Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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further structural change to take place at 385°C with the formation of

another intermediate phase (x) but as mentioned earlier that need not

concern us here.

16.52.

In spite of these heat-treatment phenomena, the main industrial

uses of aluminium bronzes depend upon other features such as:

(a) the ability to retain strength at elevated temperatures, particularly

when certain other elements are present;

(b) the high resistance to oxidation at elevated temperatures;

(d) good wearing properties;

(e) the pleasing colour which makes some of these alloys useful for

decorative purposes, particularly as a substitute for gold in imitation

jewellery.

A serious drawback to the wider use of aluminium bronze has always

been present in the difficulties arising during casting. Since aluminium

oxidises so readily at the high casting temperature necessary (above

1100

C), oxide skin and dross is persistently formed and will be drawn into

the mould during casting unless special non-turbulent methods of casting

are employed (2.33-Pt2). Such skilled casting techniques inevitably cost

more money.

16.53 The ('-phase Cold-working Alloys contain between 4.0 and

7.0% aluminium and occasionally up to 4.0% nickel, which improves cor-

rosion-resistance still further. This latter type of alloy is particularly useful

in the manufacture of condenser tubes and heat-exchangers, where high

strength and corrosion resistance are required up to temperatures of about

300

Since the composition of these a-phase alloys can be adjusted to

give a colour similar to that of 18-carat gold, rolled sheet is used in the

manufacture of imitation-gold cigarette-cases and for other decorative

articles of this type. In less permissive times large quantities of the alloy were

used for cheap 'wedding rings'. Many products originally made from alu-

minium bronze are now produced in anodised aluminium the surface of

which is further treated to give a gold-like finish at lower cost.

16.54 The a + y

Hot-working and Casting Alloys with between

7.0 and 12.0% aluminium, may also contain other elements, such as iron,

nickel or manganese. The hot-working alloys contain from 7.0 to 10.0%

aluminium and are shaped by forging or by hot-rolling according to their

subsequent application. These alloys may also contain up to 5.0% each of

iron and nickel, the iron acting as a grain-refiner. Such alloys are used for

chemical-engineering purposes (especially for components exposed to high

temperatures); also for a variety of purposes where a corrosion-resistant

forging is required.

The alloys used for sand- and die-casting contain between 9.5 and 12.0%

aluminium with varying amounts of iron and nickel up to 5.0% each, and

manganese up to 1.5%. These alloys are used widely in marine engineer-

ing, eg for pump rods, valve fittings, propellers, propeller shafts and bolts.

They are also used for valve seats and sparking-plug bodies in internal-

combustion engines and for brush-holders in generators; for bearings to

work at heavy duty; for gear wheels; for pinions and worm wheels; and in

the manufacture of non-sparking tools such as spanners, wrenches, shovels

and hammers in the potentially 'dangerous' gas, paint, oil and explosives

industries, though they are softer and hence inferior to beryllium bronze

in this direction.

Copper-Nickel Alloys

16.60 As the thermal-equilibrium diagram (Fig. 9.9) shows, the metals

copper and nickel are soluble in each other in all proportions, forming a

continuous series of solid solutions. In the cast condition cored crystals will

be present, but, though coring may be extensive, it can never lead to the

precipitation of a brittle secondary phase, as may happen with the other

copper alloys dealt with. On annealing, all copper-nickel alloys consist of

uniform solid solutions.

16.61 Being completely solid solution in structure, the copper-nickel

alloys are all ductile. They are also very accommodating with regard to the

methods of manufacture, and can be hot or cold-worked by rolling, forging,

stamping, pressing, drawing and spinning. The absence of any second

phase in the microstructure eliminates the possibility of electrolytic cor-

rosion (21.70) in the presence of an electrolyte, and this contributes to the

high corrosion-resistance of copper-nickel alloys. This corrosion-resistance

reaches a maximum with Monel Metal. Cladding of ships' hulls and parts

in off-shore structures with 90-10 cupro-nickel is now widely used as a

protection against marine corrosion. Details of the more important cop-

per-nickel alloys are included in Table 16.4.

16.62 Nickel Silvers are alloys containing from 10 to 30% nickel, 55

to 63% copper and the balance zinc. They are all completely solid solution

in structure, and comparable with similar brasses as far as mechanical

properties are concerned. They are, however, white in colour, which makes

them admirably suitable for the manufacture of spoons, forks and other

table-ware. Being ductile, the alloys can be cold-formed for these applica-

tions and are usually silver-plated (the stamp EPNS means 'electroplated

nickel silver'). The current fashion for stainless steel table-ware has meant

a decline in the popularity of the more expensive EPNS. A lot of brassware

carrying a very thin 'flashing' of silver also now masquerades as EPNS.

The addition of 2.0% lead makes nickel silvers easy to engrave and such

alloys are used in the manufacture of Yale-type keys where the presence

of lead increases the ease with which the blank can be cut.

Plate 16.5 16.5A 90/10 Aluminium bronze, cast and annealed.

Crystals of primary a (light) in a matrix of a + y

eutectoid. x 1100. Etched in acid iron(lll)

chloride.

16.5B 90/10 Aluminium bronze, water quenched from 900

Martensitic type of structure (p'). x 100. Etched in acid iron(lll) chloride.

16.5C 90/10 Aluminium bronze, water quenched from 900

C and tempered at 500

Martensitic |3' transforming to a fine a + y

structure, x 100. Etched in acid iron(lll) chloride.

16.5c.

16.5B.

6.5A.

Copper Alloys which

can be

Precipitation

Hardened

16.70 It is often assumed that 'age hardening' or, more properly, 'precipi-

tation hardening' (9.92), is a phenomenon confined to the light alloys.

Some of the copper-base alloys can nevertheless be so treated, though

these alloys have on the whole rather specialised uses.

16.71 Beryllium Bronze is one of the most outstanding of these alloys.

It contains 1.75-2.5% beryllium and up to 0.5% cobalt, the balance being

copper. It is solution-treated at 800

C (Fig. 16.6(i)), quenched, cold-

worked (if necessary) and then precipitation-hardened at 275°C for one

hour. After such treatment the alloy attains the remarkable tensile strength

of 1300-1400 N/mm

due to the formation of the coherent precipitate |3'.

It is particularly suitable for the manufacture of tools where non-sparking

properties are required (hammers, chisels and hacksaw blades in gas, oil,

paint and explosives industries and in 'dangerous' mines), since it is both

hard and tough. It also has a high fatigue strength and is therefore used

for the manufacture of springs and for diaphragms and tubes of pressure-

recording instruments.

16.72 Chromium Copper containing 0.4-0.5% chromium (Fig.

16.6(ii) is another heat-treatable alloy which finds use in electrical switch-

gear and as pressure (spot and seam) welding electrodes (20.61), since it

combines a reasonable strength of 540 N/mm

with an electrical conduc-

tivity of 80% in the hardened state. It is quenched from 1020

C (P in Fig.

16.6(ii)) and precipitation hardened at 450

16.73 Zirconium Copper The solid solubility of zirconium in copper

increases from about 0.01% at 0

C to 0.15% at the eutectic temperature

of 965°C. Consequently copper-base alloys containing approximately

liquid

Fig.

16.6

Equilibrium diagrams

for

some copper-base alloys which

can be

precipitation

hardened.

each case

the

commercial alloy

is of

approximate composition

'X' and is

solution

treated

at 'P' at

which temperature

the

microstructure

completely

unsaturated solid

solution

oc.

BeFYo)

Cr(°/o)

TiW

Table 16.4 Copper-Nickel

Alloys

Characteristics

and

uses

Has slightly better

mechanical

properties

and

corrosion-resistance

than pure copper.

Can be

regarded

toughened copper.

The alloy

lowest nickel content which

has a

less white colour

Used

for

manufacture

bullet envelopes

because

of its

high ductility

and

corrosion-resistance. Will withstand

severe

cold-working.

Used

mainly

for

coinage,

eg the

present

British

silver

coinage.

Used

for

condenser

and

cooler tubes where

good resistance

corrosion

required.

Typical

mechanical properties

Hardness

(VPN)

130

145

165

170

175

Elonga-

tion

(%)

Tensile

strength

N/mm

263

463

324

494

340

541

355

602

355

649

0.1% Proof

stress

N/mm

108

463

108

541

Condition

Annealed

Hard

Annealed

Hard

Annealed

Hard

Annealed

Hard

Annealed

Hard

Composition

(%)

Other

elements

1.2

0.5

0.25

0.4

Relevant

specifications

2870/5:

CN101

2870/5:

CN103

2870/5:

CN104

2870/5:

CN105

2870/5:

CN106

Constantan,

Largely used as a resistance

material and for thermocouples. Has a high

specific

resistance and a

very

low temperature

coefficient.

Eureka is a similar alloy.

Monel

Metal. Combines good mechanical

properties

with

excellent

corrosion-

resistance. Used in

chemical-engineering

plant, etc.

Monel. A heat-treatable modification of

monel metal. Used for motor-boat propeller

shafts.

Submarine sea-water systems—valves,

pump

casings, pipe

fittings.

190

120

220

175

225

310

190

386

649

541

726

680

757

1070

510

216

571

340

571

788

Annealed

Hard

Annealed

Hard

Annealed

Hard

Heat-treated

Fe 1.25

Mn 1.25

2.75

Fe 0.9

Mn 0.4

Ti 0.6

0.2%

Proof

stress

310

Sand cast

Cr-1.8

Fe-0.75

Mn-0.75

Si-0.3

Zr-0.1

Ti-0.15

Rem

3072/6:

NA13

3072/6:

NA18

NES*

824

Naval

Engineering Standard

0.12% Zr can be solution treated at 950

C followed by quenching to give

a super-saturated solid solution which is then precipitation treated at 450

to form a coherent precipitate (probably Cu

Zr). Mechanical properties

are similar to those of chromium copper.

Copper containing small amounts of both zirconium and chromium (0.7

Cr; 0.1 Zr) have slightly better strength after heat-treatment. These

materials are used in commutators because of their good thermal and

electrical conductivities; rocket nozzles and resistance-welding electrodes.

16.74 Titanium Copper alloys containing 2.5 and 4.0% titanium are

in production. The 4% alloy is solution treated at 850-900

C (Fig. 16.6(iii))

for one hour, quenched and then cold-worked up to 40% reduction. It is

then precipitation hardened in the range 400-525

C to give a tensile

strength of 1076 N/mm

. It has similar applications to beryllium bronze,

ie bellows and diaphragms; springs; electrical contacts and non-sparking

tools.

16.75 K Monel The mechanical properties of ordinary monel metal

(16.61) can be further improved by the addition of 2.0-4.0% aluminium

to give an alloy designated K Monel. This alloy can be solution treated,

cold-worked and then precipitation-hardened to give a tensile strength as

high as 1160 N/mm

Some brasses and bronzes which contain small quantities of nickel and

aluminium can be precipitation hardened. Thus an alloy containing 20 Zn;

6 Ni; 1.5 Al; bal.Cu is quenched from 850

C and then precipitation hard-

ened at 500

C. These alloys are used for machine parts such as gear wheels;

instrument pinions and pressings where strength, hardness and corrosion

resistance are important.

16.76 'Narcoloy' an aluminium bronze, contains 7 Al; 0.5 Co and

0.25 Sn. It is an a-type alloy and its heat-treatment does not depend upon

the formation of martensite as with the a + y

aluminium bronzes (16.50),

but upon precipitation hardening. It is solution treated at 800

C, cold-

worked up to 25% and then precipitation hardened at 450

C to give a

tensile strength of 580 N/mm

and a hardness of 250 H

. Its high impact

value coupled with excellent resistance to stress corrosion make it particu-

larly useful in marine atmospheres for cold-forged bolts and thread-rolled

studding.

Since it has an attractive gold-like appearance, takes a high polish and

resists corrosion by all household detergents and vinegar, it is suggested

that it could be used for tableware such as forks and spoons.

'Shape Memory' Alloys

16.80 'Shape memory' is a phenomenon associated with a limited number

of alloy systems and was first investigated in 1951 in some gold-cadmium

alloys.

The fundamental characteristics of these alloys is the ability to

exist in two distinct shapes or crystal structures above and below a critical

transformation temperature. Below the critical temperature a martensitic

type of structure forms and develops as the temperature falls further. This

martensitic change is unlike that observed in steels in that it is reversible,

and as the temperature is raised again the martensite is absorbed and

ultimately disappears. This reversible change in structure is linked to a

change in dimensions and the alloy exhibits a 'memory' of the high and

low temperature shapes.

16.81 The best know commercial 'memory metals' are in the copper-

zinc-aluminium system—the SME ('shape-memory-effect') brasses—the

compositions of which vary in the range 55-80% copper with 2-8% alu-

minium, the balance being zinc. By choosing a suitable composition, tran-

sition temperatures between —70 and +130

C can be achieved. The usable

force associated with the change in shape can be employed to operate a

range of temperature-sensitive devices; snap on/off positions often being

obtained by the use of a compensating bias spring. Following progress with

the SME brasses other alloys are being developed in this field, eg a Cu-

Al-Ni alloy which operates in the region of 250

C. SM alloys also exhibit

other interesting characteristics, including superelasticity where large

recoverable strains can be introduced into the material. These strains dis-

appear as the alloy is heated past its transition point. Among such alloys

is a nickel-titanium series of which Nitinol (55Ni-45Ti) is possibly the

best known. It has a transition temperature of 115°C.

16.82 The SM alloy element is generally in the form of a heavy-gauge

coiled-spiral spring and such units are used in automatic greenhouse venti-

lators,

thermostatic radiator valves, liquid-gas safety switches, de-icing

switches on air-conditioning plant, valves in solar heating systems, electric

kettle switches, boiler damper safety switches and warning devices to indi-

cate the presence of ice on roads.

Many other more sophisticated uses are currently being developed for

these alloys. Thus Nitinol is used in a number of aerospace projects, for

example in the form of a coupling ring for joining stainless steel and

titanium tubes. The ring contracts on reaching its transition temperature

and four knife-like protrusions lock mechanically on to the tube surfaces

to form a hermetic seal. Another SM nickel-titanium alloy is used for the

manufacture of spectacle frames. The martensitic alloy used results in the

frame being soft and comfortable to wear, and if it is accidentally bent

the frame can be restored to its original shape by immersion in warm

water. A more unexpected use is in the engineering of underwired 'bras'.

The low 'yield' strength and superelastic straining at relatively constant

stress apparently makes these garments very comfortable to wear and virtu-

ally impossible to bend out of shape. This contrasts with conventional

underwires which are either comfortable or durable but not both.

Exercises

By reference to Fig. 16.2 estimate as accurately as you can, the temperature

at which the Widmanstatten structure begins to develop as a cast 60-40 brass

cools slowly.

Assuming that the brass continues to cool slowly what will be the approxi-

mate composition of the phases present at 300

C and in what proportions will

they exist?

By reference to Fig. 16.4 describe, step by step, the phase changes which will

take place in a bronze containing 22% tin as it cools extremely slowly from

900

C to 350

In what proportions will the phases a and 6 be present at 350

Describe the microstructures which would be produced when an alloy contain-

ing 89.5 Cu-10.5 Al is water quenched from (i) 950

C; (ii) 700

C, after being

slowly heated to the quenching temperature. Account for the structures you

describe. (16.51)

By reference to Fig. 9.9 estimate as accurately as possible the freezing range

of the British copper-nickel coinage alloy.

Describe the effects of alloying on the following properties of copper: (a)

electrical conductivity; (b) machinability; (c) formability; (d) corrosion resist-

ance;

(e) mechanical strength.

(16.21;

16.23;

16.31;

16.34; 16.37; 16.44; 16.46;

16.70)

6. Give an account of the effects of small amounts of oxygen and other elements

on the properties and uses of copper. (16.20)

Pure copper is weak. How has it been found possible to increase the strength

of the metal and still retain a relatively high conductivity? (16.21)

8. By reference to the appropriate thermal equilibrium diagram, show how the

composition and structure of a commercial brass dictates the method by which

it will be shaped and the subsequent uses to which it will be put. (16.30)

9. With reference to the equilibrium diagram for copper-tin alloys, give an

account of the structure, properties and uses of tin bronzes. Discuss the reasons

for adding: (a) phosphorus; (b) zinc; (c) lead to these alloys.

(16.41;

16.42;

16.44;

16.45; 16.46)

10.

Discuss the constitution, structure, properties, heat-treatment and uses of the

industrially important aluminium bronzes. (16.50 to 16.54)

11.

The basic requirements of a precipitation-hardening alloy system is that the

solid solubility limit should decrease with decreasing temperature.

Discuss this statement with reference to copper alloys containing beryllium,

chromium and titanium in small quantities.

How can further strengthening be obtained with Cu-Be alloys? At what

stage should this further treatment be applied? (16.70)

Bibliography

American Society for Metals, Source Book on Copper and Copper Alloys, 1980

Higgins, R. A., Engineering Metallurgy (Part II), Edward Arnold, 1986.

West, E. G., Copper and its Alloys, Ellis Horwood, 1982.

A large number of books and leaflets dealing with the properties and uses of copper

and its alloys are available from Copper Development Association.

BS 1400: 1985 Copper Alloy Ingots and Copper and Copper Alloy Castings.

BS 2870: 1980 Specifications for Rolled Copper and Copper Alloys: Sheet, Strip and

Foil.

BS 2871: 1971 and 1972 Copper and Copper Alloys: Tubes.

BS 2872: 1969 Copper and Copper Alloys: Forging Stock and Forgings.

2873:

1969 Copper and Copper Alloys: Wire.

BS 2874: 1986 Copper and Copper Alloys: Rods and Sections.

BS 2875: 1969 Copper and Copper Alloys: Plate.