Transcript Slide 1

ENMAT101A Engineering Materials and Processes
Associate Degree of Applied Engineering
(Renewable Energy Technologies)
Lecture 8 – Alloys
www.highered.tafensw.edu.au
TAFE NSW -Technical and Further Education Commission
Mechanical Deformation of Metals
Reference Text
Section
Higgins RA & Bolton, 2010. Materials for Engineers and Technicians,
5th ed, Butterworth Heinemann
Ch 8
Additional Readings
Section
Sheedy, P. A, 1994. Materials : Their properties, testing and selection
Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and
Engineering: An Introduction, 8th Ed, Wiley, New York.
EMMAT101A Engineering Materials and Processes
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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.
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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
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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 (meltingpoints) of both bismuth and cadmium are
LOWER when alloyed to the other. A minimum
freezing-point - or 'eutectic point' - is produced.
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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.
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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: http://threeplanes.net/cementite.html
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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.
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Solid Solutions
(Higgins 8.3)
Dendritic structure
Higgins Figure 8.5 The dendritic
structure brass. This is cast 70-30
brass at a magnification of x39. The
dendrites would not be visible were it
not for the coring of the solid solution.
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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
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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
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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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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.
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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.
You Tube
Offline (mp4)
http://www.msm.cam.ac.uk/doitpoms/tlplib/dislocations/
dislocation_glide.php
Courtesy of DoITPoMS, The University of Cambridge.
Released under Creative Commons Attribution-NonCommercial-Share Alike licence
http://creativecommons.org/licenses/by-nc-sa/2.0/uk/
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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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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.
You Tube
Dislocation and the effect of
migration of interstitial atoms
Offline (mp4)
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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
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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.
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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.
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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
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GLOSSARY
Alloy
Binary Alloy
Soluble
Solvent
Solute
Substitutional
Interstitial
Carburising
Phase
Intermetallic
Crystal
Grain
Eutectic
Laminated grain structure
Phase
Dentritic structure
Cementite
Pearlite
EMMAT101A Engineering Materials and Processes
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QUESTIONS
Callister: NA
Moodle XML: 10107 Processing
1.
2.
3.
4.
5.
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?
6. In carburising, what kind of diffusion is taking place?
7. (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.
EMMAT101A Engineering Materials and Processes
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