1. Engineering Materials and Processes Lecture 8 – Alloys
Prescribed Text:
Ref 1: Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. ISBN:
Readings:
Callister: Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and Engineering: An Introduction, 8th Edition, Wiley, New York. ISBN
Ashby 1: Ashby, M. & Jones, D., 2011, Engineering Materials 1: An Introduction to Properties, Applications and Design, 4th edition, Butterworth-Heinemann, Oxford UK. IBSN:
Ashby 2: Ashby, M. & Jones, D., 2011, Engineering Materials 2: An Introduction to Microstructures and Processing, 4th edition, Butterworth-Heinemann, Oxford UK. IBSN:
Lecture (2 hrs):
Ref 1, Ch 1: Engineering materials;
Ref 1 Ch 2: Properties of materials.
Laboratory 1 (2 hrs): Hardness test
Callister: Ch 1, 2, 18-21
Ashby 1: Ch 1, 2
Ashby 2: Ch 1 1

2. Mechanical Deformation of Metals
Reference Text
Section
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann
Ch 8
Additional Readings
Section
Sheedy, P. A, Materials : Their properties, testing and selection
Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and Engineering: An Introduction, 8th Ed, Wiley, New York.
Engineering Materials and Processes

3. Alloys (Higgins 8.1)
Note: Text (Higgins) is followed very closely in this chapter.
An alloy is a mixture of two or more metals. The reason is usually to improve the properties of either metal. Often the alloy has properties not possessed by either of the metals in the pure state.
Alloys are usually stronger than the original metals. (better for engineering)
Alloys usually have a lower melting point than pure metals. (better for processing)
Pure metals are usually better at conducting electricity (and heat).
Another reason to alloy is to lower the cost – e.g. adding silver to gold.
Engineering Materials and Processes

4. Solutions (Higgins 8.1.1)
Solid solution is same idea as liquid solution – whether the 2 substances (or metals) will mix.
Water and alcohol mix in any ratio. (soluble)
Oil and water do not mix. (insoluble). Oil just floats on top.
Likewise for molten metals;
Molten lead and molten tin are soluble. (makes solder)
Molten lead and molten zinc insoluble. Zinc just floats on top.
Sn60Pb40 solder
Wikipedia
Engineering Materials and Processes

5. Eutectics
(Higgins 8.2)
If a molten alloy of this composition is allowed to cool, it will remain completely liquid until the temperature falls to 140°C, when it will solidify by forming alternating thin layers of pure cadmium and pure
bismuth (Figure 8.2) until solidification is complete.
Higgins Figure 8.1 The freezing-points (melting-points) of both bismuth and cadmium are LOWER when alloyed to the other. A minimum freezing-point - or 'eutectic point' - is produced.
Engineering Materials and Processes

6. Eutectics
(Higgins 8.2)
Lamination makes the structure strong (like plywood). If one metal is ductile and the other strong, the eutectic structure tends to get both strength and toughness. (Ideal in most engineering applications)
Higgins Figure 8.2 At a magnification of about ten million times (way too much for optical microscope), the arrangement of atoms of cadmium and bismuth would look something like that in the left-hand part of the diagram, except that the bands in the eutectic would each be many thousand atoms in width.
Engineering Materials and Processes

7. Eutectics A photo of a eutectic structure of steel known as pearlite.
The name pearlite comes from the way it reflects light (like a pearl), due to the very fine bands.
The white is iron, the black is iron-carbide (cementite), so not as simple as Cd-Bi eutectic.
Pearlite: Copyright unknown:
Engineering Materials and Processes

8. Solid Solutions (Higgins 8.3)
As alloy cools, higher Melting Point metal forms dendrites first.
Higgins Figure 8.3 The variations in composition in a cored solid solution. The coring can be dispersed by annealing.
Engineering Materials and Processes

9. Solid Solutions (Higgins 8.3) Dendritic structure
Higgins Figure 8.5 The dendritic structure brass. This is cast brass at a magnification of x39. The dendrites would not be visible were it not for the coring of the solid solution.
Engineering Materials and Processes

10. Substitutional solid solution
(Higgins 8.3.2)
A substitutional solid solution, is where atoms of one metal been substituted for atoms of the other.
This works best when the 2 atoms are nearly the same size:
Complete solubility by substitution:
E.g. copper/nickel, silver/gold, chromium/iron,
Most alloys have limited solubility by substitution:
E.g. copper/tin, copper/zinc, copper/aluminium, aluminium/magnesium.
Higgins Figure 8.6 (i) A substitutional solid
solution
Engineering Materials and Processes

11. Interstitial solid solution
(Higgins 8.3.3)
An interstitial solid solution, is where atoms of one metal squeeze between the atoms of the other.
This requires a big size difference in the atoms.
The most famous example is carbon (small atom) sqeezing into the FCC structure of hot iron – which is how we get heat treatable steel.
Heating the iron to promote diffusion of carbon into the FCC lattice is called carburising.
Higgins Figure 8.6 (ii) An interstitial solid solution
Engineering Materials and Processes

12. Diffusion The two main diffusion methods are:
Vacancy Diffusion: A new atom works it’s way into the metallic lattice by taking vacant positions.
Interstitial Diffusion: A new (small) atom migrates between atoms.
Higher temperatures increase the rate of diffusion.
Stress encourages diffusion by opening up more “gaps”.
Diffusion of new type of atom into a metallic lattice.
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13. Diffusion
(Higgins 8.3.5)
Solid solutions give the best strength, ductility and toughness, so most useful metallic alloys are basically solid solution in structure.
This is because distortions in the crystal structure hinder slip (increasing the yield strength).
Slip is still possible (ductility) but at a higher stress (now it is toughness)
Figure 8.8 Crystal lattice distortions caused by the presence of solute atoms:
(i) a large substitutional atom,
(ii) a small substitutional atom,
(iii) an interstitial atom.
In each case, the distortion produced will oppose the passage of a dislocation through the system.
Engineering Materials and Processes

14. Slip… (again) Higgins 6.2.1 Animation of slip by dislocation glide.
Dislocation glide allows plastic deformation to occur at a much lower stress than would be required to move a whole plane of atoms at once.
A perfect crystal (in theory) would be 1000 times stronger.
Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non-Commercial-Share Alike licence
You Tube
Offline (mp4)
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15. Diffusion: Carburising
Mild steel cannot be hardened unless there is carbon in the lattice.
Adding carbon to steel is called carburising.
There are several ways to do this, but the oldest and simplest is to heat the mild steel in the presence of carbon (charcoal) – for a long time at high temperature.
This allows carbon to diffuse into the surface for a mm or so.
Pack Carburising. A few minutes excerpt from BBC Video Heat Treatment:
Heat treatment [videorecording] / producer Brian Davies.
[B.B.C.], 1981.
Video: Discusses the use of heat which changes the properties of metals. Outlines different techiques including hardening, tempering, annealing, normalising as well as a non-heat process, coldworking.
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16. Diffusion to Dislocations Dislocation slip can be hindered:
Here, an interstitial atom migrates into the stress zone, hindering dislocations.
Otherwise the slip continues to the grain boundary, which distorts the grain, hence plastic deformation.
Dislocation and the effect of migration of interstitial atoms
You Tube
Offline (mp4)
Engineering Materials and Processes 16

17. Intermetallic Compounds
(Higgins 8.4)
Metallic oxides, sulphides and chlorides are ionic compounds formed by the attraction between positive and negative ions.
Sometimes two metals when melted together will combine to form a chemical compound called an intermetallic compound, where one of the two metals has strongly positive ions and the other weakly positive ions.
A normal solid solution acts similar to the parent metals, but intermetallic compounds are dramatically different (usually brittle) and have a fixed chemical formula. Tends to act like little bits of ceramic mixed into the metal.
Cementite is an intermetallic compound in steel alloys with the chemical formula Fe3C. This phase has a specific chemical formula, unlike most phases which have ranges of chemical composition. Cementite is hard and brittle.
IMAGE: Journal of Molecular Catalysis A: Chemical Volume 269, Issues 1–2, 18 May 2007, Pages 169–178
Engineering Materials and Processes

18. Alloys: Summary (Higgins 8.5)
A phase is a chemically stable single homogeneous constituent in an alloy. A phase may be a solid solution, an intermetallic compound or a pure metal. The three main types of solid phases are;
Solid solutions are subsitutional (similar atom size) or interstitial (dissimilar size atoms) larger atoms of the other metal. Solid solutions are more useful in engineering because slip is hindered, but without eliminating ductility.
Intermetallic compounds are formed by chemical combinations, and like ceramic, tend to be hard and brittle.
Eutectics are formed when two metals, soluble when molten become insoluble when in solid. They form alternate layers or bands of each metal.
This occurs at a fixed temperature, lower than the melting-point of either of the two pure metals. Eutectics can also form with layers being solid solutions, or even an intermetallic compound.
Engineering Materials and Processes

19. Alloys: Summary 2 (Higgins 8.5)
Engineering alloys can combine six or even more metals, although there is usually a dominant metal (solvent) into which the lesser metals (solutes) dissolve.
E.g. Thus, the stainless steel 347S17 is composed almost entirely of a solid solution in which iron has dissolved 18 per cent chromium, 10 per cent nickel, 1 per cent niobium and 0.8 per cent manganese - a residual 0.04 per cent carbon existing as a few scattered undissolved carbide particles.
347S17: Austenitic chromium-nickel stainless steel with moderate strength and niobium stabilised (347 type) with moderate corrosion resistance. For aerospace and defence components including weldments.
Engineering Materials and Processes

20. Online Properties Resources.
Graphical comparison of materials properties.
DoITPoMS: Dissemination of IT for the Promotion of Materials Science
Wikipedia: Materials properties
Forming: Forging, Rolling, Extrusion, Machining
Engineering Materials and Processes 20

21. Laminated grain structure Dentritic structure Cementite Pearlite
GLOSSARY
Alloy
Binary Alloy
Soluble
Solvent
Solute
Substitutional
Interstitial
Carburising
Phase
Intermetallic
Crystal
Grain
Eutectic
Laminated grain structure
Dentritic structure
Cementite
Pearlite
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22. Define all the glossary terms.
QUESTIONS
Callister: NA
Moodle XML: Processing
Define all the glossary terms.
Explain why alloys are usually more useful in engineering than pure metals.
Describe two ways that lattice distortion can occur with a binary alloy.
How does lattice distortion increase strength?
The intermetallic compound Cementite is deliberately employed in steel. Why does this hard and brittle material (which is really a ceramic) not destroy the properties of the steel?
In carburising, what kind of diffusion is taking place?
(Research) High speed steel AS 1239 grade M2 contains 0.85% carbon, 4.0% chromium, 5.0% molybdenum, 6.0% tungsten and 2.0% vanadium. What is the solvent? Which are the solutes? Research high speed steel and list the main reasons each of these solute metals is added.
Engineering Materials and Processes 22