Materials Engineering (Week 1) Flashcards
What Materials Properties are relevant to Engineers ?
An Economic Property (1)
Price and Availability
What Materials Properties are relevant to Engineers ?
Bulk Mechanical Properties (6)
Density
Stiffness (Modulus) and damping
Strength (many types)
Toughness
Fatigue
Creep
What Materials Properties are relevant to Engineers ?
Bulk non-mechanical properties (4)
Thermal properties
Optical properties
Magnetic properties
Electrical properties
What Materials Properties are relevant to Engineers ?
Surface properties (3)
Oxidation and corrosion
Friction and wear
Biocompatability
What Materials Properties are relevant to Engineers ?
Production properties (4)
Ease of manufacture
Fabrication,
joining,
finishing
What Materials Properties are relevant to Engineers ?
Aesthetic properties (3)
Appearance,
texture,
feel
Example: Optical properties
Transmittance:
Aluminum oxide may be transparent, translucent, or
opaque depending on the material structure.
-single crystal
-polycrystal: low porosity
-polycrystal: high porosity
Classification of Materials:
Metals
Polymers/plastics
Ceramics
Composites
Metals composition/examples:
Composed of one or more metallic elements (e.g. Fe, Al, Cr, Cu, Ti, Au, Ni) and small amounts of non-metallic elements (C, N, O)
Metals features (7):
Orderly arrangement of atoms
• High density, stiff, strong and ductile used in structural applications
• Nonlocalised electrons, good electrical conductors and
heat conductors
Ceramics, examples:
• Cement and concrete
• Glasses and silicates
• Alumina (Al203, emery,
sapphire)
• Silicon carbide (SiC)
• Silicon nitride (Si3N4)
Ceramics features: (11)
Compounds between metallic and non-metallic elements Frequently they are oxides nitrides and carbides
Stiff and strong (comparable to metals), but very brittle Typically insulating to heat and electricity
Can be transparent, translucent or opaque
Polymers examples:
• Polyethylene (PE), Polypropylene (PP)
• Polymethylmethacrylate (PMMA, Perspex) Nylon
• Polystyrene (PS)
• Polyurethane (PU)
• Polyvinylchloride (PVC)
• Rubbers
Polymers features: (6)
Many polymers are organic compounds based on C, H, O, N.
Usually consist of large chainlike molecular structures with a backbone of carbon atoms.
Low density, but not as strong or stiff as ceramics and metals
Can be very ductile, easily formed into complex shapes
High chemical resistance but low temperature stability.
Composites examples:
• Wood
• Bone
• Fibreglass
• Carbon-fibre re-inforced polymer
Composites features: (2)
• composed of two (or more) individual materials, which come from the other categories.
• combination of properties that is not displayed by any single material.
Advanced materials (4 examples):
Semiconductors Biomaterials
Smart materials
Nano-engineered materials
Advanced materials
Graphene example:
Graphene, one-atom-thick sheets of carbon, can carry electric charges far faster than currently used materials.
But it has proven difficult to make it behave as a semiconductor like silicon, or to attach “contacts” to the sheets.
A study in Nature Communications solves those problems by cooking up graphene from a material called silicon carbide.
Advanced materials
Zinc oxide nanorods example:
Zinc oxide nanorods are semiconducting and piezoelectric and can be used in energy harvesting devices, electronic components such as diodes, chemical sensors and bioimaging sensors.
The 4 structures of materials:
Subatomic level
Atomic level
Microscopic structure
Macroscopic structure
Subatomic level:
Electronic structure of individual atoms that defines interaction among atoms (interatomic bonding)
Atomic level
Arrangement of atoms in materials (for the same atoms can have different properties, e.g. two forms of carbon: graphite and diamond)
Microscopic level
Arrangement of small grains of material that can be identified by microscopy
Macroscopic level
Structural elements that may be viewed with the naked eye
Microstructures – Properties
(Length Scales)
macrostructure >1mm determines shape of object, macroscopic properties
microstructure
determines physico-chemical nature
molecular structure
determines physico-chemical nature
atomic structure
determines physico-chemical nature
~1 nm – 1 μm
1 nm – 1 μm
<1 nm
Generally NOT INTRINSIC – can be manipulated!!
Angstrom = 1A =
10^-10m
The Materials Selection Process (3 steps):
- Pick Application: Determine required Properties Properties: mechanical, electrical, thermal, magnetic, optical, deteriorative.
- Properties: Identify candidate Material(s) Material: structure, composition.
3.Material: Identify required Processing
Processing: changes structure and overall shape example: casting, sintering, vapor deposition, doping forming, joining, annealing.
How do you work out the atomic mass?
Atomic number (Z) and atomic mass (A)
– A ~ Z+N (N : number of neutrons)
NA: Avogadro’s number
6.023 × 10^23
1 amu/atom is the same as…
1 gram/mole
Where do you find the electronegative and electropositive elements in the periodic table
Electropositive on the very left then becomes more and more electronegative as you move along towards the right end
Name the three main bonding types
Ionic
Covalent
Metallic
Also weaker (secondary) bonds, example…
van der Waals (dipole) hydrogen bonding
Ionic (brief description): to do with electronegativity
Strong differences in electronegativity
Covalent (brief description): to do with electronegativity
Small differences in electronegativity
Metallic bonding (brief description) to do with valence electrons
Valence electrons unbound
The properties of materials are
directly related to …
give examples
…their crystal
structure.
magnesium and beryllium
are brittle - gold is not
what kind of structures tend to have lower
energies
dense, ordered packed structures
Materials and Packing
Crystalline materials:
-atoms pack in periodic, 3D arrays, examples are metals, many ceramics and some polymers
Materials and Packing
Non-crystalline materials:
atoms have no periodic packing
occurs for: complex structures, rapid cooling
Another word for Noncrystalline
“Amorphous”
Unit Cell:
smallest repetitive volume which
contains the complete lattice pattern of a crystal.
Unit cell are parallelpipeds or prisms with 3 sets
of parallel faces
The corners of the unit cell coincide with the centre of atoms.
Metallic crystal structure:
-Tend to be densely packed
-Have the simplest crystal structures.
Reasons for dense packing (Metallic Crystal Structures) :
- Typically, only one element is present, so all atomic radii are the same.
- Metallic bonding is not directional.
- Nearest neighbour distances tend to be small in
order to lower bond energy. - Electron cloud shields cores from each other
What are the 4 Metallic Crystal Structures:
Simple Cubic Structure (SC)
Body Centered Cubic Structure (BCC)
Face Centered Cubic Structure (FCC)
Hexagonal Close-Packed Structure (HCP)
Simple Cubic Structure (SC): (2)
- Rare due to low packing density (only Po has this structure)
- Close-packed directions are cube edges.
Atomic Packing Factor (APF):
APF = Volume of atoms in unit cell /
Volume of unit cell
*assume hard spheres
Simple Cubic Structure (SC): APF
- APF for a simple cubic structure = 0.52
Body Centered Cubic Structure (BCC)
Atoms touch each other along cube diagonals. –Note: All atoms are identical; the center atom is shaded differently only for ease of viewing.
Body Centered Cubic Structure (BCC) examples
ex: Cr, W, Fe (α), Tantalum, Molybdenum
APF for a body-centered cubic structure is…
0.68
Face Centered Cubic Structure (FCC):
Atoms touch each other along face diagonals.
Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing.
Face Centered Cubic Structure (FCC): examples
ex: Al, Cu, Au, Pb, Ni, Pt, Ag
APF for a face-centered cubic structure is…
0.74
maximum achievable APF
FCC Stacking Sequence:
ABCABC… Stacking Sequence
Hexagonal Close-Packed Structure
(HCP): staking sequence
ABAB… Stacking Sequence
Hexagonal Close-Packed Structure
(HCP): APF
APF = 0.74
Hexagonal Close-Packed Structure
(HCP): examples
ex: Cd, Mg, Ti, Zn
Theoretical Density, ρ
Density = ρ = Mass of Atoms in Unit Cell / Total Volume of Unit Cell
ρ = n A / Vc Na
where
n = number of atoms/unit cell
A = atomic weight
Vc = Volume of unit cell = a^3 for cubic
Na = Avogadro’s number = 6.023 x 10^23 atoms/mol
Densities of Material Classes:
In general
ρ metals > ρ ceramics > ρ polymers
In general, why does the following order exist:
ρ metals > ρ ceramics > ρ polymers
Metals have…
* close-packing
(metallic bonding)
* often large atomic masses
Ceramics have…
* less dense packing
* often lighter elements
Polymers have…
* low packing density
(often amorphous)
* lighter elements (C,H,O)
Composites have…
* intermediate values
Polymorphism
Two or more distinct crystal structures for the same material (allotropy/polymorphism)
Polymorphism examples (2):
Titanium: α, β-Ti
HCP α form at room temp.
BCC β form at 882 °C
Carbon
diamond, graphite
The prevailing crystal
structure depends on
temperature and
pressure.
Atoms may assemble into … or
… structures.
crystalline
amorphous (An amorphous structure has no organization, and the atomic structure resembles that of a liquid)
Common metallic crystal structures are …
FCC,
BCC,
HCP.
Coordination number and atomic packing factor
are the same for which 2 crystal structures?
FCC and HCP crystal structures
We can predict the density of a material, provided we know what?
the atomic weight, atomic radius, and crystal
geometry (e.g., FCC, BCC, HCP).
What Materials Properties are relevant to Engineers ? (6)
-An economic properties
-Bulk mechanical properties
-Bulk non-mechanical properties
-Surface properties
-Production properties
-Aesthetic properties
