Nature of Materials

5 main levels of structure

Atomic structure

Short and Long range atomic arrangements

Nanostructure

Microstructure

Macrostructure

Structure on various ____ scales can profoundly influence a material’s properties, behavior and applications

levels/lengths

Atom

consists of a nucleus (protons & neutrons) orbited by electrons

Electrons and protons are held together by

electrostatic attraction, since both carry opposite charges

Atomic number equals to

number of protons in nucleus

Atomic mass equals to

mass of protons + neutrons in atom

travels around the nucleus in orbital path

Electrons

makes up most of the size of the atom

electron cloud

The distance between electrons and the nucleus is ____; it is better described as a ___ with a distribution.

not fixed, random variable

is the number of electrons in an atom that participate in bonding/chemical reactions

Valence

An electrically neutral (or complete) atom has equal numbers of

electrons and protons

An electrically neutral (or complete) atom has equal numbers of electrons and protons. However, an atom may either release or attract ___ to reach a more ____configuration.

electrons, stable

If an atom has a valence of 0 it is

chemically unreactive/inert

describes the tendency of atom to gain an electron to achieve a valence of zero

Electronegativity

Atoms such as chlorine are very electronegative because they have____ ,in the outer shell and need only ____more to achieve 0 valence.

7 electrons, one

Atoms with nearly empty outer shells have ____________ since they give up their electrons to achieve 0 valence.

low electronegativity

Example of an atom with nearly empty outer shell

Sodium

4 important mechanisms where atoms are bonded in engineering materials

Metallic bonds

Covalent bonds

Ionic bonds

Van der Waals bonds

Metallic bonds, Covalent bonds, Ionic bonds are

Primary bonds

Primary bonds are

strong

Van der Waals bonds are

secondary bonds

secondary bonds are

weaker

Characteristics of metallic bonds

• Valence electrons of metal ions form a sea of electrons that are not bound to any particular atom.

• Positively charge ion cores are bound together with floating valence electrons.

• Good conductivity: electrons are not fixed in place

For metallic bonds Valence electrons of metal ions form a sea of electrons that are

not bound to any particular atom

For metallic bonds positively charge ion cores are

bound together with floating valence electrons

electrons are not fixed in place

Good conductivity

Characteristics of Covalent Bonds

• are formed by sharing valence electrons among two or more atoms.

• Low conductivity: Valence electrons are locked.

formed by sharing valence electrons among two or more atoms

Covalent bonds

Valence electrons are locked

Low conductivity

Characteristics of Ionic Bonds

• form between materials with different electronegativities.

• consists of cations and anions which are formed after electron transfer between atoms.

• Oppositely charged ions are attracted to each other.

Ionic bonds form between materials with different

electronegativities

An ionic bond consists of______ which are formed after electron transfer between atoms.

cations and anions

Van der Waals forces

exist

Reasons for van der waals exist is

temporary fluctuations in electron distribution create instantaneous electric dipoles

After temporary fluctuations with van der waals the dipoles are induced in

neighboring atoms, leading to weak attractive forces

H2O is a ___molecule

polar

van der Waals forces play a significant role in

influencing materials properties

Metallic bonds

between atoms with 1, 2, or 3 valence electrons

Example of Metallic bonds

sodium, iron, aluminum,

Inorganic solids

covalent and ionic bonds between atoms with 5, 6, or 7 valence electrons

Example of Inorganic Solids

portland cement concrete, bricks, diamond, glass, aggregates

Organic Solids

Long molecules of covalent hydrogen-carbon molecules with secondary bonds between chains

Examples of Organic Solids

hydrocarbons, asphalt, plastics, wood

What are the different states of matter

Short range order and long range order

materials “without form” and exhibiting only short-range atomic order

Amorphous

materials arranged in a highly ordered and repeating pattern called a crystal lattice

Crystalline

Types of structures for metallic materials

Grain Structure

Crystal Lattice Structure

Grain Structure characteristics

Crystals grow during cooling of molten metals

Each grain is confined within a boundary

Crystal Lattice Structure characteristics

3-D geometric pattern of atoms (geometric skeleton of atoms)

Lattice

Unit Cell

repeating arrangement of points in space

Lattice

smallest repeating unit

Unit Cell

composed of small repeated entities called unit cells

Lattice structure

Characterization of Crystal structure

Atoms are arranged in a lattice or 3-dimensional array of points coinciding with sphere centers of identical environments

How to find how many atoms are in a simple cubic structure

8 corners *(1/8 atom per corner)=1 atom/cell

sphere volume

4/3 Pi*r³

Cell volume simple cube

2r³

Packing fraction

vatoms/vcell

Packing fraction for simple cubic structure

pi/6 or .52

How to find how many atoms are in a body centered cubic structure

2 atoms/cell

How to find fraction of the unit cell volume occupied by atoms

sqr(3)*a0=4r

a0=4r/sqr3

Cell volume for Body Centered Cubic Structure BCC

a0³=(4r/sqr3)³

volume of atoms for BCC

2 atoms*4/3*pi*r³=8/3*pi*r³

Packing fraction for BCC

sqr3*pi/8=.68

How to find how many atoms are in a Face centered cubic structure

4 atoms/cell

Packing fraction for FCC

.74

Perfect lattice structures only exist under ____and ______ of material.

ideal conditions , small quantities

Defects

• Point (missing atoms)

• Line (rows of missing atoms, edge dislocation)

• Area (grain boundary)

• Volume (cavities)

Point

missing atoms

Line

rows of missing atoms, edge dislocation

slip is important in understanding

mechanical behavior of metals

Slip explains why strength of metals is

103-104 times lower than predicted from the metallic bond

slips provide ____in metals which enables ____

ductility, plastic deformation

Mechanical properties of metals/alloys can be controlled by

interfering with dislocation movement

Total length of dislocation line per unit volume in a material

Dislocation density

Higher dislocation density increases the

strength of a metal because it makes it harder for dislocations to move which increases the stress required to cause further plastic deformation

the boundaries, or planes, that separate a material into regions

Surface defects or grain boundaries

Grain boundaries are ___zones where the atoms are not properly ____.

narrow, spaced

Mechanical properties of metals/alloys can be controlled by

interfering with grain sizes

How does grain size influence strength?

Smaller grains mean more grain-boundary area. Dislocations travel only a short distance before hitting a boundary, where slip planes no longer line up. The boundary blocks their motion, increasing the metal’s strength.

Grain size mainly depends on

rate of cooling of the molten metal

Smaller grains are formed by

rapid cooling which increase strength