Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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Fig.

18.5 The aluminium-tin equilibrium diagram.

and recrystallisation the tin network is broken up to some extent so that

the aluminium forms a continuous strong matrix. Bearing shells of this

type are generally carried on a steel backing strip and have been widely

used in automobile design both for main and big-end bearings.

18.44 Polytetrafluoroethene (PTFE) and Nylon as non-metallic

bearing materials are competing with metals in many fields. The coefficient

of friction of PTFE (\i = 0.03-0.1) is lower than for any other solid

material. Unfortunately its mechanical properties are poor so that it has

to be strengthened by the use of some 'filler' material. PTFE/bronze com-

posites (Plate 18.2) are useful up to temperatures of 300

C—the upper

working limit for PTFE. They are used in the automobile industry as

bearings in windscreen wipers and steering system bushes. Nylon can be

reinforced with bronze particles. An important feature of both materials

is that they function well as 'dry' bearings. Hence they are useful in machin-

ery for the preparation of food and textiles where contamination by a

lubricant would be unacceptable. They are also useful in some cases where

a bearing is so inaccessible as to make its lubrication difficult.

TIN

(Ph)

+ liquid

liquid

Plate 18.2 The microstructure of a PTFE filled sintered bronze bearing. (Courtesy of The

Glacier Metal Co Ltd, Northwood Hills, Middlesex).

SURFACE

PTFE

(black)

-containing

particles

SINTERED

BRONZE

'SOLDER'

ZONE

STEEL

BACKING

Fusible Alloys

18.50 These contain varying amounts of lead, tin, bismuth and cadmium,

and are generally ternary or quaternary alloys of approximately eutectic

composition. A number of such alloys will melt below the boiling point of

water, and in addition to such obvious uses as the manufacture of teaspoons

for practical jokers, they are used for metal pattern work, for the fusible

plugs in automatic fire-extinguishing sprinklers, for bath media in the low-

temperature heat-treatment of steels and for producing a temporary filling

which will prevent collapse of the walls during the bending of a pipe.

Bismuth-rich alloys such as Cerromatrix expand slightly on cooling, and

are useful for setting press-tool punches in their holders.

Table 18.7 Fusible Alloys

Composition (%)

Other Melting point/range

Type of Alloy Bi Pb Sn Cd elements CC)

Tinman's solder - 38 62 - - 183

Cerromatrix 48 28.5 14.5 - Sb-9 102-225

Rose's Alloy 50 28 22 96

Wood's Alloy 50 24 14 12 - 71

Cerrolow117 45 23 8 5 ln-19 47

Ternary eutectic - - 12 - Ga-82 17

Zn-6

Titanium and its Alloys

18.60 Mention of the use of small amounts of titanium in some steels and

aluminium alloys has already been made. In recent years titanium has

become increasingly important as an engineering material in its own right,

both in the commercially pure and alloyed forms.

Titanium is not a newly discovered element (it was discovered in Corn-

wall in 1791 by W. Gregor, an English priest), and is only rare in the sense

that it is a little-used metal, and one which is expensive to produce. In

Nature it is very widely distributed, being the tenth element in order of

abundance in the Earth's crust. Of the metallurgically useful metals only

aluminium, iron, sodium and magnesium occur more abundantly (Table

1.2). Nevertheless it is unlikely that titanium will ever be an inexpensive

metal because of its high chemical affinity for all non-metals except for the

noble gases. For this reason it can only be produced by reducing its chlor-

ide,

TiCU, with magnesium—obviously an expensive process—and the

titanium so produced will react with all orthodox crucible materials. Hence

it can only be remelted in vacuum by using an electric arc process in

which the electrode consists of the titanium 'sponge' (produced by chloride

reduction) and in which the crucible is virtually of titanium. This seemingly

impossible feat is achieved by using a water-cooled copper crucible on to

which a layer of titanium freezes—a thermodynamically inefficient but

necessary situation.

Very strong titanium alloys can be produced and since the relative den-

sity of the metal is only 4.5 then these alloys have a very high specific

strength (2.36). Its high melting point (1725°C) has contributed to its use

in jet-engine construction. Moreover the creep properties and fatigue

strengths of titanium alloys are very satisfactory so that they are finding

increasing use in aerospace industries. As an example the four engines of

Concorde contain some 16 tonnes of titanium alloys.

18.61 Although titanium is chemically very reactive it has an excellent

corrosion resistance. Like aluminium it coats itself with a dense protective

oxide skin. It is now very extensively used, generally in the 'commercially

pure'

form, in chemical plant where it will resist attack under extremely

hostile conditions such as are provided by strong acids, chloride solutions

and bromine. By adding 0.15% of the noble metal palladium the corrosion

rate can be reduced by up to 1500 times because of 'anodic passivation'.

18.62 Titanium is a bright silvery metal and, when polished, resembles

steel in appearance. The high-purity metal has a relatively low tensile

strength (216 N/mm

) and a high ductility (50%), but commercial grades

contain impurities which raise the tensile strength to as much as 700 N/

and reduce ductility to 20%. It is an allotropic element. The a-phase,

which is hexagonal close-packed in structure, is stable up to 882.5°C, whilst

above this temperature the body-centred cubic |3-phase is stable. The poly-

morphic change point is affected by alloying in a similar manner to the A3

point in iron (13.11). Thus, alloying elements which have a greater solu-

bility in the a-phase than in the (3-phase tend to stabilise a over a greater

temperature range (Fig. 18.6(i)). Such elements include those which dis-

solve interstitially—oxygen, nitrogen and carbon—and also aluminium

°c

'martensite

°h AKSn) °h Mo,V, Nb <&Fe, Mn, Si, Cu

Fig.

18.6 The effects of alloying on the a -> (3 transformation temperature in titanium.

which dissolves substitutionally. Elements which tend to dissolve in |3, and

consequently stabilise it, are usually those which, like (3, are body-centred

cubic in structure. These are mainly the transition elements iron, chro-

mium, molybdenum, etc., and the resulting equilibrium diagrams are of

the types (ii) or (iii) (Fig. 18.6). Those alloys represented by an equilibrium

diagram of type (iii) could be expected to undergo a martensite-type trans-

formation (12.22) if quenched from the (3-range, and indeed they do.

However, unlike the hard martensite produced by quenching steel the

martensite formed here is relatively soft because the solute atoms are

substitutionally dissolved and do not produce the same degree of lattice

distortion as do the interstitially dissolved carbon atoms in steel. Due to

considerable sluggishness the eutectoid transformation in type (iii) alloys

is rarely complete and can be neglected so that most alloys can be regarded

as belonging to group (i) or (ii) as far as subsequent heat-treatment is

concerned.

18.63 The (3-phase in alloys tends to be hard, strong and less ductile

than the relatively soft a-phase. Nevertheless, the (3-phase forges well so

that most of the commercial alloys, which are of the a + |3 type, are

hot-worked in the (3 range. Those elements like aluminium and tin which

stabilise a tend to strengthen alloys by normal solid solution effects but

the a + (3 alloys formed when molybdenum, vanadium, niobium, silicon

and copper are added are strengthened by heat-treatment. Low alloy

additions of these group (ii) and (iii) elements tend to give a martensitic

structure when quenched from the (3-range but with higher alloy additions

a completely (3 structure is retained. Suitable ageing treatment produces a

finely-dispersed precipitate of a within the (3 structure and the best all-

round combination of mechanical properties. However, during the ageing

treatment a very brittle intermediate phase oo is first precipitated but pro-

longed treatment causes this to disappear and be replaced by a. Conse-

quently ageing treatment must be carried out in the region of 500

C for 24

hours to ensure the removal of all oo and its subsequent replacement by

finely-divided a. Most of the heat-treatable titanium alloys are first solution

treated in the range 850-1050

C according to composition, followed by

ageing at 500

C for 24 hours.

18.64 Titanium alloys are now extensively used in both airframe and

engine components of modern supersonic aircraft because of the attractive

combination of specific strength and excellent corrosion resistance. Ex-

pensive precision cameras now contain titanium-alloy parts requiring low

inertia such as shutter blades and blinds, whilst the high corrosion resist-

ance of titanium has led to its use as surgical implants as well as in the

chemical industries. Some alloys of titanium are given in Table 18.8.

Table

18.8

Some Titanium Alloys

Typical

mechanical

properties

Composition

(%)

Typical

uses

chemical plant for

resistance

acids

and

chloride solutions.

Air

frame—gear box

fittings,

engine nascelles,

fuselage skinning, wings,

etc.

Jet

engines—blades

and

discs.

Marine—heat

exchangers, scrubbers,

sonar, yacht fittings.

Surgery—joint

replacements, bone

plates

and

screws.

Stress

produce

0.1%

strain

10Oh

(N/mm

)

550

300

Elongation

(%)

Tensile

strength

(N/mm

)

650

1120

1200

0.2%

Proof

stress

(N/mm

)

460

900

970 min.

Heat-treatment

Annealed

at 650-750

Annealed

700-900

Heat

to 870-950

quench

and

heat

to 400-510

for

hours.

Other

elements

Fe-0.2

max.

H-0.0125

max.

Commercially-pure

titanium

Relevant

specification

(B.S.)

2TA6

2TA10

(Sheet

and

strip)

2TA28

(Forgings)

Air

frame—flaps and slat

tracks,

brackets in wings,

engine pylons.

Jet

engines—discs and

blades, fan casings, etc.

Supplied as bars for

machining components.

Jet

engines—discs and

blades.

Jet

engines—rotary and

static

components, fan

casings.

Sheet and strip.

465 at

400

385 at

450

300 at

520

10 min.

1125

1350

1225

1020

700

1075

810

920

1095 min.

970

850

480

960

550

Heat to

900

and soak; cool in

air.

Heat at

500

for 24 hours.

Heat to

900

for 1 hour, cool

air. 'Age' at

500

for 24 hr.

Heat

1050°C

for 30 minutes,

quench in oil. Heat at

550

for

24 hours.

520

520°C

Solution

treated

and precipitation-

hardened.

Solution-treated at 1025°C—air

cooled or quenched according to

section.

Aged for 2 hours at

700

Heated to

790-820

cooled in

air.

C-0.13

Nb-0.7

C-0.06

Cu-2.5

3.5

0.5

0.25

0.35

0.5

2.25

5.8

TA48

TA38

(IMI 685)

(IMI 834)

TA52

Uranium

18.70 The mineral pitchblende was formerly thought to be an ore of

tungsten, iron or zinc, until in 1789 M. H. Klaproth proved that it contained

what he called 'a half-metallic substance' different from the three elements

named. He named this new element Uranium in honour of Herschel's

discovery of the planet Uranus a few years earlier. It was not until 1842,

however, that E. M. Peligot, by isolating the metal

itself,

showed that

Klaproth's 'element' was in fact the oxide.

18.71 In the solid form uranium is a lustrous, white metal with a bluish

tint, and capable of taking a high polish. It is a very heavy metal, having

a relative density of 18.7. Pure uranium is both malleable and ductile, and

commercially it can be cast and then fabricated by rolling or extrusion,

followed by drawing. It is a chemically reactive metal and oxidises at

moderately high temperatures. Consequently, it must be protected during

fabrication processes. For atomic energy purposes it is usually vacuum

melted in high-frequency furnaces, and cast in 25 mm diameter rods.

Chemically, uranium is similar to tungsten and molybdenum. Like them,

it forms stable carbides which led to the experimental use of uranium in

high-speed steels. These steels have not survived, since they showed no

advantage over the orthodox alloys.

18.72 Uranium is one of a number of elements which undergo natural

radioactive disintegration. During such spontaneous disintegration three

different types of emission proceed from the source—the so-called a, (3

and

'rays'. The phenomenon generally referred to as a-rays is in fact the

effect produced by a stream of moving particles—a-particles. An

a-particle is, in effect, of the same constitution as the nucleus of a helium

atom. Similarly, (3-rays consist of a stream of fast-moving |3-particles (in

actual fact these are electrons). Only y-rays are in fact true electromagnetic

radiation, and since this radiation is of short wavelength it is able to pen-

etrate considerable thicknesses of metal.

All three forms of radiation are emitted at some stage in the natural

radioactive disintegration of uranium,

238

U. Obviously the loss of an a-part-

icle from the nucleus of a

238

U atom will cause a change in both its atomic

mass number and atomic number and consequently in its properties. In

this general way

238

U changes in a series of steps to radium, and finally to

a stable, non-radioactive isotope of lead (

206

Pb). Each stage of the radio-

active decay is accompanied by the emission of one or more of the types

of radiation described above. A somewhat simplified representation of this

process is indicated in Fig. 18.7

18.73 Some stages in the process of radioactive disintegration take

place more quickly than others, and each separate decay process is gov-

erned by the expression—

Fig.

18.7 Stages in the radioactive disintegration of uranium, H

where n is the number of atoms of the species present and X is a constant

for that species. Integration of this expression and evaluation of the time

necessary for half of the atoms present, initially, to decompose gives what

is called the half-life period of the species.

Thus—

0.6932

^=-r~

Each radioactive species is characterised by its half-life period. Thus it

takes 4.5 x 10

years for half of the

238

U atoms present in a mass of uranium

to change to the next uranium isotope in the series, whilst it takes only

1590 years for half of the radium atoms present to change to the radioactive

gas radon.

18.74 Until its development as a source of fissionable material,

uranium was little more than a chemical and metallurgical curiosity so that,

before 1943, there was no commercial extraction process operating with

the object of uranium as the sole product. Its industrial use, before the

Second World War, was limited to the manufacture of electrodes for gas-

URANlUM

HALF-LIFE

THORI

QTH

ETC.

TC.

RADIUM

236-

4.5 x IC? YEARS

24 DAYS

RADON

RADIOACTIVE

GAS

POLONIUM-™Po

ETC

ETC.

LEAD

206

3.8 DAYS

3 MINUTES

STABLE

I59O YEARS

KEY

Of-RARTlCLE

/3-PARTICLE

'(ELECTRON)

Y-RAYS

MASS NUMBER

238

ATOMIC NUMBER

discharge tubes; whilst

its

compounds were used

in the

manufacture

incandescent

gas

mantles.

18.75

The

reader will,

doubt, associate uranium with

the

production

of nuclear power. This element

has two

principal isotopes (1.90),

the

atoms

of which contain

protons

and 92

electrons

each case. However,

one

isotope

has 146

neutrons giving

total mass

238,

whilst

the

other

has

143 neutrons giving

total mass

235. These isotopes

are

generally

rep-

resented

by the

symbols 92

and

U respectively. Here

the

lower index

represents

the

number

protons

(92)

the

nucleus

(the

atomic number,

and the

upper index represents

the

total number

protons

and neu-

trons

(the

atomic mass,

A, of

the

isotope).

Fig.

18.8 The

fission

atom

uranium,

Here,

isotopes

the

metal barium, 55

Ba,

and the

'noble'

gas

krypton,

Kr,

have been

produced together with three neutrons,

Jn.

About thirty similar reactions

are

known

take

place during

the

fission

of a

atom,

the two

isotopes produced varying

mass number

accordingly between

72 and 162. One

such radioactive isotope

the

notorious 'strontium

ninety',

§jjSr.

Since

neutron

has no

electrical charge,

and

hence suffers neither attrac-

tion

nor

repulsion

either

the

negatively charged electrons

positively

charged protons,

can

fired into

the

nucleus

of an

atom. When

neutron enters

the

nucleus

atom

the

latter becomes unstable

and splits into

two

approximately equal portions (Fig.18.8).

the

same

time

small reduction

in the

total mass occurs,

for

although

the

total

number

particles (protons

and

neutrons) remains

the

same after fission,

the

sum

the

masses

the

resultant nuclei

slightly less. This

is a

measure

the

mass equivalent

the

nuclear binding energy. Einstein's

Theory

Relativity states that mass

and

energy

are not

distinct entities

but

are

really different manifestations

the

same thing.

The two

are

the

velocity

light

expression:

= me

Since

the

velocity

light

2.998

m/s

follows that even

small

loss

mass

can

result

in a

great release

energy.

often

the

case

engineering problems this energy

emitted

the

form

heat.

can

calculated that

the

energy produced

by the

loss

one

gramme

matter

in this

way

sufficient

heat about

200

tonnes

water from

100

NEUTRON

(UNSTABLE)

HEAT

I4O

Fig.

18.9

Chain fission

U atoms.

Fission

also accompanied

by the

emission

neutrons

and if

the overall

mass

great enough, other atoms will absorb some

these

neu-

trons

that

chain reaction occurs leading

to an

'atomic explosion'. There

'critical size'

for a

mass

U. Below this critical size neutrons will

tend

escape rather than

absorbed

U nuclei,

and a

chain reaction

will

not,

therefore,

promoted.

18.76

The

rate

production

nuclear energy from this source

can

be controlled

introducing into

'atomic pile' rods

some element

which will absorb unwanted neutrons

and

thus control

the

rate

disinte-

gration

other 92

U atoms.

natural uranium total disintegration will

not occur since only 0.7% 92

present, mixed with

the

more common

isotope 92

U. Isotope

will however, absorb

neutron

give

atom

of nuclear mass 239. This undergoes further change

produce plutonium,

U, which

can be

used

as a

concentrated fuel

such 'fast reactors'

that

Dounreay.

Some Uncommon Metals

18.80

Of the

sixty-nine metallic elements which occur naturally

in the

surface

of our

Earth only about half find

any

metallurgical applications.

Some—like those

Group I—are

far too

chemically reactive, whilst

others lack mechanical properties which interest

the

engineer.

The

tech-

nology

some

'new'

metals

has

been developed because

properties

which

are of

value

nuclear engineering. Others would

be of

greater

interest

to the

aerospace industries were they

not so

expensive because

scarcity

high extraction costs. Several

these metals have very high

"TRIGGER*

NEUTRON

'Uu

'Su

T,u