Science of Eng. Materials : Exam 1 : Chapters 1-6

Characteristics of metals

Strong, ductile, tough, high density, conductors, have valence electrons detached from atoms in an "electron sea" that glues ions together

Characteristics of ceramics

Strong, brittle, low density, insulators, have covalent bonding (electrons shared between atoms), e.g. glass/porcelain

Characteristics of polymers

Weak, ductile, low density, insulators, are bound by covalent and weak van der Waals forces, usually based on C and H, e.g. plastic/rubbers

Characteristics of composites

Strong, ductile, low density, conductors

Characteristics of semiconductors

Weak, brittle, low density

In crystals, atoms have ______

Long range periodic order

In glasses (amorphous), atoms have ______

Short range order only

Electronegative elements readily ______ electrons to become ______ ions

Acquire, negative

Electropositive elements readily ______ electrons to become ______ ions

Give up, positive

Types of bonding (strongest to weakest)

1. Primary bonding - electrons are transferred/shared2. Secondary bonding - weak at about 10 kJ/mol, no electrons are transferred/shared

Types of primary bonding (strongest to weakest)

1. Covalent2. Ionic3. Metallic

How do ionic bonds form?

Oppositely charged ions are attracted to each other, large difference in electronegativity, non-directional

How do covalent bonds form?

Electrons are shared between atoms with similar electronegativity, highly directional

How do metallic bonds form?

Atoms are ionized and lose some electrons, which form an "electron sea" and bind charged nuclei in place, non-directional

Types of secondary bonding (strongest to weakest)

1. Permanent dipole bonds2. Polar molecule-induced dipole bonds3. Fluctuating induced dipole

What is a thermite reaction?

Reaction of aluminium with iron oxide to make aluminium oxide and iron, highly exothermic

Bonding characteristics in ceramics

Ionic and covalent bonding, large bond energy, large melting temp (T), large bond energy (E), small coefficient of thermal expansion (a)

Bonding characteristics in metals

Metallic bonding, variable bond energy, moderate melting temp (T), moderate bond energy (E), moderate coefficient of thermal expansion (a)

Bonding characteristics in polymers

Covalent and secondary bonding, secondary bonding dominates, small melting temp (T), small bond energy (E), large coefficient of thermal expansion (a)

Characteristics of mixed bonds

Greater difference in electronegativity means bond is more ionic, smaller difference in electronegativity means bond is more covalent

Potential well is deeper if the melting temp (T) is ______ , elastic modulus (Em) is ______ , and coefficient of thermal expansion (a) is ______

Larger, larger, smaller

Potential well is shallower if the melting temp (T) is ______ , elastic modulus (Em) is ______ , and coefficient of thermal expansion (a) is ______

Smaller, smaller, larger

Melting temp (T) is larger if bond energy (E) is ______

Larger

Types of crystal structure

1. Simple cubic2. Body centered cubic3. Face centered cubic

Miller Indices Procedure

1. Sketch plane in unit cell that does not pass through origin2. Record intercepts of planes using crystallographic axes as coordinate system3. Take reciprocals of these numbers and clear fractions4. Reduce reciprocals5. Put in proper format, e.g. (110)

Metal crystal structures tend to be ______ packed and have the ______ crystal structures

Densely, simplest

Three common structures for metals

1. Face-centered cubic (FCC)2. Hexagonal close-packed (HCP)3. Body-centered cubic (BCC)

APF

Atomic Packing Factor: APF = (volume of atoms in unit cell) / (volume of unit cell)

Characteristics of SC structure

Coordination # = 6 (# nearest neighbors), APF = 0.52

Coordination # of BCC (# nearest neighbors)

Coordination # = 8 (# nearest neighbors), APF = 0.68

Coordination # of FCC (# nearest neighbors)

Coordination # = 12 (# nearest neighbors), APF = 0.74

In crystalline materials, atoms pack in ______ , ______ arrays, typical of ______ , ______ , ______

Periodic, 3D, metals, many ceramics, some polymers

In noncrystalline (amorphous) materials, atoms have ______ packing, occurs for ______ , ______

No periodic, complex structures, rapid cooling

Avogadro's number

6.023 x 10^23 atoms/mol

Most engineering materials are ______-crystalline

Poly

Grain sizes typically range from ______ to ______

1 nm, 2 cm

Single crystals have properties that ______ with ______ ; polycrystals have properties that may or may not ______ with ______

Vary, direction, vary, direction

If material properties vary with single crystal orientation, they are called ______

Anisotropic

If material properties are generally non-directional, they are called ______

Isotropic

What is polymorphism?

Two or more distinct crystal structures for the same material

What is a common method of structure determination?

X-ray diffraction, aka crystallography, used to determine unique crystal structure and interplanar spacing of substances

Types of defects

1. Point (0-D)2. Line (1-D)3. Area (2-D)4. Volume(3-D)

Types of point defects

1. Vacancy atoms2. Interstitial atoms3. Substitutional atoms

What are vacancies?

Vacant atomic sites in a structure

What are self-interstitials?

Extra" atoms positioned between atomic sites

What are impurities?

Atoms which are different from the host

Types of line defects

1. Dislocations

What are dislocations?

When slip occurs between crystal planes when they move, causing permanent plastic deformation

Types of area defects

1. Grain boundaries2. Surfaces3. Twins4. Stacking faults (FCC)5. Interphase boundaries

What are grain boundaries?

Boundaries between crystals that have a change in orientation across them, impede dislocation motion

What are twin boundaries?

Low energy mirror planes caused by shear, arise from mechanical deformation (mechanical twins, BCC/HCP) or heating (annealing twins, FCC)

Types of volume defects

1. Extended defects, e.g. pores/cracks

Pores, a type of volume defect, can greatly affect ______ , ______ , and ______ properties

Optical, thermal, mechanical

Cracks, a type of volume defect, can greatly affect ______ properties

Mechanical

Foreign inclusions, a type of volume defect, can greatly affect ______ , ______ , and ______ properties

Electrical, mechanical, optical

All defects cost ______

Energy

What is second phase?

As solute atoms are added to an alloy, local precipitates are formed

Hume-Rothery Rules for Mixing

1. Atomic Size Factor (15% Rule): solid solution will not be favored if size difference of elements is greater than +-15%2. Crystal structure: similar elemental crystal structures are better3. Electronegativity: greater electronegativity difference between elements is better4. Valency: a metal more likely to dissolve in another metal of higher valency than one of lower

What is a slip system?

A combination of close-packed planes and close-packed directions on those planes where slip occurs

Materials can be strengthened if dislocation motion is ______ ; dislocation motion can be ______ by ______ , ______ , and ______ ; however, properties will ______

Impeded, impeded, point defects, other dislocations, grain boundaries, change

Optical microscopy is useful up to ______ magnification

2000X

Types of electron microscopes

1. Scanning electron microscope (SEM)2. Transmission electron microscope (TEM)

Characteristics of SEM

Provides top view of sample, great depth of field, magnification from 10X to 50,000X

Characteristics of TEM

Sees" through a thin foil of a specimen, used to study dislocations, magnification up to 10^6X

Characteristics of scanning probe microscopy (SPM)

Gives 3-D images with surface topography info, magnification up to 10^9X

What is self-diffusion?

In elemental solid, atoms migrate on their own over time

What is substitutional diffusion?

Atoms exchange places with vacancies over time, rate depends on number of vacancies and activation energy to exchange

What is interstitial diffusion?

Requires small impurity atoms to fit into interstices in host, faster than vacancy diffusion

What is diffusion flux?

The number or mass of atoms diffusing through a unit area per unit time, can be measured for vacancies/host atoms/impurity atoms

Fick's 1st Law

Stead-State diffusion: diffusion flux along direction x is proportional to concentration gradient (change in C / change in x)

Fick's 2nd Law

Nonsteady-State Diffusion: in most real situations the concentration profile and concentration gradient are changing with time

Diffusion is faster for:

1. Open crystal structures2. Lower melting temp materials3. Materials with secondary bonding4. Smaller diffusing atoms5. Cations (positive ions)6. Lower density materials

Diffusion is slower for:

1. Close-packed structures2. Higher melting temp materials3. Materials with covalent bonding4. Larger diffusing atoms5. Anions (negative ions)6. Higher density materials

Types of loading

1. Tensile2. Compressive3. Shear4. Torsion

What is shear?

Unaligned forces pushing one part of body in one direction and another part of body in a different direction

What is torsion?

A variation of shear but caused by torque rather than forces

Engineering stress (sigma)

sigma = F / A, where F is load applied perpendicular to specimen cross section and A is cross-sectional area BEFORE load is applied

Engineering strain (epsilon)

epsilon = (change in I / Io) x 100%, where Io is the initial length of the specimen

Hooke's law

sigma = E * epsilon, where sigma is engineering stress, E is modulus of elasticity, and epsilon is engineering strain

What is elastic deformation?

Deformation that is reversible, follows Hooke's law

Comparison of modulus of elasticity of ceramics, metals, and polymers

Ec > Em >> Ep

Modulus of elasticity is dependent on ______

Temperature

What is anelasticity?

Time-dependent elastic behavior, small effect on metals but can be significant for polymers (visco-elastic behavior)

Materials subject to tension shrink ______ ; those subject to compression ______

Laterally, bulge

Poisson's ratio (v)

Ratio of lateral and axial strains

What is plastic deformation?

Deformation that is irreversible, shift of single atoms and slipping of planes, does not follow Hooke's law

For structural applications, yield stress is a ______ important property than tensile strength because a structure is deformed beyond acceptable limits once the ______ stress has passed

More, yield

What is tensile strength?

Maximum stress on stress-strain curve

Comparison of yield strength of ceramics, metals, and polymers

sigmaC >> sigmaM >> sigmaP

Comparison of yield strength of ceramics, metals, and polymers

TSc ~ TSm >> TSp

What is ductility?

Measure of the deformation at fracture, ductile materials can support a relatively large amount of plastic deformation while brittle materials fail early

In stress-strain curves, stress appears to ______ , after going through the maximum point, where the ______ starts to form ; this occurs because the area on which stress acts ______ due to ______

Decrease, neck, decreases, necking

What is hardness?

Resistance to permanently indenting a surface, resistance to plastic deformation

Moh's Scale

Determined by ability of material to scratch another material, from 1 (softest, talc) to 10 (hardest, diamond)

Hardness is proportional to ______

Tensile strength

What is toughness?

Energy to break a unit volume of material, can be measured with impact test

(Exam 1 2017) For a particular crystal structure, the slip direction is the direction in the slip plane having the ______

Highest linear density

In interstitial solid solutions, the solute atoms occupy ______

Unoccupied space between atoms

(Exam 1 2017) The diffusion rate for a substitutional atom is ______ that of an interstitial atom

Less than

(Exam 1 2017) Atoms in the slip plane are ______ atoms in other planes

Closer together than

(Exam 1 2017) What materials may form crystalline solids?

Metals, ceramics, polymers

(Exam 1 2017) The surface energy of a material depends on the ______

Planar orientation

(Exam 1 2017) The preferred slip planes in a crystal are those that have the ______ atomic packing

Densest