Materials Final Review

8/2/2019 Materials Final Review

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Materials Final Review

General Terms

Composition: Chemical makeup of a material Structure: arrangement of atoms Microstructure: Structure of the material of the material at the microscopic scale Thermal conductivity: ability to conduct heat Yield strength: stress needed for noticeable onset of plastic deformation Maximum tensile strength occurs only on an engineering stress-strain curve and it is the point

when necking begins to become a bigger factor than the strain hardening

Necking concentrates stress on a metal by reducing the cross sectional area Necking strengthens polymers by aligning the fibers with the applied force Toughness: area under the curve of a stress-strain diagram, energy required to break the

material

o Small toughness in ceramics as they dont display any plastic deformationo Large toughness in metals that have plastic deformation and medium tensile strengtho Small toughness in polymers which are unreinforced as they have a very low tensile

strength and plastically deform a lot

Hardness is the resistance to plastic deformations in compression Ductility is the plastic strain at failure, if this is high the material can plastically deform a lot


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Atomic Bonding

Interatomic spacing is the energy well graph Force vs Distance graph shows the elastic modulus Second derivative of the bottom of the energy well is the elastic modulus of the material Stress-strain graph has the slope of the elastic modulus Once plastic deformation begins, if the material is released it will travel back down the same

slope to zero where plastic strain can be measured

Amorphous: only short-range atomic arrangements, with a random arrangement on the longrange

Crystalline: both short and long range order are present in the atomic order Allotropes: Elements that can be made from the same material but have different material

properties

Polymorphic: Compounds exhibiting more than one type of crystal structure Large electronegativity means that there is a high tendency to acquire electrons Metallic, covalent, and ionic bonds are primary while van der waals bonds are secondary Metallic bonding forms a sea of electrons surrounding the atoms

o This promotes good electrical conductivity because there isnt a band gap between theconduction and valence bands

o Ductility is good because the bonds are non-directional Covalent bonding is the formation of a bond by sharing a valence electron among two or more

atoms of similar electronegativity

o There is a directional relationship formed when they are covalently bonded whichresults in the materials being strong and hard but show low ductility

Ionic bonding is when more than one type of atom is present in a material, one atom maydonate its valence electrons to a different atom

o Forms a cation: positive charge and an anion: negative chargeo The opposing ions are attracted to each other to produce an ionic bond

Relative strengths:o Ionic is the strongest and is typically ceramics (non-directional)o Covalent is variable strength and is directional bonds (semiconductors, ceramics,

polymers)

o Metallic is variable strength and is non-directional bonds (metals)o Secondary are smallest strength and have directional bonds (inter-chain polymers)

Coefficients of Thermal expansiono Polymers, metals, ceramics (large - small)o Opposite of the relative bond strengths. As bond strength increases coefficient of

thermal expansion decreases


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Crystal Structures

Short-Range order is an arrangement that extends only to the atoms nearest neighbor Long-Range order is an arrangement that extends through the material

o Found in metals, alloys, ceramics, and some polymers Crystalline materials have lower energies since they are packed closer and have shorter

interatomic bond lengths

Unit cell is the smallest repetitive volume which contains the complete lattice pattern of thecrystal

Basis is the arrangement of lattice sites Simple Cubic is rare due to low packing density, close packed directions are cube edges, with 6

nearest neighbors coordination number is 6, one atom per unit cell

Atomic Packing Factor is the volume of the atoms vs total volume of the cell, this is used to seehow much space is taken up by atoms and how much is air

Body Centered Cubic atoms touch each other along cube diagonals, coordination number is 8and there are 2 atoms per unit cell

Face Centered Cubic atoms touch each other along face diagonals, coordination number is 12,there are 4 atoms per unit cell

Maximum APF is FCC and it is 74%, SC has 52%, and BCC has 68% Single crystals have anisotropic properties as they have slip planes that they deform on most

easily

Polycrystalline have isotropic properties as they are averaged over many different directionalgrains to produce a seemingly equal properties on any orientation

How to determine point locationso 1. Vector repositioned (if necessary) to pass through origin.o 2. Read off projections in terms of unit cell dimensions a, b, and co 3. Adjust to smallest integer valueso 4. Enclose in square brackets, no commas [hkl]

Families of directions are denoted by Linear Density is the number of atoms, over the length of the direction vector in the unit cell Crystallographic plane indices

o 1. Read off intercepts of plane with axes in terms of a, b, co 2. Take reciprocals of interceptso 3. Reduce to smallest integer valueso 4. Enclose in parentheses, no commas i.e., (hkl)

Family of planes is denoted by {hkl}o {100} = (100),(010),(001),(-100),(0-10),(00-1) ** note values should have the bar over

the number not in front of it

Planar density is the atoms in the 2D repeat unit, over the total area of the repeat unit


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Dislocations and Imperfections

Imperfections and dislocations lower strength of materials from their theoretical value to theeffective value

Dislocations are only possible if there is long range order, ie crystalline materials There is no slip in the elastic range, therefore there is no dislocation motion unless bonds are

broken which means you are in the plastic range

Hardness is related to dislocation induction through plastic deformation measured by anindenter

Vacancy, interstitial, substitutional are point defects Dislocations (edge and screw) are line defects Grain boundaries Area Defect Frenkel defects are extra atoms shoved into a lattice Schottky defects are 2 holes created to maintain charge neutrality in an ionic material Edge dislocation the perfect crystal is cut and an extra half-plane of atoms are inserted which

results in an edge dislocation

Burgers vector is required to close the loop of equal atom spacing around the edge dislocation Screw dislocation Perfect crystal is cut and sheared one atom spacing away, along this line is a

screw dislocation where a burgers vector is necessary to close the loop

Dislocation motion when a shear stress is applied to the dislocation and the atoms aredisplaced, the dislocation moves one burgers vector in the slip direction

o Continued movement of the dislocation creates a step and the crystal is deformedwhich is plastic deformation

Slip plane contains both the dislocation line and the Burgers vector, preferential slipdirection, usually contains the highest planar density and large interplanar spacing

Slip direction direction of movement, highest linear density FCC slip system is {111} plane and the direction Ionically bonded materials dont typically slip because this requires atoms of like charge to pass

each other which isnt favorable

o Ceramics are ionically bonded, this is why they are too brittle to slip Peierls-Nabarro stress [] the stress required to move the dislocation from one equilibrium

position to the next

o is the angle of the applied load to the normal stress; is the direction of the appliedload to the slip plane

Surface defects grain boundaries lead to areas with surface energyo Twin boundaries, Stacking faults, domain boundaries, small angle grain boundaries

Dislocations interfere with electrical, optical, and magnetic properties adversely, as dislocationcontent increases in a material its electrical conductivity decreases, and optical properties are

reduced


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Diffusion and Atomic movement

Diffusion net flux of atoms moving through the material so differences in concentration areminimized

Interdiffusion atoms of different type migrate from regions of high concentration to regions oflower concentration, effectively mixing the materials

Activation energy is higher for substitutional atoms than it is for interstitial atoms becausehigher energy is required to squeeze atoms past one and other since substitutional are typically

larger than the interstitial

Diffusion rate increases with temperature increase Interstitial diffusion occurs much faster than substitutional or vacancy due to the lower

activation energy required

Diffusion is faster in less densely packed structures with secondary bonding


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Mechanical Properties

Elastic deformation is the stretching of bonds, when the stress is released they go back to theiroriginal position

Plastic deformation is the planes slipping causing them to hold their new configuration once thestress is released

Engineering stress relies on the original cross sectional area, therefore it reduces with theonset of necking although the material is getting stronger

Engineering strain dimensionless relation between change in length over the original length Modulus of elasticity is defined at zero strain Poisons ratio is the lateral strain over the axial strain As temperature increases maximum engineering stress decreases while the material is more

ductile, the material becomes tougher

o Yield strength and Tensile strength decrease while elongation increaseso Elastic modulus changes due to the interatomic spacing, and bond strength differences

Ductile to brittle transition temperature is where a material acts brittle below a certaintemperature but ductile above it

True stress-strain accounts for the changing cross sectional area of the material and thusdisplays a constantly increasing curve because the material is getting stronger as the cross

sectional area is decreasing

Ductile fracture is accomplished by significant plastic deformation, and is failure by internalshear

Brittle fracture is accomplished by little to no plastic deformation, it is a cohesive failure of acrack propagating through the material

o Leaves a flower pattern called Chevron from the crack propagation pointo Fracture along intergranular, between the grainso Fracture through the grains transgranular

Flaws are areas of stress concentration which is where cracks typically form and propagateo Fracture toughness refers to the ability of a material to withstand a load with a flaw

Fatigue is failure under repeated stresses, initiates at the surface of a part and is responsible for90% of mechanical engineering failures

Increasing temperature causes materials to creep more Annealing after cold working

o First is Recovery which is where dislocations are aligned which increases the electricalconductivity of the material

o Next is Recrystallization where new stress free grains form, smaller than the originalthrough nucleation at the narrow radii of the elongated grains

Recrystallization is the temperature required for the material to completelyrecrystallize in one hour. Boundary between hot and cold working the material

o Finally Grain growth is the growth of the new stress free grains, where larger grainsgrow from the absorption of smaller ones


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Hot workingo Deformation above the recrystallization temperatureo Flow stress is low and elongation is higher as dislocations are annealed away as they are

formed

o No strengthening of the material


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Strengthening Mechanisms

Strain hardening cold worko Increases strength by inducing dislocations into the grains, these dislocations impede

dislocation motion which increases strength

o Increases yield strength, and ultimate tensile strength while reducing ductilityo Strain hardening exponent is higher on materials that are more ductileo Higher strain rate exponent M results in a greater resistance to necking

Solid-solution strengthening (alloying)o Intentionally introducing substitutional or interstitial atoms into the lattice which

increase stress within the lattice, impeding dislocation motion

Grain size reductiono Increases strength by blocking dislocation motion with grain boundaries through the

misalignment of slip planes

Dispersion strengtheningo Uniform of coherent, small precipitates in a more ductile matrix increases strengtho Exceeding the solubility limit causes this second phase to be present in the materialo Precipitate must be coherent and form a definite relationship between the crystal

structure and the matrix which increases stress in the lattice

o Precipitation hardening Age hardening Heat to solution treat the solution to get a solid solution alpha Quench to retain the supersaturated solid solution Reheat to nucleate small dispersant particles throughout the matrix Requirements

Must display decreasing solid solubility with decreasing temperature Must be quenchable Matrix should be relatively soft and ductile and the precipitate should

be hard and brittle

A coherent precipitate must formo Eutectic

Intermetallic compounds are strong but brittle, cementite is an intermetalliccompound

Smaller lamellar spacing increases strength of the material Eutectic composition has the highest tensile strength while hyper and hypo

eutectic alloys have lower gains

Martensitic Strengtheningo Increases strength by a non-uniform contraction to BCT by trapping carbon within the

crystal structure by not allowing diffusion of the atoms out

Known Relationshipso Decreased size of dispersion leads to increased strengtho Uniform dispersion throughout the grain increases strength

Not at grain boundaries


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o Spherical dispersant to prevent cracking increases strengtho Harder dispersant in a softer more ductile matrix increases strength


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Solidification

Nucleation are seeds which act as templates for crystal growth, rate of addition of atoms tonucleus must be faster than the rate of loss

Supercooling drives nucleation as the energy to form a solid is lower than to stay liquid, ordifferent phase of solid

Large supercooling leads to smaller crystals as nucleation is favored over grain growth Homogeneous nucleation spherical nucleus forming within the bulk of that material

o Free energy is dependent of surface area and volume of the nucleio Requires significant supercooling

Heterogeneous nucleationo Energy to nucleate on a surface is lower than for a sphere therefore it is easier to

nucleate heterogeneously than homogeneously

o Nucleating surface is at a mold wall, impurity, or grain boundaryo Less supercooling is required

Kinetics: the relationship between nucleation and growth, reason that the TTT diagram has itsshape

Growth reduces the system energy once the embryo forms Rapid cooling results in finer dendrites and smaller arm spacing


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Solid Solutions

Phase: any portion of a system which is physically homogeneous within itself and bounded by asurface

Components: pure materials or compounds that are present Solubility: The amount of one material that will completely dissolve in a second without

creating a second phase

Unlimited solubility: A second material will dissolve into the first without creating a secondphase regardless of the amount added

o Nickel and Copper Gibbs phase rule:

o P + F = C + No P: number of phaseso F: number of degrees of freedomo C: number of componentso N: number of non-compositional variables (temp and pressure)

Solution: Solid, liquid, or gas in a single phase Mixture: solid, liquid, or gas in multiple phases As temperature increases the solubility limit increases For solid solubility the components must be of similar radii, electronegativity, crystal structure,

and valence electrons

Solid solutions are either interstitial or substitutional, this adds stress on the lattice thereforestrengthening the material

o Increased size difference or alloy concentration causes the strength to increase more Through solid solution strengthening: yield strength, creep resistance, and tensile strength are

improved while electrical conductivity and ductility are reduced

Isomorphous: complete solubility of one component in another, wt% ranges from 0 to 100 Composition of each phase is the wt% of the alloying ingredient

o In a region of 2 phases they are equal to the liquidus or solidus lines composition Weight fraction is the relative weight of one to another

o Use the tie lines to divide by the total difference in composition to determineo This is a percentage

In a region of liquid + alpha the greatest strength is achieved in the greatest difference in phaseregions

For an isomorphic alloy freezing temperature is a range based on the latent heat of fusion beingremoved, not an isothermal hold Non-equilibrium solidification leads to shells around a central nucleation, this is due to the

difference in solidus temperature required for solidification at nonequilibrium cooling rates

o Composition gradient across the gains due to changing composition through cooling Microstructure is the pictures in the circles that show what the material would look like during

each step in cooling


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Cooling curve of an isomorphous alloy


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Steel

Composition = wt% C Amount = Wx % Microconstituents = proeutectoid, and eutectoid Phase = ferrite, austenite, cementite Ferrite () an interstitial solid solution of carbon in BCC iron

o Maximum solubility (0.0218 wt% C) Austenite () an interstitial solid solution of carbon in FCC iron

o Maximum solubility of (2.11 wt% C) Cementite (Fe3C) a stoichiometric intermetallic compound of iron and carbon

o Composition (6.67 wt% C) Pearlite: The eutectoid microconstituent

o Composition (0.77 wt% C) Eutectoid transformation forms pearlite as its microconstituent

o Coarse pearlite with large Interlamellar spacing is formed at high temperatures which isrelatively soft pearlite high temperatures so that growth is favored

o Fine pearlite forms with small Interlamellar spacing is formed at lower temperaturesand its relatively hard pearlite low temperatures so that nucleation is favored

Controlling the eutectoido Amount of the eutectoid microconstituento Prior austenitic grain size

The new grains will form within the old ones therefore if the grains are initiallysmall the resulting eutectoid grains will also be small

o Cooling rate and the transformation temperature By cooling at lower temperatures finer pearlite can be formed which increases

strength

Isothermal Transformation Diagrams (TTT)o Curves which show the start and finish of different microconstituentso Important because they show the effects of kinetics instead of a phase diagram that

only shows equilibrium and slow cooling times

Off-eutectoid alloys form at the grain boundaries as they enter the single to multiple phaseregion

Once the eutectoid temperature is reached the remaining initial phase will transform to theeutectoid microconstituent while the second phase will be the proeutectoid microconstituent

Bainite is elongated Fe3C particles in a ferrite matrix, it forms because the lamellae in pearlitewould be too fine to be favorable

Spheroidite: microstructure of steel, heating Bainite or pearlite just below the eutectoid for longperiod of time forms spherical cementite particles in a ferrite matrix

Martensite: Non-equilibrium transformation product where carbon interferes with a phasetransformation from FCC to BCT


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o Diffusionless transformation must be quenched rapidly to achieveo Strongest and hardest that steel can be, but extremely brittle

Must be tempered to allow growth of small cementite particles to increasetoughness but reduces strength

o 1060 is plain carbon steel with 0.6 wt% Co Austenitizing: heating a steel to allow for a homogeneous austenite phase to formo Annealing: heating a steel for a full anneal, cooling in a furnace to form coarse pearliteo Normalizing: heating a steel to austenitize it, then cooling in air to form a fine pearliteo Spherodizing: Heating below the eutectoid temperature for a long time to form coarse

spheroidal cementite particles in a ferrite matrix

o Increasing the carbon content increases the yield and tensile strength and reducesductility

o Increasing the carbon content increases the hardenability as the nose is shifted to theright which allows more time or martensite to form

o Adding alloying ingredients can form a bay region in the TTT diagram which allowsBainite to be formed through simple cooling without an isothermal hold

o Ausforming: deformations done to the material within the bay region of the TTTdiagram then allowed to transform to Bainite or martensite

o Air, oil, water listed in increasing quenching speed and hardness


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Nonferrous Alloys

Specific strength is strength over density O Annealed, H Cold Worked, W Solution Treated, T Age hardened

o Increasing strength of increasing x in Tx and Hx Wrought alloys can be plastically deformed where cast alloys are to be melted and poured Refractory Metals begin to oxidize between 200 and 425 and are rapidly contaminated and

embrittled, they display a ductile to brittle transition temperature


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Ceramics and Glasses

Ceramics are crystalline, inorganic, considered a combination of metallic and non-metallic atom,typically form ionic bonds, and are categorized as oxides, nitrides, and carbides

Cation size must just fit or be larger than the space in between the anions to be stable Hydroplasticity of clay, water molecules can fit in between layered sheets which when external

force is applied they are free to move past each other

Drying: water is removed which causes the interparticle spaces to decrease leading to shrinkageof the part

Firing, makes Vitrification fuse clay and flux in SiO2 Sintering reduces pore size, by diffusion forming of particles together Ceramics are typically weak in tension because they contain inherent porosity which causes

stress concentrations through the material and lots of sites for crack growth

Apparent porosity: are the pores that can be reached by a viscous flow on the surfaces True porosity is the total porosity in the material, those that can be reached from the surface

and the internal pores

Glasses are the amorphous form of ceramics Crystalline materials have melting temperatures, which is the point where the crystal structure

is fully broken down, since glasses are amorphous and dont have a crystal structure to begin

with the melting temperature is a level of viscosity

Glasses have a glass transition temperature which is where the material transitions from actingin a ductile manner above, to a brittle manner below it. This is due to the bonds not being

allowed to rotate in the chains

Glass transition temperature can be seen on a specific volume vs temp graph as a slope change Low viscosity means that it is liquid and can flow Limitation of glasses are in that surface flaws are easy to create and are detrimental to the

strength of the material

Tempering of glasses puts the surface in compression by cooling the surface quicker than thecenter which when the center shrinks upon cooling it pulls on the surface to place it in

compression

o When a force is applied to the material the compressive force must first be overcomebefore the crack can be pulled open in tension

Glass-ceramics have a high degree of crystallinity but are not fully crystallineo The results are tough, thermal shock resistant materials

Ceramics dont have slip systems because they dont have crystalline structure, and they resistmotion of ions of like charge past one and other

In semicrystalline ceramics slip is possible but is resisted by ions of like charge that must pass,this also causes the burgers vector between equilibrium positions to be much larger


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Polymers

Poly (many) mer (repeat unit) considered macromolecules Monomer molecule from which a polymer is formed (this is the mer unit) Oligomer low molecular weight molecules that contain two (dimers) or three (triers) mers Wood, cotton, leather, rubber, wool, silk are considered natural polymers Linear molecules are long chains with only secondary bonding linking them together Branched molecules have branches of primarily bonded chains off the main chain and increases

mechanical interlocking

Cross-linked chains have primary bonds formed between chains which increases their strengthbut also prevents recycling because the whole molecule melts at the same temperature

Thermoplastics: polymers with linear or branched molecules that can be recycled by heating tobreak the secondary bonds, this allows them to be reshaped and then cooled

o Polyethylene Thermosets: polymers with cross-linked chains which are stronger and require a chemical

reaction to cure which sets the primary bonds between chains and they cannot be recycled

o Polyurethane Elastomers: partially cross-linked polymers that allow the material to uncoil under stress and

pull them back to the original shape once the stress is removed

o Rubber Polymers are composed of hydrocarbons, saturated for example ethane C2H6 Polymerizing can be done by addition or condensation

o Addition (chain) is where an end bond is broken then allowed to propagate then isterminated by another end

o Condensation (step) is where one group is reacted with another to form a polymer butresults in a byproduct of alcohol

Molecular weight, is the mass of a mole of chainso Low molecular weight have short chains, and high molecular weight have long chains

As molecular weight or degree of polymerization increases the tensile strength, creepresistance, impact toughness, wear resistance, and melting temperature all increase

Addition of side groups make it more difficult for the chains to rotate, uncoil, disentangle, anddeform by viscous flow when a stress is applied

o Increasing complexity causes this result to be amplified Tacticity is the stereoregularity or special arrangement of side groups added to a chain

o Isotactic all side groups are on the same side of a chaino Syndiotactic side groups alternate sides on the chaino Atactic side groups are randomly positioned in the chaino Atactic is the most difficult to crystallize, followed by syndiotactic, then isotactic is the

easiest to crystallize

Crystallinity is formed in terms of unit cells, and are formed by folding a chain into a repeatingorder


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Polymers are rarely 100% crystalline therefore crystallinity is a % crystallinity Copolymers are formed by two or more monomers polymerized together

o Random A and B randomly through the chaino Alternating A and B alternate through the chaino Block A and B are alternating in large blocks of eacho Graft chains of A are grafted onto a B back bone

ABS is a copolymer blend of styrene and butadiene in a linear copolymer and styrene andacrylonitrile, these two are alloyed together to improve toughness

Liquid polymers allow translation and rotation of bonds and chains If a crystalline polymer forms: chain movement is difficult and a melting temperature exists If an amorphous polymer forms: movement of chains is possible under stress by rotation, a

glass transition temperature is present, no real melting temperature

Glassy amorphous polymer is below the glass transition temperature where there is no easyrotation or translation of polymer chains

THERMOPLASTICS ONLY Increasing strain rate or decreasing temperature have a similar effecton a polymer, as strain rate increases there isnt time allowed for the chains to rotate or slip

which causes them to be more brittle, decreasing temperature makes it more difficult for

chains to rotate or translate which has the same effect of brittle properties

Crystalline solids only have a melting temperature with constant sloping specific volume overtemperature

Semicrystalline solids have both a melting and glass transition temperature, there is a slopchange at the glass transition temperature where the amorphous region allows rotation while

the crystalline structure exists until the melting temperature is reached

Amorphous solids have only a glass transition temperature where the slope changes and theviscosity decreases

Complex side groups increase the melting and glass transition temperatures Strain relaxation: strain in tension and hold, observe decrease in stress with time caused by the

chains adjusting to a lower stress state through rotation, only occurs when temperature is above

the glass transition temperature

In a tensile test, necking propagates across the entire length as this is the strongest portion ofthe material due to an alignment of the polymer chains with the applied load


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Composites

Constituents remain discretely separate Interfacial properties between constituents affects performance Whiskers are single crystals, dendritic Anisotropic: properties are different depending on the direction Inhomogeneous: properties are different at different points in the material Matrix is to transfer stress to the dispersed phases, and protect the dispersed fibers from the

environment

Dispersed phase is different depending on the classificationo MMC (metal metal comp.) to increase yield strength, creep resistance,o CMC (ceramic metal comp.) to increase K1C which is resistance to crackingo PMC (polymer metal comp.) to increase elastic modulus, yield strength tensile strength

Rule of mixtureso Strain is equal for both materialso Stress is equal to the combination of the stress in each of the components weighed by

their volume fraction

o Modulus is equal to the combination of the moduli of the components weighted by theirvolume fraction

Ceramic Metal Composites bonding between the dispersant and the matrix should be weak toallow for crack blunting but absorption of energy as the bond breaks

Polymer Metal Composites bonding between the dispersant and the matrix should be strongbecause the fibers are in place to take the applied stress and the matrix is to transfer stress to

the fibers


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Electronic Materials

Superconductors exhibit zero resistance at low temperatures Resistivity is a function of area and current Conductivity is the inverse of resistivity Charge carriers

o Metallic: electrons are the charge carriers, the shorter the mean free path the higherthe conductivity of the material. Heating the material increases the mean free path,

which reduces conductivity

o Covalent: bonds must be broken for an electron to move, impurities can be added toserve as charge carriers

o Ionic: Ions may diffuse by vacancy diffusion Energy band gap is the difference between the energy levels, in conduction the importance is a

gap between the valence and conduction band

o Smaller band gap increases conductivity metals have effectively zero band gapo Insulators have a very large band gap and to push charge carriers up to the conduction

band the material itself breaks down

o Semiconductors: have dopants added to the material to create donor/acceptor energystates which make it easier to promote charge carriers to the conduction band and

therefor they exhibit increasing conduction with added temperature

Free electrons have negative charge and are in the conduction band Holes have positive charge and are in the valence band These move at different speeds, or drift velocities Large electronegativity difference increases the energy gap Extrinsic semiconductors

o N-type semiconductors are extra valence electrons which are easily donated Saturation when all extra valence are donated

o P-type semiconductors are missing a valence electron which easily accept electrons Exhaustion when all missing holes are filled

Intrinsic semiconductors have the same number of holes as they do free electrons n = p P-n rectifying junction: when the extra electrons are on the negative side current will flow as the

holes and electrons move toward the junction


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Magnetism

Magnetic moments arise from electron motions and spin Ferromagnetic, ferrimagnetic, paramagnetic, diamagnetic are the main types of magnetism

o Ferromagnetic has all poles aligned in a single directiono Ferrimagnetic has all poles in the same axis but some point in opposing directionso Paramagnetic has poles which are randomly orientedo Diamagnetic has poles which are separated by 45 deg angles (negative inductance)

Magnetic responseso Ferro and ferri are already aligned with no applied field, when there is a field this

alignment increases

o Paramagnetic has random poles with no applied field, when there is a field they aligno Diamagnetic has no poles with no applied field, when there is a field they oppose the

field

The curie temperature is where ferro and ferri magnetics act as paramagnetic with a randompole orientation

There are typically different magnetic domains within grain boundaries Domains can be aligned in a certain way for ferro and ferri magnetics through exposure to a

magnetic field

Hysteresis phenomenon in permanent magneticso Applied field aligns domains, once the field is gone the alignment remains, coercivity is

the field required to demagnetize the material

Hard magnets have large coercivities, they are used for permanent magnets Soft magnets have small coercivities and have little losses from switching polarization which

make them ideal for electric motors and computer components

By decreasing grain size magnetic storage media can hold more information


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Thermal


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Corrosion and Wear

Oxidation and reduction reactions are required to corrode materials Oxidation is the addition of electrons / material Reduction is the loss of electrons / material Anode undergoes an oxidation reaction which donates ions Cathode undergoes reduction reaction which accepts ions The smaller the Vometal is the more likely it is to corrode as the anode Galvanic series, ranking reactivity of metals in sea water Ellingham diagram shows the standard free energy of formation for an oxide, this shows how

likely metals will react with oxygen at different temperatures

o Rate of corrosion or plating is given by Faradays equation

o Types of corrosion

o Uniform, stress, erosion, pitting, crevice, galvanic, intergranularo Pitting is a downward propagation of small pitso Crevice is narrow and confines spaceso Galvanic is dissimilar metals that are physically joined in the presence of an electrolyteo Intergranular is corrosion along grain boundaries where free energy is higher

Corrosion is accelerated by stress, concentration cells, and joints Corrosion is protected against by coatings, lower temperatures, inhibitors added to decrease

reactivity by adding an element that will react more easily, cathodic protection sacrifices more

anodic material to protect the other


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Oxidation types can be formed depending on the volume of the oxide verses the volume of themetal

o As this difference increases the oxide is more likely to flake off and be nonprotectiveo This is known as the pilling-bedworth ratioo If PB ratio is less than one oxide occupies a small volume which results in a coating that

is porous and oxidation continues rapidly

o If PB ratio is between one and two the size are similar and the oxide will be adherent,non-porous, and protective

o If the PB ratio is larger than 2 the oxide is much larger and will flake off the surfaceexposing fresh material to continue oxidation

o

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Materials Final Review