Metals materials properties

The most familiar phases are solid, liquid and gasWithin the solid-state, a metal may exist in several different solid phases Pure iron has three different solid phases α, γ and δ-Fe at

METALS

 What is a metal ?

 General properties

 Structure and bonding

 Phases and phase transformations

 Structure – property relationships

 Chemical properties

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No

 Opaque

 Lustrous

 High melting point

 Good conductors of heat

 Good conductors of electricity

Other Types of Bonding

-Ionic bonding is often stronger than metallic bonding

Ceramics tend to have higher melting points than metalsCovalent 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

Inter-molecular bonds can be rather weak (e.g thermoplastic polymers)

Melting Points

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

Ceramics and polymers tend to be good electrical insulators

The smallest repeating array of atoms is called the

“primitive” unit cell

The lengths of the sides of the unit cell are called the lattice parameters

Packing of Atoms

A B A B A B

Hexagonal close-packed (hcp)

Titanium (Ti) - hcp

A B C A B C A B C

Face centred cubic (fcc)

8 atoms at corners of the unit cell

1 atom centered on each of the faces The atom on the face is shared with the adjacent cell

In total, the unit cell contains 4 atoms: 8 x 1/8 at each corner

6 x 1/2 at each face

Gold (Au) - fcc

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 2 atoms

1 x 1 in the centre

8 x 1/8 at the corners

Iron (Fe), Vanadium (V), Chromium (Cr) - bcc

Crystal structures of some metals (rt)

The crystal structure of a metal can determine some of its

mechanical properties

E.g ductility

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 (α, γ and δ-Fe) at different temperatures

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

α-Fe has the bcc structure

γ-Fe has the fcc structure

Phases

Phase Equilibria and Phase Diagrams

A phase diagram is a chart which shows which

phases are stable under which conditions

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

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

Single Crystal Metals

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

Both microstructure and crystallography influence the properties of a metal

Microstructure

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 fill the space completely

Grain Shapes

If a molten metal is cooled very rapidly, the atoms do 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

Amorphous Metals

Metallic crystals are not perfect

Crystal Defects

Perfect Vacancies Interstitials

1 Edge dislocation: a missing half plane of atoms

2 Screw dislocation: layers twisted with respect to each other

3 A combination of the two

Imperfections, grains and grain boundaries, determine

many of the mechanical properties of metals Dislocations are a localised imperfection in the alignment of the layers of atoms in the lattice

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

Alloys

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

3 At certain ratios of the number of valence electrons to the

number of atoms in a structure

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

This is called an “interstitial

solid-solution”

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

Adding the solute does not change the crystal structure In contrast, an intermetallic compound often has a different crystal structure to that of the parent metals

For alloys, composition is variable

Binary alloys contain two components

Assuming pressure is not a variable, 2 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 diagram, but three phases cannot

Binary Alloys

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

More Complex Phase Diagrams

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 (σ) is defined as σ = 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

AF

TensionTensile stress σ = F/A

Compression

Materials respond to stress by straining

Nominal tensile strain εn = u/l

u = elongation, l = original length

Strain is dimensionless

ll

σ

uv/2v/2

“Inward” shrinkageNominal lateral strain εn = −v/l

Poisson’s ratio, tensile strain

strain lateral

= ν

Hooke’s Law

For many materials, when strains are small the strain is

very nearly proportional to the stress

The greater the value of the stiffness, the more difficult it will

be to produce elastic deformation

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 deformationOnce a certain stress (the “yield stress”, σy ) is exceeded the stress - strain curve ceases to be linear

The material begins to undergo “plastic” deformation

Stress - strain curves for a “typical” metal

Elastic and Plastic Deformation

σy

σ

ε

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 Deformation

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

uF=0 F=0

l0

l0 u

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

τ

τ

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 do not move easily because

of the very strong localised bonding

This explains why metals are ductile, while ceramics are brittle

b

The Force Acting on a Dislocation

A shear stress, τ, 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 = τbper unit length of the dislocation

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

Note: materials that are normally ductile can be embrittled

by contaminants

Yield Strength and Tensile Strength

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

σ0.1% 0.1% Proof stress (F/A0 at a permanent strain of 0.1%)

σTS Tensile strength (F/A0 at onset of necking)

εl (Plastic) strain after fracture, or tensile ductility The broken pieces are put together and measured and εl is calculated

from (l-l0)/l, where l is the length of the assembled pieces

σy

σ

ε

σf

“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=3σy

True hardness = F/AprojVickers hardness = F/AtotF

A

“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

Toughness

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

pre-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)

Cleavage failure has little advanced warning

Brittle Failure

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

As deformation continues, the microvoids eventually coalesce and final failure occurs

Metal Failure

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 hardeningPrecipitate and dispersion strengtheningWork hardening

Grain size effects

Treatments to Alter Properties

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

Thus τy increases with solute concentration as C½