enmat101a engineering materials and processes€¦ · substitutional solid solution (higgins 8.3.2)...

TAFE NSW -Technical and Further Education Commission

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ENMAT101A Engineering Materials and Processes Associate Degree of Applied Engineering (Renewable Energy Technologies)

Lecture 8 – Alloys


Mechanical Deformation of Metals

EMMAT101A Engineering Materials and Processes

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.


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.


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


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.


Eutectics (Higgins 8.2)


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.

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)


Eutectics


Pearlite: Copyright unknown: http://threeplanes.net/cementite.html

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.

http://threeplanes.net/cementite.html

http://threeplanes.net/cementite.html


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.


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.


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


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


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.



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.


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.

http://www.msm.cam.ac.uk/doitpoms/tlplib/dislocations/

dislocation_glide.php

Courtesy of DoITPoMS, The University of Cambridge.

Released under Creative Commons Attribution-Non-

Commercial-Share Alike licence

http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

You Tube

Offline (mp4)


http://www.msm.cam.ac.uk/doitpoms/tlplib/dislocations/dislocation_glide.php

http://www.msm.cam.ac.uk/doitpoms/tlplib/dislocations/dislocation_glide.php







http://www.youtube.com/watch?v=kk2oOxSDQ7U

videos/Slip by dislocation glide.mp4

http://www.youtube.com/watch?v=kk2oOxSDQ7U


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.


http://tafecat.tafensw.edu.au/uhtbin/cgisirsi/?ps=mvCVPvdvnM/MOU/3510123/18/XBATH4/XTITLE/Heat+treatment


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)


http://www.youtube.com/watch?v=RUuLusenhfA&feature=related

videos/Dislocation motion and migration of interstitial atom.mp4

http://www.youtube.com/watch?v=RUuLusenhfA&feature=related


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

http://www.sciencedirect.com/science/journal/13811169

http://www.sciencedirect.com/science/journal/13811169


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.


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.



Graphical comparison of materials properties.

Wikipedia: Materials properties

Online Properties Resources.

Forming: Forging, Rolling, Extrusion, Machining

DoITPoMS: Dissemination of IT for the Promotion of Materials Science

http://openlearn.open.ac.uk/mod/oucontent/view.php?id=399740&section=3.1


GLOSSARY

Alloy

Binary Alloy

Soluble

Solvent

Solute

Substitutional

Interstitial

Carburising

Phase

Intermetallic

Crystal

Grain

Eutectic

Laminated grain structure

Phase

Dentritic structure


Cementite

Pearlite


QUESTIONS Callister: NA

Moodle XML: 10107 Processing

1. Define all the glossary terms.

2. Explain why alloys are usually more useful in engineering than pure metals.

3. Describe two ways that lattice distortion can occur with a binary alloy.

4. How does lattice distortion increase strength?

5. 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.


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