cu + tin, Al, Si, Ni
What is the structure of silica glasses like?
Si tetrahedron as basic unit, otherwise disordered
What do “network modifiers” and “intermediates” do in silica glasses?
Cations and other oxides that stabilize network, lower melting point, and lower viscosity by spreading out network
Frankel Defect
Cation from normal place to interstitial place (vacancy + interstitial)
Schottky Defect
One cation + one anion removed, placed externally
Why is slip in ceramics difficult?
few slip system because of electrically charged ions that repel dislocation motion, complex dislocation structures
soda-lime glass
70 wt% SiO2, Na2O (soda) + CaO (lime)
What are the main types of bonding in polymers and where are they located?
Covalent bonding between monomers, van der Waals and hydrogen bonding between molecules
What is the difference between low molecular weight and high molecular weight polymers?
Molecular weight - measure of how long on average a chain is
Monodisperse vs. Polydisperse polymers
Monodisperse - each polymer about the same size
Polydisperse - each polymer different lengths
Describe polymer properties depending on crystallinity, non-crystallinity, melting temp, and glass transition temperature
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
Difference between number average molecular weight and weight average molecular weight?
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)
What is the polydispersity index?
Mw/Mn >=1
if PDI =1, all chains are the same MW
Difference between chain growth and step growth?
Chain growth: use a initiator to create active sites that snakes around.
Step growth: functional groups react together
Tacticity in polymers and how they impact properties?
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
How do different structures of polymers impact crystallinity?
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
Impact of Tg and Tm on properties?
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
Draw DSC curves for amorphouse and semi-crystalline polymers
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
Factors impacting Tg
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
