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Aerospace Materials
Fundamentals of the Structure of Metals
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Behaviour and Manufacturing Properties
of Material
Behaviour and Manufacturing Properties of Material
Structure of the
material
Mechanical
Properties
Physical and Chemical
Properties
Property
Modification
Atomic Bonding
Crystalline
Amorphous
PartiallyCrystalline
Polymer Chain
Strength
Ductility
Elasticity
Hardness
Fatigue
Creep
Toughness
Fracture
Density
Melting Point
Specific Heat
Thermal and ElectricalConductivity
Magnetic Properties
Oxidation
Corrosion
Heat Treatment
Precipitation Hardening
Annealing
Tempering
Surface treatment
Alloying
Reinforcements
Composites
Laminates
Fillers
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Atomic Structure and Interatomic
Bonding
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Periodic Table
Why is so important to understand the periodictable?
It classifies the elements according to their electron
configuration.
Each column or group have similar valence electronstructures, as well as chemical and physical properties.
The elements situated on the right-hand side of the
table are electronegative. They accept electrons to
form negatively charged ions, or sometimes they shareelectrons with other atoms
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Chemical Bonding
An understanding of many of the physical properties
of materials is predicated on a knowledge of the
interatomic forces that bind the atoms together.
Three different types of primary or chemical bondare found in solids: IONIC, COVALENT, ANDMETALLIC
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Chemical Bonding
Ionic BondAtoms of a metallic
element easily give up their
valence electrons to thenonmetallic atoms
Covalent BondStable electron configurations
are assumed by the sharing of
electrons between adjacentatoms
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Chemical Bonding Metallic bondingis found in metals and
their alloys. Metallic materials haveone, two, or at
most, three valence electrons.
These valence electrons are not bound
to any particular atom in the solid and are
more or less free to driftthroughout theentire metal.
They may be thought of as belonging to
the metal as a whole, or forming a sea
of electronsor an electron cloud.
The remaining nonvalence electrons and
atomic nuclei form what are calledion
cores,which possess a net positive
charge equal in magnitude to the total
valence electron charge per atom.
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Crystal Structures
Atoms situated in repeating or periodic array
All metals, many ceramics and certain
polymers form crystal structures
Hard sphere model, unit cells. For metals,
three simple crystal structures:
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BCC
Metallic Crystal Structures
Body-centered cubicChromium, iron,
tungsten, as well as
several other metals.
Face-centeredcubiccopper, aluminum,silver, and gold
FCC
Hexagonal close-packed
The HCP metals includecadmium, magnesium,titanium, and zinc.
HCP
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Bravais
Lattices
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Lattice Parameter Relationships
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Lattice Parameter Relationships
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Assignment 2
Define and give some examples of these differentstructures:
Amorphous
Partially Crystalline
Polymer Chain Define and explain the different tests needed to
obtained the materials mechanical properties.
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Solidification of Polycrystalline Material
Diagram of the various
stages in the solidificationof a polycrystalline material
(square grids depict unit
cells):
a) Small crystallite nuclei.
b) Growth of thecrystallites.
c) Upon completion of
solidification, grains
having irregular shapes
have formed.d) The grain structure as it
would appear under the
microscope.
a) b)
c) d)
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Solidification of Polycrystalline Material
Single crystals have periodic, repeated atomic
arrangement and this can be also produced
artificially under controlled conditions.
More common is polycrystalline structures,composed of small crystals or grains.
Generally, rapid cooling produces smaller
grains and slow cooling larger grains.
At room temperature, large grains generally
low strength, hardness and ductility.
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Microstructure of Common Materials
Cartridge brass
- grain boundaries
- annealing twinsStructure in 1045 steel: Ferrite (light) +
pearlite (lamellar)
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Anisotropy
The physical properties of single crystals of somesubstances depend on the crystallographic directionin which measurements are taken.
For example, the elastic modulus, the electrical
conductivity, and the index of refraction may havedifferent values in the [100] and [111] directions.This directionality of properties is termedanisotropy.
Substances in which measured properties areindependent of the direction of measurement areisotropic.
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Anisotropy
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a) Interstitial impurity atom, b) Edge dislocation, c) Self interstitial atom,
d) Vacancy, e) Precipitate of impurity atoms, f) Vacancy type dislocation
loop, g) Interstitial type dislocation loop, h) Substitutional impurity atom
Crystal Defects
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Point Defects (lattice irregularities)
Impede dislocation motion Vacancies form during solidification, and as
a result of atomic vibrations. Equilibriumnumber of vacancies increases exponentiallywith temperature.
Self interstitial is atom crowded intointerstitial void . In metals this induces large
distortions due to high packing factors, and isnot highly probable (will exist in lowconcentrations).
Impurities always exist. Alloys have higher"impurities"
Solvent is host element, solute iselement in minor concentration.
Solid solution if random, uniformdispersal of impurities.
Substitution
Interstitial atoms must be much smallerthan host atoms (carbon .071nm isinterstitial when added to iron .124nm,with max. concentration ~2%).
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Line Defects (Dislocations) Dislocations motion allows sl ip(plastic deformation
wherein interatomic bonds are ruptured and reformed). Edge dislocations allow slip at a much lower stress
than in a perfect crystal
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Line Defects (Dislocations)
Dislocation motion is analogous to the movement of a
caterpillar
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Line Defects (Dislocations)
Dislocation motion is analogous to the movement of a
caterpillar
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Line Defects (Dislocations)
Another type of dislocation, called a screw dislocation.
The screw dislocation derives its name from the spiral or
helical path or ramp that is traced around the dislocation
line by the atomic planes of atoms.
Most dislocations found in crystalline materials areprobably neither pure edge nor pure screw, but exhibit
components of both types; these are termed mixed
dislocations.
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Motion of Dislocations
Edge Dislocation:
Dislocation moves in
direction of applied shear
stress
Screw Dislocation:Dislocation motion is
perpendicular to applied
shear stress
Direction of motion of
mixed dislocations is
somewhere between
parallel and perpendicular
to the applied stress.
Formation of a step on the surface of a crystal due to:
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Slip
Slip is dominant mechanism fordeformation.
Slip occurs due to dislocation motion, and
the slip plane is the preferred plane for
dislocation motion.
Slip bands are collection of parallel slip
planes.
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Slip lines on the surface of a
polycrystalline specimen of
copper which was polished and
subsequently deformed.
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Slip Planes
Slip planes are those with most dense atomicpacking Metals with FCC or BCC structures have relatively large
number of slip systems, and thus are quite ductile (plasticdeformation possible along slip systems)
Slip systems >5 indicate ductile material
HCP metals tend to be brittle due to few slip planes
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Slip Planes
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Plastic Deformation of Polycrystalline
Materials More complicated than single crystals because direction of slip
varies by grain.
Strength generally increased due to tangled dislocations.
Anisotropic behaviour (preferred orientations).
Alteration of grain
structure by plastic
deformation:
a) before: equiaxed
grainsb) after: elongated grains
Hot working would not
produce elongated
grains
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Plastic Deformation of Polycrystalline
Materials
Plastic deformation of grains in
compression (e.g. rolling, forging) Alignment along horizontal
preferred direction
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Slip planes
In single crystals there are preferred planes wheredislocations move (slip planes). There they do notmove in any direction, but in preferredcrystallographic directions (slip direction).
The set of slip planes and directions constituteslip systems.
The slip planes are those ofhighest packingdensity.
How do we explain this? Since the distance between atoms is shorter than the
average, the distance perpendicular to the plane has to belonger than average. Being relatively far apart, the atomscan move more easily with respect to the atoms of theadjacent plane.
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Slip planes
BCC and FCC crystals have more slip systems, that is
more ways for dislocation to propagate. Thus, those
crystals are more ductile than HCP crystals (HCP
crystals are more brittle).
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Strengthening
Size of grains influences the mechanical properties
Generally, rapid cooling produces smaller grains and
slow cooling larger grains
At room temp., large grains generally low strength,
hardness and ductility
The ability to plastically deform depends on the
ability ofdislocations to move, so to increase
strength one must impede dislocation motion:
by alloying -- introducing point defects and more grains
by "tangling" dislocations through "working"
by making smaller grains when cooling the melt
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Strengthening by Grain Size Reduction
Grain boundaries (planar
defects) generally impededislocation motion
more grains produce higher
strength
High Temperature: grains can slide against one
another under load (creep)
some alloying elements &
impurities can cause
embrittlement
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Strengthening by Grain Size Reduction
Grain boundaries (planardefects) generally impede
dislocation motion
more grains produce
higher strength Grain boundary barrier to
dislocation motion
slip planes discontinuous
slip planes have different
orientations
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Strengthening by Grain Size Reduction
At High Temperature:
grains can slide against one
another under load (creep)
some alloying elements &
impurities can causeembrittlement
Figure shows the influence of
grain size on yield strength of
70% Cu - 30% Zn brass alloy.
grain diameter increases right
to left
scale not linear
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Solid Solution Hardening
Another technique to strengthen and harden metals is
alloying with impurity atoms that go into eithersubstitutional or interstitial solid solution.
This is called solid-solution strengthening.
High-purity metals are almost always softer and weakerthan alloys composed of the same base metal.
Increasing the concentration of the impurity results in anattendant increase in tensile and yield strengths, asindicated in Figures a and b for nickel in copper; thedependence of ductility on nickel concentration ispresented in Figure c.
Dislocation movement is restricted due to lattice strainfield interactions between dislocations and these impurityatoms
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Strain hardening
Strain hardening is the phenomenon whereby a ductile
metal becomes harder and stronger as it is plastically
deformed.
It is also called wo rk hardening, or, because the
temperature at which deformation takes place is coldrelative to the absolute melting temperature of the metal,
cold working.
Most metals strain harden at room temperature.
The price for this enhancement of hardness andstrength is in the ductility of the metal.
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Strain hardening
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Recovery, Recrystallization and Grain
Growth Some of the energy from the deformation process is
stored within the structure as STRAIN ENERGY around
dislocations and shear zones.
Properties such as strength, ductility, conductivity etc.
are all affected by cold work.
The properties may be returned to the pre-cold work
values through the processes ofRECOVERY AND
RECRYSTALLIZATION. This can be followed by GRAIN
GROWTH.
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Recovery: Internal stresses are relieved and subgrain boundaries formed
(polygonization) in highly worked regions -- with no appreciable changein mechanical properties
Recrystallization:
New, strain-free grains are formed between 0.3 Tm and 0.5 Tm Depends on prior cold work (lower temp. reqd. due to stored
energy)
Grain Growth: Big grains are soft and low in strength (no tangled dislocations)
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Recovery,
Recrystallization
andGrainGrowth
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Recrystallization and grain growth of brass
Cold-worked (33%CW) grain structure
Initial stage of recrystallization after heating
3s at 580CPartial replacement of cold-worked grains
by recrystallized ones (4 s at 580C)Complete recrystallization (8 s at 580C)
Grain growth after 15 min at 580C
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Numerical Simulation of the Grain
Growth The grain grows form small
nuclei (shown in the 50s and
100s in grey)
Each color represents
different crystallographic
orientation.