Material Science Exam 3

Eutectic Definition

Liquid transforms to two solid phases

Eutectoid Definition

One solid phase transforms to two other solid phases

Peritectic Definition

Liquid and one solid phase transform to a second solid phase

What does a heat treatment do to steel?

Heating and cooling properties of steels vary the mechanical properties

Martensite definition

Metastable phase consisting of super saturated solid solution of C in BCC or BCC tetragonal iron]

Caused by rapid cooling of austenitic steel into room temperature

What are the 3 types of heat treatment types?

1. Full Annealing

2. Quenching (no transition phase)

3. Tempering (Tempered Martensite)

What is hardenability?

measure of the ability to form martensite

it is measured using the jominy end quench test

Martensite by Rapid Quenching makes 2 types of martensite which are

1. Lath Martensite: Less than 0.6% C and consists of domains of lath of different orientation

2. Plate martensite: More than 0.6% C and have fine structure of parallel twins

Facts are martensite and its transition

• Transfer to martensite is diffusionless

• No change of relative position of carbon atoms after transformation

• Strength and hardness increase with carbon content

• Strength is due to high dislocation concentration and interstitial solid solution strengthening.

Isothermal Decomposition of Austenite (Bainite formation)

• If hot quenching temperature is between 550 degrees C to 250 degrees C , an intermediate structure Bainite is produced

• Bainite constrain nonlamellar eutectoid structure of alpha ferrite and cementite.

• Upper Bainite: 550-350

• Lower Bainite: 350-250

In images Ferrite versus Cementite

Ferrite = light

Cementite = dark

Fast facts about Ceramics

• Keramikos = “burnt stuff”

• Compounds: Metal-nonmetal

• Bonds: Ionic mostly and some covalent

• Insulator (no free electrons)

• Hight melting point

• Brittle

• Low tensile strength

• high compressive strength

• high chemical stability

Traditional versus engineering ceramics

traditional: Basic Components (Clay and silica)

engineering: pure compounds (Al2O3, Sic)

Electronic Properties of ceramics

Bounds electrons, exhibit band gap (not very conductive)

Optical properties of ceramics

optical band gap mostly transparent, colorful with atomic defect

Magnetic Properties of ceramics

Magnetic in presence of defect due to unpaired electrons.

Why are ceramics brittle?

• Unlike metals ceramics have bound electrons between atoms

• In metals, non-directional nature of electrons, so no resistance in moving dislocations

• in ceramics, covalent bonds have directional nature, also stronger bond energy

• Bond energy in ionic bonds is high. Also produce strong attractive and repulsive forces.

Structure of ceramics

Ceramics are constructed by ions: Cations (+) and Anions (-)

Crystal structure is influenced by 2 factors…

1. Charge neutrality: All the actions chargers much be balanced by an equal number of anions charges

2. Size: Cations are ordinarily smaller than anions: (if the anions does not touch the cations, then the arrangement in unstable.)

Radion Ration = rCar/rAn

Critical radius Ratio for stability coordination numbers 8,6, and 3 are >0.732, >0.414, and >0.155

NaCl Crystal Structure

• Highly ionically bonded with Na+ ions occupying interstitial sites between FCC and Cl- ions.

• Radion Ratio = 0.56

• CN = 6

• Mg, CaO, NiO, and FeO have similar structures

CsCl Crystal Structure

• CsCl is ionically bonded with radius ration = 0.94

• CN = 8

• Eight chloride ion surround a central cesium cation at the (1/2,1/2,1/2) position

• CsBr, TiCi, and TiBr have similar structure

What is Amorphous?

Amorphous: Non-crystalline solid, lacking long-range order

Amorphous structure occurs by adding impurities

impurities interfere with formation of crystalline structure and decreasing melting point.

Point defect in Ceramics

Vacancies: Vacancies exist in ceramics for both cations and anions

Interstitials: Interstitials exist for cations. interstitials are not normally observed for anions because anions are large relative to the interstitial sites.

Frankel Defect: A cation vacancy-cation interstitial pair.

Schottky Defect: A paired set of cations and anion vacancies

Polymers (poly=?) (mer=?)

poly = many

mer = repeat unit

Ancient polymers

• Originally natural polymers were used (wood, cotton, leather, rubber, wool, silk)

• Oldest known uses (rubber balls used by incas, use of pitch to seal boat seams)

Polymer Composition

Most polymers are hydrocarbons

Saturated hydrocarbons:

each carbon singly bonded to found other atoms (ex: C2H6)

Unsaturated Hydrocarbons

double and triple bonds somewhat unstable- can form new bonds

double bond found in ethylene (C2H4)

triple bond found in acetylene (C2H2)

Isomerism

Two compounds with same chemical formula can have quite different structures

Free radical polymerization

is a chain-growth polymerization process where reactive free radicals initiate the formation of long polymer chains from monomers.

Initiator

An initiator is a chemical compound that starts the polymerization process by generating reactive species, such as free radicals or ions, which then react with monomers to form the initial polymer chains.

Molecular Weight

Molecular weight, “M” is mass of a mole of chains

An average molecular weight give a measurement of physical properties such as viscosity

Degrees of polymerization = average # of repeat units

Copolymers

Two or more monomers polymerized together

• random: A and B randomly position along chain

• alternating: A and B alternate in polymer chain

• block: large blocks of A units alternate with large blocks of B units

• Graft: Chains of B units grafted onto A backbone

3 Types of Polymers

1. Elastomers: extensive elastic strains, chain coiling/uncoiling

2. Thermoplastics: soften upon heating, rigid to soft transition, chain slippage by heating, can be reheated and reformed

3. Thermosets: extensive cross-linking, no chain slippage, most rigid, cannot be reheated and reformed, set to final shape by chemical reaction

Composite

• Combination of two or more individual materials

• Design goal: obtain a more desirable combination of properties (principle of combined action)

Terminology/Classification of Composite

Composite: Multiphase material that is artificially made

Phase types: Matrix=continuous, dispersed= is discontinuous and surrounded by matrix

Concrete compound

gravel, sand, cement, water

Reinforced concrete

added steel rebard to increase strength even if cement matric is cracked

Prestressed concrete

• rebar placed under tension during setting of concrete

• release of tension after setting placed constant in a state of compression

• to fracture concrete, apples tensile stress must exceed this compressive stress.

Posttensioning

tightening nuts to place concrete under compression

Corrosion

deterioration of metal resulting from chemical attack by its environment

rate of corrosion

depends upon temperature and concentration of reactants and products

About metals and corrosion

Metals have free electrons that setup electrochemical cells within their structure

Metal have tendency to go back to low energy state by corroding

Ceramics and Polymers suffer corrosion by direct chemical attack

Standard electrode half-cell potential of metals

Oxidation/reduction half cell potentials are compared with standard hydrogen ion half cell potential

Anodic to Hydrogren:

More tendency to corrode

Cathodic to Hydrogen

Less tendency to corrode

Types of corrosion

• Uniform Corrosion

• Galvanic Corrosion

• Intergranular Corrosion

• Pitting Corrosion

• Crevice Corrosion

• Stress Corrosion

• Erosion and cavitation corrosion

• Fretting corrosion

• selective leaching or dealloying

Uniform Corrosion

Uniform or general attack corrosion: Reaction proceeds uniformly on the entire surface. controlled by protective coatings, inhibitors and cathodic protection

Galvanic Corrosion

Galvanic or two metal corrosion Electrochemical reaction leads to corrosion of on metal. Two dissimilar metals in contact in water or humid air. Visible activity at the junction. Metal loss of accumulation of corrosion product (rust) at the anode side of the junction.

Pitting Corrosion

pitting localizes corrosive attacks that produce holes or pits in a metal. Results in sudden unexpected failure as pits go undetected. Pitting requires an initiation period and grows in direction of gravity. Pits initiate at structure and compositional heterogeneities.

Crevice Corrosion

Localized electrochemical corrosion in crevice and under shielded surfaces where stagnant solution can exist. Occurs under valve gaskets, rivets, and bolts in alloy systems like steel, titanium, and copper alloys.

Intergranular Corrosion

Grain boundaries are more anodic hence get corroded. Localized corrosion at and/or adjacent to highly reactive grain boundaries resulting in disintegration. When exposed to corrosive environment, the region next to grain boundaries become anodic and corrode.

Stress Corrosion

Stress corrosion cracking. cracking caused by combined effect of tensile stress and corrosive environment. Stress might be residual and applied. Only certain combination of alloy and environmental causes SCC. Crack initiates at pit or other discontinuity. Crack propagates perpendicular to stress. Crack growth stops if either stress or corrosive environment is removed.

Erosion Corrosion

Erosion corrosion is defined as the acceleration in the rate of corrosion attack in a metal due to the relative motion of corrosive fluid is rapid, the effects of mechanical wear and abrasion can be severe and expedite the erosion corrosion. Appearance in metal surfaces of grooves, valleys, pits, rounded holes, in the direction of the flow of the corrosive fluid,

Cathodic Protection

Corrosion control can be achieved by a method called cathodic protection in which electrons are supplied to the metal structure to be protected. Thus is electron are continually supplied to the steel structure, corrosion will be suppressed.