presentacion del aluminio

7/27/2019 presentacion del aluminio

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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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Dislocation motion is analogous to the movement of a

caterpillar


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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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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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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.

presentacion del aluminio

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