Mech 473 Lectures
Professor Rodney Herring
Copper and its Alloys
Why Copper?
• Copper was the first metal made by man
• Copper alloys, bronze and brass, were the first alloys
made by man
• Copper alloys enabled currency for trade because of its
corrosion resistance and it is still used in this manner
although diminishing because it is too expensive.
• Copper is used for conducting electricity and thus for
electronic interconnects
• Copper is still widely used for marine applications
because of its corrosion resistance
• A knowledge of the properties of copper alloys will be
useful for Victoria’s ship building industry.
Copper Alloys
Cupronickel:
Brass:
Cu + 0-100%Ni
(the Ni-based alloys are Monels)
Cu + 0-40%Zn (a and a+b)
Tin Bronze:
Cu + 0-10%Sn (a and a+b)
Aluminum Bronze: Cu + 0-11%Al (a and g2)
Silicon Bronze:
Cu + 3%Si (a)
Beryllium Copper Cu + 0-2%Be (a+g)
a is fcc; b is bcc; g is bct
Note: Strengthening is by solid solution strengthening, grain size reduction,
strain hardening and age hardening.
Shape Memory Alloys
Cu-based Alloys
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Cu-Al-Ni with 14/14.5 wt.% Al and 3/4.5 wt.% Ni
Cu-Sn approx. 15 at.% Sn
Cu-Zn 38.5/41.5 wt.% Zn
Cu-Zn-X (X = Si, Al, Sn)
Other shape memory alloys include:
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Ag-Cd 44/49 at.% Cd
Au-Cd 46.5/50 at.% Cd
Fe-Pt approx. 25 at.% Pt
Mn-Cu 5/35 at.% Cu
Fe-Mn-Si
“Red” denotes major shape memory alloys
Pt alloys
Co-Ni-Al
Co-Ni-Ga
Ni-Fe-Ga
Ti-Pd in various concentrations
Ni-Ti (~55% Ni)
Ni-Mn-Ga
Shape Memory Alloys
What are Shape Memory Alloys?
• Shape memory alloys (SMA's) are metals, which exhibit
pseudo-elasticity and the shape memory effect.
• The most effective and widely used alloys include CuAlNi,
CuZnAl, and NiTi
• The shape change involves a solid state phase change
involving a molecular rearrangement between Martensite
and Austenite.
• A temperature change of only about 10 oC is necessary to
initiate this phase change
Shape Memory Alloys
Athermal reaction with no diffusion.
Shape Memory Alloys
The Shape alloys are currently being used in:
• Coffee pots
• The space shuttle
• Thermostats
• Vascular Stents
• Hydraulic Fittings (for Airplanes)
Cupronickel Alloys
Cu and Ni form a continuous series of solid solutions – as
they are both FCC and have similar electronic structures
and atomic sizes: Cu = 0.2556 nm; Ni = 0.2492 nm
(These fit the Hume-Rothery rules of mixing of two
elements for the formation of a single phase)
Hume – Rothery Rules
The Hume-Rothery rules give the tendency of two substances
to form a substitutional solid solution.
The formation of substitutional solid solutions is encouraged
by the following characteristics of materials:
• 1) the atomic diameters of the elements must not differ by
more than about 15%
• 2) the crystal structure of the atoms must be the same
• 3) there should be no appreciable difference in the
electronegativities of the elements so that compounds will
not form
• 4) the two elements should have the same valence.
Cupronickel
When Cu-Ni alloy of composition “a” is cooled to the liquidus
temperature – solid of composition “b” is formed.
On further cooling to a’– as solid phase is precipitated- the
composition of the solid should shift from “b” to “d” –
while the liquid composition shifts from “a” to “c”.
Cupronickel
However – as diffusion is very slow in this alloy system –
even at high temperatures – the solid composition only
changes to the point of d’.
On cooling to the solidus temperature – the solid should be
at composition a” – but it only moves to f.
The solid thus forms in successive layers of composition b,
d’ and f – around the initial particles – or dendrites – and
this is called “coring”.
Micrographs of Cu-Ni Alloys
The cupronickels are single phase alloys – but coring causes
their as-cast microstructures to show gradients of
composition across the dendrites.
Cu-15% Ni: As cast showing coring
Cu-15% Ni: annealed for 9 h at 950 oC –
showing homogeneous a-phase.
Properties of Cu-Ni Alloys
Note – maximum electrical resistance, re, is ~50 Ni.
Properties of Cu-Ni Alloys
Three Cu-Ni alloys are of industrial importance
Cu - 66.7%Ni – known as Monel metal – (really a Ni-based
alloy) – has the highest strength in the series – which can
be further increased to 613 MPa by precipitation
hardening with Ni3Al.
2) Cu - 30%Ni – has slightly lower strength than Monel – but
it is preferred for marine applications – due to its high
resistance to sea water corrosion.
3) Cu – 45%Ni – known as constantan – has the highest
electrical resistivity – combined with a very low
temperature dependence of resistivity – and is used for
precision standard resistors* and rheostats.
* - for heating elements it is coated with SiO2 to reduce
oxidation.
1)
Mechanical Properties of Cu-Ni Alloys
Note: The peaks in strength and hardness occur on the Ni
side of 50% - because the smaller sized Ni atoms in a
copper matrix cause more elastic misfit strain – than the
larger Cu atom in a nickel matrix.
Both strength
and ductility can
be improved with
more Ni.
Brasses
The FCC a-solid solution extends up to 40% Zn – at 400oC but
– due to low diffusion – very little of the b-phase
precipitates on cooling to RT.
Brasses
The BCC b-phase forms by a peritectic reaction at 905oC.
In this reaction – previously formed crystallites of a-solid
extract additional Zn from the liquid to form a
surrounding layer of b – i.e., a + liquid  b (see arrow)
Brasses
The b-phase is a random solid solution – but at temperatures
below 460oC it becomes ordered, b’ – that is, Cu atoms
and Zn atoms occupy designated sites in the crystal.
Brasses
Because of the slope of the a/b phase boundary between
500-900oC – Cu 40%Zn, a+b brasses are composed of
a-plates within a b-phase matrix.
Brasses
The complex structures of g and d cause these to be brittle –
so the practical limit of Zn composition is 50%.
Micrographs of Cu-Zn Alloys
Cu-40%Zn: Extruded and air cooled – showing Widmanstatten plates of aphase precipitated within b grains and the b grain boundaries.
Properties of Wrought a-phase Cu-Zn Alloys
The colour of the a-brasses depends on the Zn content:
Guilding metal: 5%Zn – has a golden colour – used for cheap jewelry
Commercial bronze: 10%Zn – has a bronze colour – similar to Sn
bronze.
Red Brass: 15%Zn – has a bright red tint like copper
Cartridge brass: 30%Zn – has a bright yellow colour Used for what?
The UTS increases only gradually across the a-phase – because of the
similarity in the atomic sizes of Cu and Zn.
In contrast to most other alloy solid solutions – the ductility of the aphase increases with Zn concentration – so that 30%Zn – the strongest
alloy – is also the most ductile!
Properties of Wrought a-phase Cu-Zn Alloys
The a-phase has excellent deep drawing properties – and is used for
making gun cartridges from flat rolled strip – but it is also very
susceptible to stress corrosion cracking in alkaline environments.
The (a+b) 40%Zn alloy is stronger than the a-phase alloys – but has
reduced ductility equivalent to the 10%Zn a-brass.
It is commonly alloyed with small amounts of Sn, Pb and Mn to increase
its corrosion resistance in saline solutions.
Work Harrdening of Cu-Zn Brasses
Note: The 30% Zn alloy has a greater strength and elongation
compared to the 15% Zn alloy.
It’s rare to increase strength and ductility at the same time.
Micrographs of Cu-Zn Alloys
Twin boundary
Cu-30%Zn: Showing a-phase twins formed during annealing – and stress
corrosion cracking (arrow) – by exposure to NH3 vapor under tensile
load.
Note: no b-phase and the cracking along the grain boundary indicated by arrow.
Properties of Wrought (a+b)-phase Cu-Zn
Alloys
Muntz Metal – the pure 40% Zn alloy – has greater strength – but lower
ductility – than the 30% Zn a-phase alloy
The addition of 2% Pb to Muntz metal does not affect the annealed
strength – but increases its machinability – this alloy is used for
manufacturing brass screws and fittings in high speed automatic
machines.
Naval brass – with 1% Sn – has increased strength compared to Muntz
metal in both the annealed and cold worked condition – and improved
corrosion resistance – it is used for forgings and marine machinery.
Properties of Wrought (a+b)-phase Cu-Zn
Alloys
Mn bronze (is misnamed as it’s really a brass) – with 1%Fe + 0.03% Mn –
has even greater strength in both the annealed and cold worked
condition.
Brasses for casting are (a+b) Muntz metal alloys with additional alloying
elements – to control strength, shrinkage, castability and machinability
For comparison – their properties are listed with the cast bronzes – which
are also two-phase Cu-based alloys.
Tin Bronzes (Cu-Sn alloys)
The FCC a-solid solution extends up to 15% Sn at 500 oC –
but at lower temperatures the solubility of Sn drops to
almost zero.
Tin Bronzes
However - at normal cooling rates Cu3Sn (e-phase – circled
on phase diagram) does not precipitate from a-phase
alloys – due to the very low diffusion rate of Sn in Cu.
Tin Bronzes
A succession of eutectoid
reactions occur in the
Cu-Sn system.
At 586 oC the bphase decomposes to
form (a+g)
At 520 oC the gphase decomposes to
form (a+d)
At 350 oC the dphase decomposes to
form (a+e)
Tin Bronzes
The d-phase is both
hard and brittle –
but the later is
offset to some
extent – because in
the two phase
alloys this phase
forms as relatively
fine dispersions –
located in the
spaces between
the original aphase dendrites.
Aluminum Bronzes
Alloys of Cu-Al and Cu-Si are also referred to as
bronzes – because they have similar colouration to
the tin bronzes.
Single phase wrought alloys contain 5.0% Al – while
heat treatable cast alloys contain 11% Al.
Aluminum Bronzes
In the Cu-Al system,
the a-solid solution
extends to 9.4%Al at
550 oC– but in
contrast to the CuSn system –
multiple eutectoid
decompositions do
not occur – because
the g2 phase –
formed by the
decomposition of b
– is stable on
cooling down to
room temperature.
a
Cu + 20%Al
a
g2
a + g2 phases
Aluminum Bronzes
• Aluminium bronzes are most valued for their higher
strength and corrosion resistance as compared to other
bronze alloys.
• These alloys are tarnish-resistant and show low rates of
corrosion in atmospheric conditions, low oxidation
rates at high temperatures, and low reactivity with
sulfurous compounds and other exhaust products of
combustion.
• They are also resistant to corrosion in sea water.
Aluminium bronzes' resistance to corrosion rests in the
aluminium component of the alloys, which reacts with
atmospheric oxygen to form a thin, tough surface layer
of alumina, which acts as a barrier to corrosion of the
copper-rich alloy.
• The addition of tin can improve corrosion resistance
Aluminum Bronzes
• Another notable property of aluminium bronzes
are their biostatic effects.
• The copper component of the alloy prevents
colonization by marine organisms including
algae, lichens, barnacles, and mussels, and
therefore can be preferable to stainless steel or
other non-cupric alloys in applications where
such colonization would be unwanted.
• Aluminium bronzes tend to have a golden color
Aluminum Bronzes
• Aluminium bronzes are most commonly used in
applications where their resistance to corrosion makes
them preferable to other engineering materials.
• These applications include plane bearings and landing
gear components on aircraft, engine components
(especially for seagoing ships), underwater fastenings
in naval architecture, and ship propellers.
• The attractive gold-toned coloration of aluminium
bronzes has also led to their use in jewelry
Silicon Bronzes
The phase diagram for
Cu-Si is similar in
form to the Cu-Al
system – but the asolid only extends up
to 4.65 wt% Si (9.95
at% Si).
Single phase wrought
alloys in this system
contain 1.5% Si and
3.0 % Si.
Beryllium Copper
This alloy is another
misnomer – it is properly
called beryllium bronze.
Only 2.2% Be dissolves in
Cu at 864 oC – due to the
large difference in atomic
sizes. Cu = 2.556 A and
Be = 2.225 A
Beryllium Copper
An alloy containing 1.8 % Be
(and 0.25% Co) can be
solution treated at 800 oC quenched to retain the Be
in solution – and tempered
at 400 oC to harden by
precipitation (aging) of the
g phase.
This alloy has high hardness
and excellent stiffness – so
it is used for miniature
springs – e.g., for current
and future MEMS
applications.
Copper Beryllium Alloys
Cu – Be alloys typically contain 0.6-2% Be and 0.2-2.5%
Co.
• By precipitation hardening and cold working, extremely
high strength alloys are possible (~215,000 psi), which
is the highest of any commercial copper alloy
• Non-sparking, excellent fatigue resistant
• Used for springs, gears, diaphrams, valves, welding
nozzles – relatively inexpensive.
Properties of Wrought Bronzes
Wrought bronzes are a-phase alloys – prepared as rolled
sheet and bar as they have good fabricability.
After shaping by hot rolling – the sheets are tempered cold
rolled to improve strength and stiffness – with the degree
of temper depending on the intended application.
Properties of Wrought Bronzes
The strength of the Sn bronzes increases more rapidly per
unit of solute content – compared to the Zn brasses – and
once again the increase in strength is accompanied by an
unusual increase in ductility.
The strength of the Al and Si bronzes is also increased with
solute content – but at the expense of ductility
The alloys containing 5%Al, 3%Si and 10%Sn all have much
the same strength – but the Sn bronze has significantly
greater ductility.
Properties of Cast Copper-Based Alloys
In contrast to wrought alloys – Cu-based casting alloys are
mainly two-phased and contain additional alloy elements.
“Three-Fives” – 5%Sn 5%Zn and 5%Pb – is the most widely
used casting alloy – it is a combined Zn brass and Sn
bronze – with good mechanical casting and corrosion
properties and excellent machinability (due to Pb).
Properties of Cast Copper-Based Alloys
The Mn bronzes – actually (a+b) brasses because of their
high Zn contain Mn Fe and Al. These are both strong and
ductile – with exceptional resistance to corrosion – and are
used for ships propellers and hydraulic machinery
Properties of Cast Copper-Based Alloys
Bearing bronzes – 10Sn-10Pb – is used for heavy bearings –
the Sn bronze gives a strong tough matrix – while the dphase provides hardness and wear resistance – and the Pb
gives antifriction properties.
Properties of Cast Copper-Based Alloys
Ni-Sn bronze is a 5% Sn bronze with the addition of 2% Zn
and 5% Ni. In the solid solution treated condition – the
strength is the same as gunmetal (i.e., 30% Zn) – but after
quenching and tempering (635 C) to precipitate a Ni-Sn ephase – the strength is increased by almost 50%.
Properties of Cast Copper-Based Alloys
The heat treatment of 11% Al bronze is equivalent to the
quench & temper in steels. Above 565 oC – this alloy is
100% b-phase – on quenching the (a+d2) eutectoid
transformation is suppressed and a distorted martensitic b’
structure is formed – on tempering the b’ breaks down to
fine grained a + d2 – causing an increase in strength but
with some loss of ductility.
Machinable Copper Alloys
As seen, the machinability of copper alloys can be improved
by the addition of lead.
Pb dissolves in liquid copper – but is insoluble in solid
copper – so it remains as liquid globules at temperatures
above its melting point – and when the alloy is cooled
below 326 oC – the globules solidify.
The addition of Pb has very little effect on mechanical
properties.
Micrograph showing the
form of Pb globules
located at grain
boundaries in Cu-based
alloys.
The End
Any questions or comments?