ME115 - Structures and Properties of Materials

Why do metals generally have higher electrical conductivity than ceramics?

-Free electrons (cloud/sea)

-move from atom to atom

-non directional bonding

What material properties would be ideal for gear ? (surface and internal)

Surface: wear resistance, low friction, machining to high tolerances with excellent surface finishes

Internal: Tougher but softer than surface to avoid any brittle behavior

What material properties are vital for the gear of an outdoor clock?

-No requirements of demanding mechanical performance

-long term wear resistance

-outer coating to protect from environmental effects

What does a lattice parameter represent?

Side length of a cube for a unit cell

(IAE GRAPH) What results in higher bonding energy?

Due to a deeper minimum point

(IAE GRAPH) What results in a higher melting point

Due to higher bonding energy

(IAE GRAPH) What results in higher stiffness?

Proportional to Young modulus → stronger Bonding → higher Stiffness

(IAE GRAPH) What results in higher separation?

Look at raverage to see if one curve has higher separation at any point compared to the other

Elastic Behavior

the material deforms under a load but then returns to the original shape when the load is removed

Specific Modulus

-Stiffness per unit mass (Elastic Modulus/Density)

-Used to aid material selection with both mass and stiffness are important to design criteria

Specific Strength

-Yield stress/density

If you could cut a piece of wood with grains perpendicular to your sample, how do you expect the value to change? Why?

Wood has the highest strength when the load is applied parallel to the grain. It has the lowest strength when the load is applied perpendicular to the grain.

Lower elastic modulus.

Metallic Bonding

-Sea of electrons, 1-3 donated valence electrons

-Non-directional bonds, atoms can move

-wide range in bond strength

Covalent Bonding

-Shared electrons

-Directional bonding occurs between atoms with similar electronegativities

-Strong and inflexible bond

Ionic Bonding

-Occurs between positive and negative ions

-Directional attraction when large difference in electronegativity

-Very strong, inflexible bond, electrons are tightly bound

Secondary Bonding

-Weak interaction between atoms already bound by metallic, covalent, or ionic bonds as primary bond

-Weak strength

-Examples: van der waals, london, debye, keesom

Properties of metallic bonding

-Good electrical and thermal conductors

-Good ductility (non-directional)

-Nearly isotropic properties

-Bonding is weaker than ionic or covalent

Properties of covalent bonding

-Sharing of valence electrons among two or more atoms

-Directional relationship between atoms that form specific angles, depending on the material

-Mechanically strong, hard, poor conductors

Ionic bonding

-Transfer of valence electrons

-Creates cation and anion that are attracted to each other (strong bonding)

-Usually between elements with large difference in electronegativity

-Hard, brittle, high melting temperature

Secondary Bonds

Results from temporary fluctuating dipoles

London Force

both are attractions induced dipoles

Keesom Interactions

-One polarization

-One end is slightly positive, other end is slightly negative

Debye interaction

-Arises between a polar molecule and a non-polar molecule

Interatomic spacing

-Balance of attractive and repulsive forces

-Occurs at minimum total interatomic energy or when no net force is acting

-Minimum energy is the "binding" energy, affects melting point and strength

Monoatomic gases

No order

Amorphous materials

-No long range order

-Only short range order

Liquid Crystals

-Short range order and long range order in small volumes

Crystalline materials

short and long range order

Lattice

A collection of points that divide space into smaller equally sized segments

Basis

A group of atoms associated with a lattice point (same as motif)

Unit Cell

-A subdivision of the lattice that still retains the overall characteristics of the entire lattice

-Smallest group of atoms that can be used to describe the arrangement of atoms in a crystalline solid

Atomic Packing Factor

-Fraction of space occupied by atoms

-(#atoms*volumeAtoms)/volumeUnitCell)

FCC Stacking sequence

ABCABCABC

HCP Stacking sequence

ABABAB

Linear Atomic Density (LD)

equivalent length of atoms centered on the direction vector/length of direction vector

Planar Atomic Density (PD)

area of atoms centered on a plane/area of a plane

Interstitial site

a location between the normal atom or ions in a crystal into which another atom can be placed (in-between spots)

Allotropy

The characteristic of an element being able to exist in more than one crystal structure depending on temperature and pressure

Polymorphism

Compounds exhibiting more than one type of crystal structure

Vacancies

Produced when an atom or an ion is missing from its normal site in the crystal structure. (has a formula)

interstitial atoms

-Smaller than the host atom and can fit into the interstice

-Generally somewhat larger than the interstitial position and push the surrounding atoms out a bit creating a much larger volume of distorted lattice around it

Substitutional Atoms

-could be larger or smaller

-both distort lattice and strain the bonds over a large surrounding volume

Fundamental Observation of applied tensile force

All deformation is a result of shear

Point Defects

Localized disruptions in otherwise perfect atomic or ionic arrangements in a crystal structure

Interstitial Defect

Formed when an extra atom or ion is inserted into the crystal structure at a normally unoccupied position

Substitutional Defects

Introduced when one atom or ion is replaced by a different type of atom or ion

Frenkel Defect

when an ion jumps from a normal lattice point to an interstitial site

Schottky Defect

-unique to ionic materials and commonly found in ceramic materials

-When vacancies occur in an ionically bonded material, a stoichiometric number of anions and cations must be missing from regular atomic positions if electrical neutrality is to be preserved