Final ME3010

Primary Bonding types

Includes Ionic, Covalent, Metallic

Secondary Bonding Types

Van der waals bonding

Ionic bonding example

ceramics

Covalent bonding example

polymers, diamonds

Metallic bonding example

metals, alloys

Ionic bonding is between

Metals, Nonmetals

Covalent bonding is between

Nonmetals, nonmetals & metalloids

Secondary bonding is between

all atoms

How does ionic bonding happen

Transferring of valence electrons => IONIC

How does metallic bonding happen

Free valence => “Sea” of electronsH

How does Van der Waals bonding happen

Attraction between atomic or molecular dipoles

Ionic bonding characteristics

• Hard & Brittle

• Electrical & thermal insulators

Metallic bonding characteristics

• Good ductility

• Good electrical conductivity

Crystalline materials

Repetitive three dimensional pattern over large atomic distance

Noncrystalline materials

Lacking regularity over large atomic distances

Face Centered Cubic

Body Centered Cubic

HCP

FCC number of atoms

4

FCC equation

a = 2Rsqrt2

FCC APF

0.74

APF ratio

Vs / Vc

BCC number of atoms

2 atoms

BCC equation

a = 4R / sqrt3

BCC APF

0.68

HCP APF

0.74

Single Crystal materials

Crystalline solid w/ periodic and repeated arrangement w/o interruption

Polycrystalline materials

Composed of more than one crystal or grain. GB is the region where crystals meet

Bonding in ceramics

mostly ionic, some covalent

Crystal structures of Ceramics

Composed of electrically charged ions

Structure of polymers, decreasing strength

Network, Cross-linked, Branched, Linear

Molecular weight

Mass of moles of chain

Polymer chains

many kinks and coils that form naturally

Polymers - two main types

Plastics & rubbers

Plastics

• synthetic

• molded into shape while soft

• set into rigid, slightly elastic form

Thermoplastics

• Softens when heated & hardened when cools

  • Mostly van der waals

  • ductile, flexible, easy to recycle

Thermoplastics example

polyethylene, nylon

Thermosets/ Thermosetting plastics

Polymers become hard when heated, does not soften upon subsequent healing

• mostly covalent

• hard, brittle, difficult to recycle

Thermosets/ Thermosetting plastics examples

epoxy, polyurethane

Rubber

Material that may be elastically stretched to at least 2x the original length. It returns after force is removed

Composites

Combination of two or more materials to obtain a new material w/ desirable properties

Composite properties

function of constituents

-relative amounts & geometry of phases

Matrix

Continuous phase of composite

Dispersed

Reinforced phase of composite

Purpose of matrix

Transfers load from fiber-fiber

Protects phases form environment

Purpose of dispersed phase

Primary load carrier'

Enhanced matrix properties

Fiber-reinforced

3 types

• continuous and aligned

• discontinuous and aligned

• discontinuous and misaligned

Continuous and aligned

long aspect ratio, along the piece

Discontinuous and aligned

short aspect ratio, along the piece

discontinuous and misaligned

Short aspect ratio, random orientation

Example of particle reinforced

Tires.

Matrix: rubber

Particle: Carbon

Example of structural reinforcement

Laminar, sandwich panels

0 - Dimensional defects are also called

Point defects

Types of Point Defects

Vacancies

Interstitial

Substitutional

1 - Dimensional defects are also called

Linear defects

Types of Linear Defects

Dislocations

3 - Dimensional defects are also called

Bulk Defects

Types of Bulk Defects

Pores

Inclusions

Second Phases

Cracks

The Red Arrow represents what type of defect

Vacancy

The Blue Arrow represents what type of defect

Interstitial

The Orange Arrow represents what type of defect

Substitutional

Vacancy Defect

Vacant lattice sites; absence of an atom

How are alloys created

impurity atoms are added in order to impart desirable characteristics on the materials

Second Phases

impurity atoms added to host

• crystal structures change and a new structure is formed

Solid Solutions

In the event of the addition of impurity atoms NOT being added to host, crystal structure is maintained and no new structures are formed.

SolVENT

element present in greatest concentration

soLUTE

presents in the least common concentration

How can you recall Solvent & Solute?

solVENT - VENT = modern → common today

soLUTE - LUTE = olden → not common today

Substitutional Defects

Solute or impurity atoms replace or substitute for host aotmsi

Interstitial Defects

Impurity atoms fill voids(interstitials)

Requirements for interstitial impurity

Atomic diameter of the interstitial impurity is less than host atoms

Dislocations

Introduced during 1-D defects associated with the edge of the an extra half-plane

Dislocation movement produces…

plastic deformation

If there is no dislocation movement

no plastic deformation

Grain Boundaries exist iff

• Boundary between two grains (polycrstalline)

• Needs to have an orientation transition (misorientation)

How do grain boundaries strengthen metals?

The act as barriers to dislocation movement

What is grain boundary energy, what causes it?

Energy stored in boundary due to atomic disorder. Temperature up = less; disorder up = more

Higher chemical reactivity (GB)

allows etchants to reveal microstructure

Impurity segregation (GB)

material properties altered

Grain size

average diameter of grains in polycrystalline material

Bulk defects

larger than atoms :)

Metallographic prep steps

Sample, Mount, grind, Polish, etch, Capture image

Optical microscope use case

crystallographic orientation variation

chemical potential variation

• some phases may be revealed by etchant, but not all

SEM use case

useful for analyzing fracture surfaces (recall cup & ball)

may reveal fine microstructural features & second phases

may reveal elemental composition through use of Energy Dispersion Spectroscopy