advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
ACTIVITY BRIEF
Metals The science at work Metals have numerous uses. Often their properties are modified to maximise their suitability for a particular purpose. Understanding the structure of metals helps. You’ll find out more when you try Study sheet: Metal structures. A metal’s properties determine what it can be used for. Metallurgists can modify the structure of metals to make their properties better suited to a particular use. This is done by processing metals using treatments such as cold working and heat treatment (including annealing, quenching and tempering). Broadly speaking, metals with small grains are stronger but less ductile than metals with large grains. So, the trick is being able to manipulate the grain size and orientation. Your brief You need to:
Use fully annotated diagrams to describe the structures of more than two metals and relate their structures to physical properties (you will need these for the poster and accompanying leaflet you must produce as part of this unit).
Carry out tests on samples of metals you have work-hardened, annealed and tempered. Compare the effects against control samples and write a report on your findings. Include an evaluation of whether the treatments have produced the result you anticipated. This report is part of the assessment requirements for the unit.
Task 1 The relationship between the properties of metals and their structures By completing the Study sheet: Metal structures you will make notes that you need to produce the poster and write the accompany leaflet. Task 2 Modifying the mechanical properties of metals By completing Practical sheet: Treating metals you will carry out the practical investigation required as part of the assessment for this unit.
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
STUDY SHEET
Metal structures Structure at the atomic level Describe the bonding in metals and sketch a diagram to show it. Make sure you use and explain the terms cation and delocalised electrons.
The metal atoms are arranged in a regular lattice. There are three types. In each case describe the arrangement of atoms (you might it find it helpful to sketch diagrams):
Hexagonal closed packed (hcp)
Face centred cubic (fcc)
Body centred cubic (bcc)
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
Structure at the micro level Metals are made from their ores at very high temperatures. They are liquid at these temperatures. Liquid metal is cast by pouring into moulds where it cools and solidifies. At the freezing temperature, metal atoms pack together to form tiny crystals. These tiny crystals begin to form at lots of places throughout the molten metal. They grow until they reach a neighbouring growing crystal. So the solidified metal is not a single crystal. It is made up of lots of tiny crystals; it’s polycrystalline. These tiny crystals are often called grains. Where one grain meets another is called a grain boundary. Sometimes grains can be seen under a light microscope, sometimes even with the naked eye. The diagram below shows this. Add labels and descriptions so that somebody who has not read the text above can understand what they show.
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
The photograph below is of the surface of galvanised iron (i.e. coated with zinc to protect against corrosion) used in the tallest building in the world, the Taipei 101 Tower in Taiwan. It shows the grain structure. 1
Label the photograph
6 cm Photo: Professor Harry Bhadeshia, University of Cambridge
2
Estimate the size of the grains.
3
Grains are not always this size. It’s rare to see them with the unaided eye, often you need a light microscope or even an electron microscope. Do a web search to try to find other examples. Estimate the range of grain sizes you come across.
Dislocations The crystals aren’t perfect, however. Dislocations are a common defect. Dislocations occur where a few atoms in a layer are missing. Here is a diagram to illustrate a dislocation. Label the point of the dislocation.
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
Structure and mechanical properties Mechanical properties include tensile and compressive strength, ductility, toughness and hardness. For metals, these are influenced by:
the type of crystal lattice
the average size of the grains
the number of grain boundaries
dislocations in the grains.
As grain size decreases a metal becomes:
stronger
harder
stiffer (less ductile).
This is because dislocations in a crystal cause neighbouring layers to be displaced slightly to minimise the strain. The more grain boundaries there are the more difficult it is for the dislocations to move and for the metal to change shape. The micro-structure of a metal can be affected by various ‘treatments’. These include coldworking and heat treating metals, including annealing, quenching and tempering. Most metals respond to heat treatment but the temperatures used are specific for different metals. Heating a metal usually increases grain size. Atoms move between grains. Small grains fuse with one another to form larger grains. Cooling very hot metal slowly gives large grains; cooling it very quickly gives small grains. Predict the effect of the following treatments on the strength, hardness and ductility of metals:
Bending a piece of metal backwards and forward (work hardening; cold working).
Heating a metal and letting it cool slowly (annealing).
Heating a metal and cooling it very rapidly (quenching).
Re-heating a quenched metal and allowing it to cool slowly (tempering).
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
PRACTICAL SHEET
Treating metals Purpose In this practical you will investigate how different treatments affect the mechanical properties of a number of metals. Equipment and materials
Several strips of metal (about 150 mm x 10-15 mm x 0.3-0.7 mm)
Copper rod (about 200 mm x 5 mm) [other metal rods may be available]
2 x wooden blocks (about 100 mm x 100 mm x 30 mm thick)
G clamp
Ruler
Hacksaw
Felt tip pen
Leather gloves or mole grips to hold metal strips
Bunsen burner
Heat-resistant mat or ceramic tile
You may need access to a workshop as well as a laboratory for this practical work. Health and Safety The metal strips may have sharp edges so take care when handling them. Wear leather gloves or use mole grips to hold the metal strips to bend them. Take extreme care with the metals when heating them. Metals retain heat well, so make sure they have plenty of time to cool down before handling them. Wear eye protection when quenching. A risk assessment must have been done before starting work. 1
The effects of work-hardening a
Use mole grips or pliers to hold one end of the copper rod. Heat it in a Bunsen flame until it glows a dull red. Start moving the rod in and out of the flame. The idea is to let the metal cool down slowly. After another few minutes, lay the metal rod on a heat resistant surface and leave to cool to room temperature.
b
Bend the rod backwards and forwards a few times. Note your observations.
c
Use mole grips or pliers to hold the rod on a hard surface such as a vice. Hammer one half of its length while rotating it. This is called cold working. Describe the effect of this cold working.
d
Repeat the heating processing and, after cooling, test the stiffness of the rod again.
e
Describe your observations and try to explain them.
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
2
The effects of heat treatments There are several metal strips to investigate and three different heat treatments. You will also check the reliability of measurements, so it may be helpful to work in a small team and share the workload. Procedures for heat treatments Annealing Hold the metal strip at the end to be clamped between the wooden blocks (see Procedure below). Heat the whole length of the metal strip for five minutes in the hottest part of a roaring Bunsen flame. Half close the hole in the Bunsen burner and keep the metal strip moving in the non-luminous flame. After a minute or so, start moving the strip in and out of the flame so that the strip begins to cool slowly. After another few minutes, lay the metal strip on a heat proof surface and leave to cool to room temperature. Quenching Wear eye protection. Fill a 250 cm3 beaker with cold tap water. Hold the metal strip at the end to be clamped between the wooden blocks. Heat it in the hottest part of a roaring Bunsen flame until it is red hot. Immediately plunge it into the beaker of water. Swirl it around vigorously for about 30 seconds. Tempering Take a strip of metal that has been quenched. Reheat until it until it glows a dull red colour (do not heat as strongly as for quenching). Now gradually remove the strip as you did for the annealing process and allow to cool to room temperature. Comparing the effects of heat treatments Use the procedure below to compare the properties of:
untreated metal strips (the control samples)
annealed metal strips
quenched metal strips
tempered metal strips.
Procedure a
Using the felt tip pen and ruler make a mark on one side of each strip 30 mm from one end.
b
Place the shorter end of the strip between the two pieces of wood so that the pen mark on the side is just visible. Clamp the blocks of wood firmly to the bench so that the pieces of wood are level with the bench and the end of the strip sticks out horizontally (picture A).
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
picture A
picture B
picture C
c
Wearing leather gloves or using mole grips bend the metal strip upwards through 90° until the strip is lying along the side of the top wooden block (picture B). Make sure that the metal is bending at the place where you made the pen mark – or as close as possible to this.
d
Now bend the strip downwards through 180° until it is along the edge of the bottom wooden block (picture C). This counts as is two bends in total.
e
Continue bending the metal strip up and down counting each bend until the metal breaks in two. Take care when handling the sharp edges of the broken metal.
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
f
Record in a table: i
the metal used
ii
how easily it bent relative to the control sample
iii
how the ease of bending changed, if at all, as it was bent several times
iv
the number of bends until fracture.
Interpretation Summarise your findings. Try to explain your observations in terms of the effect of treatments on the microstructure of the metals (see Study sheet: Metal structures).
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
Teacher notes This activity links to OCR A2 Unit 11 Materials for a purpose. Metal structures One assessment requirement for the unit is that students must produce:
a poster and accompanying leaflet outlining the structures of polymers, metals, ceramics or glasses, and composite materials with the highest marks gained if they
produce a description, with diagrams, of the structures of more than two examples of each of polymers and metals, relating their structures to physical properties.
This study sheet may be copied and given to students to complete. Its successful completion will enable students to gather sufficient information to be able to complete the parts of the poster and leaflets relating to metals. Structure at the atomic level Students should be familiar with the idea of metal cations in a sea of delocalised electrons. However, it may be useful to spend a little time recapping this by asking them to say what they know about the properties of metals and how these relate to their structure and bonding. They should be able to find diagrams for the three types of lattice in most A level chemistry or physics books, or on the Internet. Structure at the micro level Importantly, though, it is the crystal size and arrangement that has the biggest effect on mechanical properties. Students might be familiar with how rate of cooling affects crystal size. If they have grown crystals such as alum they will know that, to obtain large crystals, solutions need to be evaporated slowly. Rapid evaporation gives small crystals. In metallurgy, these crystals are usually referred to as grains. The four pictures (a)-(d) represent the formation of small crystals from a molten metal. Each crystal has a regular arrangement of atoms. As they grow they eventually meet another crystal. A grain boundary if formed. In the picture (d) the representation of atoms in a crystal is removed and we are left with a diagram that represents crystal size and shape. Students should identify and label grains and grain boundaries in the photograph. Grains are of the order 0.1 - 1 cm across. Possible demonstrations: Seeing crystals If you look on the Institute of Physics’ Practical Physics website you will find a nice demonstration called the ‘lead tree’. Lead crystals are grown in an electrolytic cell. Lead compounds are generally toxic. Another simple demonstration is to place a copper wire in a solution of silver nitrate. Silver nitrate solid and solutions of concentration greater than 0.5 M are corrosive, greater than
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
0.2 M are an irritant and less than 0.2 M stain skin and clothing. A displacement reaction takes place and beautiful silver crystals form on the wire. Structure and mechanical properties
Bending a piece of metal backwards and forward (work hardening; cold working).
Answer might include: Cold working increases the number of dislocations. Initially these
can move through the metal structure and the shape of the material changes. However, as the number of dislocations increase their movement becomes more difficult. The metal becomes stronger and less ductile. We say that the metal has become work hardened. Annealing makes it more ductile again.
Heating a metal and letting it cool slowly (annealing).
Answer might include: Slow cooling gives larger grains and fewer grain boundaries. The metal, therefore, usually becomes softer and more ductile.
Heating a metal and cooling it very rapidly (quenching).
Answer might include: Rapid cooling gives small grains and, therefore, more grain
boundaries. The treatment has the opposite effect to annealing; the metal becomes stronger, harder and less ductile.
Re-heating a quenched metal and allowing it to cool slowly (tempering).
Answer might include: This is most common with alloys. It is difficult to generalise, but broadly its effect is to harden the metal and make it stronger. Careful control of temperature allows an alloy to be manipulated (the balance between strength and ductility) so that it has appropriate properties.
Treating metals The assessment requirement for the unit says that students must produce a:
report on tests to show the effect of your work-hardening, annealing and tempering treatments with the highest marks gained if they
produce a report on tests you have carried out on samples you have work-hardened, annealed and tempered and control samples, including an evaluation of whether the treatments have produced the expected result.
Successful completion of this practical sheet, together with Study Sheet: Metal structures, will enable students to gather sufficient information gain the highest marks for this part of the unit. Predictions By completing Study Sheet: Metal structures students make predictions about the expected results from treating metals in various ways. To gain the highest marks students need to include an evaluation of whether the treatments
have produced the expected result.
Equipment and materials
Several strips of metal (about 150 mm x 10-15 mm x 0.3-0.7 mm)
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advanced applied science: GCE A2 UNITS © The Nuffield Foundation 2008
Notes: Aluminium, copper, brass and steel were obtained as sheets from a local hardware shop and cut to size using a hacksaw. Others may be used. There are, of course, many types of steel and it would be interesting to compare some of these. Make sure a mixture of ‘pure’ metals and alloys are used. Alloys are affected more significantly by the heat treatment than the ‘pure’ metals due to their composition. The specification does not require that students know about alloys, but it would be appropriate to have a brief discussion about them in view of their huge importance.
Copper rod (about 200 mm x 5 mm) [other metal rods may be available]
2 x wooden blocks (about 100mm x 100 mm x 30 mm thick)
G clamp
Ruler
Hacksaw
Felt tip pen
Leather gloves or mole grips to hold metal strips
Bunsen burner
Heat-resistant mat or ceramic tile
It may be helpful if students could access a workshop as well as a laboratory for the practical work. Eye protection should be worn for the quenching activity.
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