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corrosion enhances the rate of growth of fatigue cracks in most metals and alloys if grain boundaries corrode preferentially, cracks may be formed and propagate. preferential[r]

Outline

 What is a metal ?  General properties

 Structure and bonding

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Lr Rf Db Sg Bh Hs Mt Uuu Uub Uub

What is a Metal ?

Non-metals Metals

 Opaque  Lustrous

 High melting point

 Good conductors of heat

 Good conductors of electricity  Dense

 Strong  Malleable  Ductile

Bonding

e- e- e- e- e- e

-e

-e

-e

-e

-e

-e

-e

-e- e

-e

-e

-e- e- e- e- e

-e- e- e- e- e

Other Types of Bonding

Covalent Bonding

e.g salt

e- e-

C

C

Ionic Bonding

e.g polymers

-Ionic bonding is often stronger than metallic bonding

Ceramics tend to have higher melting points than metals Covalent bonds can also be extremely strong

Covalently bonded materials may also have higher melting points than metals, e.g diamond

However, many covalently bonded materials have very low melting points due to the existence of molecules

Electrical Conductivity

Freely moving electrons can conduct electricity

Metallic materials tend to be good electrical conductors Some metals are better conductors of electricity than others, e.g copper is a better electrical conductor than tin

Crystallinity

In a crystalline solid, the “lattice” is made up of repeating units of atoms

A repeating array of atoms in a lattice is called the “unit cell”

The smallest repeating array of atoms is called the “primitive” unit cell

A B A B A B

Hexagonal close-packed (hcp)

Each unit cell here contains atoms x 1/8 at each corner

A B C A B C A B C

Face centred cubic (fcc)

8 atoms at corners of the unit cell atom centered on each of the faces The atom on the face is shared with the adjacent cell

Body centred cubic (bcc)

A third common packing arrangement in metals is body centred cubic

The BCC unit cell has atoms at each of the eight corners of a cube plus one atom in the center of the cube

The unit cell contains a total of atoms

Aluminum FCC Nickel FCC

Cadmium HCP Niobium BCC

Chromium BCC Platinum FCC

Cobalt HCP Silver FCC

Copper FCC Titanium HCP

Gold FCC Vanadium BCC

Iron BCC Zinc HCP

Lead FCC Zirconium HCP

Magnesium HCP

Crystal structures of some metals (rt)

The crystal structure of a metal can determine some of its mechanical properties

The most familiar phases are solid, liquid and gas

Within the solid-state, a metal may exist in several different solid phases

Pure iron has three different solid phases (,  and -Fe) at

different temperatures

Each of these phases has its own distinctive structure and properties, although all three are made up of iron atoms

-Fe has the bcc structure -Fe has the fcc structure

Phase Transformations

As iron is heated from room-temperature to above its melting point, the following changes in phase occur:

 transforms to ;  transforms to ;

 transforms to liquid iron

Phase Equilibria and Phase Diagrams

Stable Metastable Unstable

For a single element material, the variables that influence phase stability are temperature and pressure

Extremely high pressures are generally required to significantly change phase equilibria for solid metals

Under normal conditions, the phase diagram for a pure metal generally needs only a temperature axis

The pure iron phase diagram at constant pressure (not to scale)

The phase diagram consists of single phase regions Two phases are only found together at a point

Phase Diagrams

T Liquid

Single crystal: lattice extends the edges of the material, e.g a diamond

Metal single crystals are possible: e.g Ni alloy turbine blades used in aero gas-turbine engines (“jet engines”) can be produced as single crystals

Above their melting points, metals are liquids The atoms are randomly arranged and relatively free to move

On cooling to below the melting point, the atoms rearrange forming the ordered, crystalline solid structure

Polycrystalline Metals

In most cases, solidification begins from multiple sites, each of which can produce a different orientation

The result is a “polycrystalline” material consisting of many small crystals of “grains”

The “microstructure” of a material is the portion of the material’s structure that can be observed under a microscope A good quality light microscope will produce a magnification of around 1,000 x

A modern electron microscope is capable of magnifications of 1,500,000 x or more

Microstructure features are in the range

~100 pm (i.e. x 10-10 m, 0.1 nm, or Å)

to ~100 m (i.e. x 10-4 m, or 0.1 mm)

Feature such as grains count as microstructure

Both microstructure and crystallography influence the properties of a metal

In 3D, the grains in a polycrystal are usually polygonal

The interfaces between the grains (“grain boundaries”) have an interfacial energy associated with them

Matter always tries to adopt the lowest energy condition possible

The interfacial energy of the sample can be reduced by minimising the total interfacial area present

Spherical grains would give the lowest surface area to volume ratio, but it is impossible to completely fill space by packing spheres together

The surface area to volume ratio of polygons is nearly as low as that of spheres, but polygons can stack together to

If a molten metal is cooled very rapidly, the atoms not have time to rearrange to form an orderly crystalline lattice

Instead, a random “amorphous” arrangement is produced and the result is a non-crystalline material

The best known amorphous material is window glass: amorphous materials are often referred to as glasses

Metals can usually crystallise even at very high cooling rates, but under extreme conditions metallic glasses can be produced in some alloys

NB metallic glasses are not transparent

The lack of long range order in metallic glasses produces unusual properties which may have specialist applications

Metallic crystals are not perfect

Crystal Defects

Dislocations

1 Edge dislocation: a missing half plane of atoms

2 Screw dislocation: layers twisted with respect to each other A combination of the two

Imperfections, grains and grain boundaries, determine many of the mechanical properties of metals

An alloy is a mixture of a pure metal and one or more other elements

Often, these other elements are metals

For example, brass is an alloy of copper and zinc Metals can also be alloyed with non-metals

In many cases, metals are quite soluble in other metals

In other cases, instead of a solid-solution a new phase, an “intermetallic compound”, with a structure different from that of any of its constituent metals can be produced

Intermetallic Compounds

Hume-Rothery rules

1 “Size factor” compounds Only a limited amount of the solute can be dissolved in the solvent

2 Large difference in electronegativity between the solvent and solute Bonding is more ionic than metallic

Solid Solutions

Even when intermetallic formation does not occur, there may not be perfect solid-solubility

If two metals have different crystal structures then at some intermediate composition there will have to be a change from the crystal structure of the one metal to that of the other In such a case the result would be, on gradually changing the composition of an alloy from pure metal A (forming the

phase) to pure metal B (forming the phase): single-phase  solid-solution;

two-phase mixture of  solid-solution and 

solid-solution;

The nature of solid-solutions depends on the size of the solute atoms, relative to that of the solvent

Structures of Solid Solutions

When the solute atoms are much smaller than those of the solvent, the solute will sit in such empty spaces (“interstices”) as are available between the solvent atoms

Substitutional Solid Solutions

When the solute atom is fairly similar in size to that of the solvent, then solute atoms will substitute for some of the solvent atoms and the result is called a “substitutional solid-solution”

In both substitutional and interstitial solid-solutions the sites occupied by specific atoms are random

For alloys, composition is variable

Binary alloys contain two components

Assuming pressure is not a variable, axes are required

The vertical axis represents temperature and the horizontal axis composition

In the case of binary alloy phase diagrams, the following key features are observed in the alloy:

 Single phase regions are separated by two phase regions  Both single and two phase regions can occupy an area on

The simplest binary phase diagram is that in which there is perfect solid solubility

Nickel and copper are mutually soluble according to the Hume-Rothery rules for the formation of intermetallics

Since copper and nickel are FCC with almost the same lattice parameter a two phase mixture is not expected

Simple Binary Phase Diagrams

The Ni-Cu binary phase diagram

T

100

Liquid (L)

L + S

Solid (S)

More Complex Phase Diagrams

A

Composition

B T

 + 

 + L  + L

L

In a “tensile test” a sample is gradually elongated to failure and the tensile force required to elongate the sample is measured using a load cell throughout the test

The result is a plot of tensile force versus elongation

Structure – Property Relationships

Stress () is defined as  = F/A

F = force applied to the sample

A = cross-sectional area of the sample Stress has units of Pa (i.e. N m-2)

Stress

A F

A F

Tension

Tensile stress = F/A

Strain

Materials respond to stress by straining Nominal tensile strain nu/l

u = elongation, l = original length Strain is dimensionless

l



u

v/2 v/2

“Inward” shrinkage

Nominal lateral strain nv/l

Hooke’s Law

For many materials, when strains are small the strain is very nearly proportional to the stress

 = E

E is Young’s modulus and is a measure of “stiffness”

 

Gradient =  / 

= E

Initially, the stress-strain curve is linear

In this region, Hooke’s law is obeyed and the material is said to behave “elastically”, i.e it undergoes elastic deformation Once a certain stress (the “yield stress”, y) is exceeded the

stress - strain curve ceases to be linear Stress - strain curves for a “typical” metal

Elastic and Plastic Deformation

y



Elastic Deformation

Simplified view of metal bar

When the load is released

Elastic deformation involves bond stretching Stiffness is independent of microstructure

With an applied force

Plastic deformation involves the breaking and making of bonds

The mechanism of plastic deformation involves sliding layers of atoms over each other

The more closely packed together the atoms are, the easier the layers of atoms will be to slide

Hence, shearing takes place in the close-packed plane and along the close-packed direction that are nearest to the location of maximum shear stress

In contrast to stiffness, the yield and ultimate tensile strengths of metals and alloys are extremely sensitive to microstructure

Plastic Behaviour and Ductility

Load-extension curve for a bar of ductile metal in tension

The greater the extent of plastic deformation, the higher the “ductility”

F

u

F=0 F=0

l0

Plastic Deformation and Dislocations

Dislocations can serve as a means of producing the shearing involved in plastic deformation

When a shear stress is applied, bonds are made and broken

locally, reducing the yield stress



Motion of Dislocations

b is the unit of slip (the Burger’s vector)

Dislocations move easily in metals, due to delocalised bonding Dislocations exist in ceramics, but not move easily because of the very strong localised bonding

This explains why metals are ductile, while ceramics are brittle

The Force Acting on a Dislocation

A shear stress, , exerts a force on a dislocation, pushing it

through the crystal

For yielding to occur, the force must be large enough to overcome the resistance to the motion of the dislocation

The magnitude of the force, f, is given by

f = b

per unit length of the dislocation

f



b

Ductility and Structure

FCC metals and alloys are usually ductile at all temperatures

The atoms in FCC metals are closely packed and can slide over each other easily

BCC materials tend to become brittle at low temperatures

The atoms in BCC metals are less closely packed and cannot slide over each other so easily

Yield Strength and Tensile Strength

y Yield strength (F/A0 at the onset of plastic flow)

0.1% 0.1% Proof stress (F/A0 at a permanent strain of 0.1%) TS Tensile strength (F/A0 at onset of necking)

l (Plastic) strain after fracture, or tensile ductility The broken

pieces are put together and measured and l is calculated

y





“Hardness” is a measure of resistance to plastic deformation

Hardness is measured by determining the depth or projected area of an indentation produced by a standard indentor

The higher the hardness of the material, the shallower the indentation for a given load and the smaller the projected area

Hardness

Hardness is related to yield strength: H=3y

True hardness = F/Aproj Vickers hardness = F/Atot F

“Toughness” is a measure of how much energy can be absorbed by the material before failure

The material is subject to an impact from a swinging hammer and the amount of energy absorbed from the swing is measured (the less energy is absorbed, the higher the hammer will swing after fracturing the sample) Energy is absorbed by plastic deformation, so ductile materials such as metals show a high toughness

Brittle materials can have a high strength, but have negligible toughness

In brittle materials, final failure generally initiates at pre-existing defects such as cracks (originating, for example, from fatigue), or notches

Since the cross-sectional area is lower in a the region with a crack than in uncracked regions, for a given applied load, the stress is higher in regions with cracks than without

If the load is increased and/or the cracks are made larger, then a point will be reached at which the stress can no longer be borne and the material will cleave into two pieces Cleavage cracks can move very quickly (around the speed of sound)

Metals can also fracture if placed under too large a stress

In a ductile material, however, plastic deformation tends to blunt cracks and cleavage failure does not occur

The most common reason (about 80%) for metal failure is fatigue

Through the application and release of small stresses as the metal is used, small cracks (microvoids) in the metal are formed and grow slowly

Microvoids often form due to decohesion between precipitates and the matrix, or fracture of precipitates

In industry, molten metal is cooled to form the solid

The solid metal is then mechanically shaped to form a particular product

How these steps are carried out is very important because heat and plastic deformation can strongly affect the mechanical properties of a metal

Methods of hardening / strengthening: Solid solution hardening

Precipitate and dispersion strengthening Work hardening

Grain size effects

Solid Solution Hardening

Metals may be hardened by making them impure E.g Adding zinc to copper to make the alloy brass

Zn atoms replace Cu atoms in the lattice to make a random substitutional solid solution

Since Zn atoms are larger than Cu atoms, they introduce stresses into the structure which “roughen” the slip planes

For a solid solution of concentration C, the spacing of dissolved atoms on the slip plane varies as C½

Bar Chart of Yield Strengths

Ceramics 0.1 10 102 103 104 105 Metals Polymers DIamond SiC

Al2O3

Precipitate and Dispersion Strengthening

If an impurity is dissolved in a metal at high temperature and the alloy cooled, the impurity may precipitate as small particles Approach situation

Sub-critical situation

Critical situation

Escape situation

Force b

per unit length

T b T

bL

L

Yielding

The dislocation escapes and yielding occurs when

Since f = b, the obstacles exert a resistance of f = 2T/L

The greatest hardening is produced by strong, closely spaced precipitates

y

L

bL

T

y

2

Annealing is a softening process in which metals are heated and then allowed to cool slowly

Most steels may be hardened by heating and quenching (cooling rapidly)

Quenching results in a metal that is very hard but also brittle Gently heating a hardened metal and allowing it to cool slowly will produce a metal that is still hard but also less brittle

This process is known as tempering

In steel, it results in many small Fe3C precipitates, which block dislocation motion thereby strengthening the metal

Metals and alloys undergo work hardening

Undeformed metals and alloys contain some dislocations as random growth defects

During plastic deformation, new dislocations are formed

The more dislocations that are trying to move at once, the greater the probability of dislocations becoming entangled or “pinned”

The result is dislocation pile-ups that make further plastic deformation more difficult

Hence, the stress required to produce further deformation is increased and work hardening occurs

Unlike plastic deformation, work hardening is reversible

By heating to a sufficiently high temperature (“annealing”) the dislocations are able to re-orient themselves into networks, in a process called “recovery”

Further heating actually allows the growth of new grains with a low dislocation density This is called “recrystallisation”

Both recovery and especially recrystallisation remove dislocation pile-ups and hence reduce the hardness of the material

If a material is cooled immediately after recrystallisation, the new grains will be quite small

With prolonged heating, grain growth occurs

A grain can grow larger by atoms migrating from another grain that may eventually disappear

Grain size is very important in plastic deformation, because there is an inverse relationship (the Hall - Petch relationship) between grain size () and yield stress:

yy

where y and 0 are constants For a single crystal  is large

and so y = 0

Dislocations cannot cross grain boundaries easily, so the size of grains determines how easily the dislocations can move

As expected, metals with small grains are stronger but they are less ductile

A fine grain size is not only makes the material stronger, but also enhances toughness (unlike other methods of strengthening that reduce ductility)

The only time that a fine grain size is not desirable is for high-temperature service, because the presence of grain boundaries enhances creep

0

Oxidation

2M + O2

2MO

E

If E < 0,

oxidation will occur Ceramics 1000 500 -500 -1000 -1500 Metals Polymers Silicon KCl Si3N4 SiC Au Be Most polymers PTFE Foamed polymers E ne rg y / k J m o l -1 NaCl LiCl Diamond / Graphite MgO/SiO2/ Al2O3

Al , Zr Ti

Rate of Oxidation

An oxide layer forms at the surface of the metal

The amount of oxidised material formed, m, varies with

time t

linear oxidation

m = kLt (kL is constant)

parabolic oxidation

m)2 = kPt (kP is constant)

W

ei

gh

t

ga

in

,



Rate of Oxidation

Oxidation rates follow Arrhenius’ Law kL = ALe-QL/RT k

P=APe-QP/RT

1/T

Ln

k L

,

ln

k P

t



m

T increasing

Parabolic Oxide Growth - Mechanism 1

O2- diffuses in through oxide

Oxide grows at metal-oxide interface e.g Ti, Zr, U

Metal Oxide Air

O2- 2e- + O

+ M MO

-Mechanism 2

M2+ diffuses out through oxide

Oxide grows at air-oxide interface e.g Cu, Fe, Cr, Co

Metal Oxide Air

+ O M

MO M2+

+2e

Mechanism 3

Electrons move only very slowly

Formation of oxide at metal-oxide or air-oxide interface depends on whether Mn+ diffuses faster than O

2-Metal Oxide Air

-Protection

The parabolic rate constant kP depends on the diffusion coefficient D

KP  cD0e-Q/RT where c is the concentration of oxygen

Protective films are those with low diffusion coefficients

Films which are electrically insulating are also more highly resistant to attack

Linear Oxide Growth

Such behaviour depends on the formation of cracks in the oxide film as oxidation proceeds

If the volume of the oxide formed is much less or much greater than that of the metal on which it is forming, it will crack or lift off the metal to relieve the strain

Wet Corrosion

Metals may also oxidise in water or aqueous solution

Abraded Iron Aerated Water

O2 + 2H2O + 4e- 4OH

-2Fe - 4e- 2Fe2+

Oxidation

(loss of electrons)

Reduction

(gain of electrons)

Surface Degradation

Attack on the metal is much faster than for dry oxidation

 The species formed may be soluble in water, diffusing

away into the liquid rapidly

 If the oxide is insoluble, deposition may occur away

from the surface

 If deposition on the surface does occur, the layer

Corrosion Potentials

The voltages shown are those which would just stop the metal from oxidising in aerated water

Note: these values assume unit activity concentrations In more dilute solutions, corrosion can occur more rapidly

If two dissimilar metals are joined, the one with the more negative potential will be attacked, while the other remains unattacked

Mg2+ +2e Mg

Al3+ +3e Al

Ti2+ +2e Ti

Zn2+ +2e Zn

Cr3+ +3e Cr

Fe2+ +2e Fe

Co2+ +2e Co

Ni2+ +2e Ni

Sn2+ +2e Sn

Au3+ +3e Au

Pt2+ +2e Pt

Ag+ +e Ag

Cu2+ +2e Cu

10 -10 -20

2H+ + 2e

H2

O2 + 2H2O + 4e 4OH

Corrosion Cracking

Wet corrosion often attacks metals selectively as well as, or instead of, uniformly

This can lead to more rapid failure

under stress, corrosion may enhance the growth rate of cracks

corrosion enhances the rate of growth of fatigue cracks in most metals and alloys if grain boundaries corrode preferentially, cracks may be formed and propagate

preferential attack can also occur at breaks in the oxide film or at precipitates