Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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is available the boiling ethanol bath can be dispensed with and the spotting

method used for all specimens.

A summary of the most useful etching reagents is given in Tables 10.1,

10.2,

10.3 and 10.4.

10.25 Both polishing and etching can be carried out electrolytically.

This involves setting up an electrolytic cell in which the surface of the

specimen acts as the anode. By choosing a suitable electrolyte and appro-

priate current conditions, the surface of the specimen can be selectively

dissolved to the required finish. Not only can 'difficult' metals and alloys

be attacked in this way, but a high-quality, scratch-free finish can be pro-

duced. The description of detailed techniques is, however, beyond the

scope of this book. The traditional methods of polishing and etching

already described are adequate for the successful preparation of most met-

allurgical materials.

Table 10.1 Etching Reagents for Iron, Steels and Cast Irons

Type of etchant

Nital

Picral

Alkaline sodium

picrate

Mixed acids and

glycerol

Acid ammonium

peroxydisulphate

Composition

2 cm

nitric

acid;

98 cm

ethanol (industrial

methylated spirit)

4 g picric

acid;

96 cm

ethanol

2 g picric

acid;

25 g sodium

hydroxide; 100 cm

water

10 cm

nitric

acid;

20 cm

hydrochloric

acid;

20 cm

glycerol;

10 cm

hydrogen

peroxide

10 cm

hydrochloric

acid;

10 g ammonium

peroxydisulphate; 80 cm

water

Characteristics and uses

The best general etching reagent for irons and steels.

Etches pearlite, martensite, tempered martensite and

bainite, and attacks the grain boundaries of ferrite. For

pure iron and wrought

iron,

the concentration of nitric

acid may be raised to 5 cm

. To resolve pearlite, etching

must be very light.

Also suitable for ferritic grey cast irons and blackheart

malleable irons.

Very good for etching pearlite and spheroidised

structures, but does not attack the ferrite grain

boundaries.

It is the most suitable reagent for all cast irons, with the

exception of alloy and completely ferritic cast irons.

The sodium hydroxide is dissolved in the water and the

picric acid then added. The whole is heated on a boiling

water-batH for 30 minutes and the clear liquid poured

off.

The specimen is etched for 5-15 minutes in the boiling

solution.

Its main use is to distinguish between ferrite and

cementite. The latter is stained black, but ferrite is not

attacked.

Suitable for nickel-chromium alloys and

iron-chromium-base austenitic steels.

Also for other austenitic steels, high chromium-carbon

steels and high-speed steel.

Warm the specimen in boiling water before immersion

Particularly suitable for stainless steels.

Must be freshly prepared for use.

Table 10.2 Etching Reagents for Copper and its Alloys

Type of etchant

Ammonical

ammonium

peroxydisulphate

Ammonia-

hydrogen

peroxide

Acid iron(lll)

chloride

Acid dichromate

solution

Composition

20 cm

ammonium

hydroxide (0.880);

10 g ammonium

peroxydisulphate;

80 cm

water

50 cm

ammonium

hydroxide (0.880); 20-50

hydrogen peroxide

(3%

solution); 50 cm

water

10 g iron(lll) chloride;

30 cm

hydrochloric

acid;

120 cm

water

2 g potassium dichromate;

8 cm

sulphuric

acid;

4 cm3

saturated sodium chloride

solution;

100 cm

water

Characteristics and uses

A good etchant to reveal the grain boundaries of pure

copper, brasses and bronzes.

Should be freshly made to give the best results.

The best general etchant for copper, brasses and

bronzes. Etches grain boundaries and gives moderate

contrast.

The hydrogen peroxide content can be varied to suit a

particular alloy.

Used for swabbing or immersion, and should be freshly

made as the hydrogen peroxide deteriorates.

Produces a very contrasty etch on brasses and

bronzes. Darkens the (3 in brasses. Can be used

following a grain-boundary etch with the ammonium

peroxydisulphate etchant.

Use at full strength for nickel-rich copper alloys. Dilute

1 part with 2 parts of water for copper-rich solid solutions

in brass, bronze and aluminium bronzes.

Useful for aluminium bronze and complex brasses and

bronzes. Also for copper alloys of beryllium, manganese

and silicon, and for nickel silvers.

Table 10.3 Etching Reagents for Aluminium and Alloys

Type of etchant

Dilute hydrofluoric

acid

Caustic soda

solution

Keller's reagent

Composition

0.5 cm

hydrofluoric

acid;

99.5 cm

water

1 g sodium hydroxide;

99 cm

water

1 cm

hydrofluoric

acid;

1.5 cm

hydrochloric

acid;

2.5 cm

nitric

acid;

95 cm

water

Characteristics and uses

The specimen is best swabbed with cotton wool soaked

in the etchant.

A good general etchant.

A good general etchant for swabbing.

Particularly useful for duralumintype alloys. Etch by

immersion for 10-20 seconds.

NB On no account should hydrofluoric acid or its fumes be allowed to come into contact with the skin

or eyes. Care must be exercised with all strong acids.

The Metallurgical Microscope

10.30 The metallurgical microscope is similar in optical principles to any

other microscope, but differs from some of them in the method by which

the specimen is illuminated. Most biological specimens can be prepared

as thin, transparent slices mounted between sheets of thin glass, so that

illumination can be arranged simply, by having a source of light behind the

specimen. Metals, however, are opaque substances, and since they must

be illuminated by frontal lighting, it follows that the source of light must

be inside the microscope tube

itself.

This is usually accomplished, as in

Fig. 10.5, by means of a small plain-glass reflector, R, placed inside the

tube.

With this system of illumination much of the light is lost both by

transmission when it first strikes the plate and, by reflection, when the

returning ray from the specimen strikes the inclined plate again. Neverthe-

less,

a small 6-volt bulb is usually sufficient as a source of illumination.

The width of the beam is controlled by the iris diaphragm, D. Generally

speaking, this should be partly closed so that the beam of light is just

sufficient to cover the back component of the objective lens. An excess of

light, reflected from the sides of the microscope tube, will cause light-

scatter and, consequently, 'glare' in the field of view.

The optical system of the microscope consists of two lenses, the objec-

tive,

O, and the eyepiece, E. The former is the more important and expen-

sive of the two lenses, since it has to resolve the fine detail of the object

being examined. Good-quality objectives are corrected for chromatic and

Table 10.4 Etching Reagents for Miscellaneous Alloys

Type of etchant

Ethanoic and

nitric acids

Ethanoic acid

and hydrogen

peroxide

Acid iron(lll)

chloride

Dilute hydrochloric

acid in alcohol

Iodine solution

Mixed nitric acid

ethanoic acid

Composition

3 cm

glacial ethanoic

(acetic)

acid;

4 cm

nitric

acid;

16 cm

water

30 cm

glacial ethanoic

(acetic)

acid;

10 cm

hydrogen peroxide

(30%

solution)

10 g iron(lll) chloride;

2 cm

hydrochloric

acid;

95 cm

water

1 cm

hydrochloric

acid;

99 cm

alcohol (industrial

methylated spirit)

10 g iodine crystals;

30 g potassium iodide;

100 cm

water

50 cm

nitric

acid;

50 cm

glacial ethanoic

(acetic) acid

Characteristics and uses

Useful for lead and its alloys (use freshly prepared and

etch for 4-30 minutes).

5% Nital is also useful for lead and its alloys.

Suitable for lead-antimony alloys.

Etch for 5-20 seconds.

Suitable for tin-rich bearing metals.

Other tin-rich alloys can be etched in 5% Nital.

For zinc and its alloys.1% Nital is also useful.

The best etchant for cadmium-bismuth alloys.

Suitable for nickel and monel metal.Should be freshly

prepared.

Fig.

10.5 The basic optical system of the metallurgical microscope.

This illustrates the system used in an early instrument. However, the optical system is

fundamentally similar in more sophisticated modern instruments.

spherical aberrations, and hence, like camera lenses, are of compound

construction. The magnification given by the objective depends upon its

focal length—the shorter the focal length, the higher the magnification. In

addition to magnification, resolving power is also important. This is defined

as the ability of a lens to show clearly separated two lines which are very

close together. In this way resolution can be expressed as a number of

lines per mm. Thus resolving power depends upon the quality of the lens,

and it is useless to increase the size of the image, either by extending the

tube length of the microscope or by using a higher-power eyepiece, beyond

a point where there is a falling off of resolution. A parallel example in

photography is where a small 90 mm x 60 mm snapshot, on enlarging,

fails to show any more detail than it did in its original size and in fact shows

blurred outlines as a result.

OBJECTIVE

SPECIMEN

DIFFUSING

DISC

EYEPIECE

Plate 10.2 An 'Olympus' metallurgical microscope equipped for binocular vision.

Four objectives of different focal lengths are carried on a rotatable turret. Illumination is by

either tungsten or quartz halogen light source. (Courtesy of Metallurgical Services Labora-

tories Ltd, Betchworth, Surrey).

However, there is a definite limit to the extent to which it is worthwhile

developing microscope objectives since at high magnifications we are deal-

ing with dimensions of the same order as the wavelength of light itself so

that definition then begins to suffer considerably. Consequently micro-

scope lenses are cheap compared with high-quality camera lenses where

the main restriction is the 'resolving power' of the photographic emulsion

which accepts the image formed by the lens.

The eyepiece is so called because it is the lens nearest the eye. It is a

relatively simple lens and its purpose is to magnify the image produced by

the objective. Eyepieces are made in a number of 'powers', usually x 6,

x 8, x 10 and x 15.

The overall approximate magnification of the complete microscope

system can be calculated from:

T.N

Magnification = -—

where T = the tube length of the microscope measured from the back

component of the objective to the lower end of the eyepiece;

F = the focal length of the objective; and

N = the power of the eyepiece.

Thus for a microscope having a tube length of 200 mm and using a 16-mm

focal-length objective and a x 10 eyepiece, the magnification would be

—TT

, ie 125. Most metallurgical microscopes have a standard tube

length

of 200 mm. Assuming a tube length of this value, approximate

magnifications

with various objectives of standard focal lengths will be

found

in Table 10.5.

Plate 10.3 The ultimate in optical microscopy!

Here the 'Olympus' PME microscope is of the 'inverted' type. The apparatus incorporates a

reflex screen for direct viewing and also photography on sheet film, as well as for TV viewing

in colour or monochrome. (Courtesy of Metallurgical Services Laboratories Ltd, Betchworth,

Surrey).

Table 10.5 Approximate magnifications with different

combinations of objective and eyepiece (assuming a tube

length of 200 mm)

Focal length of Power of Approximate

objective eyepiece magnification

(mm)

32 x 6 37.5

x 8 50

x 10 62.5

16 x 6 75

x 8 100

x 10 125

8 x 6 150

x 8 200

x 10 250

4 x 6 300

x 8 400

x 10 500

2 x 6 600

(oil-immersion x 8 800

objective) x 10 1000

Most modern microscope objectives are engraved with a 'multiplying

power' which is relative to the tube length of the microscope for which

they are designed. The overall magnification is then obtained simply by

multiplying the 'power' of the objective by that of the eyepiece, eg a x 40

objective used in conjunction with a x 10 eyepiece will give an overall

magnification of x 400.

Many sophisticated modern microscopes contain built-in refinements ful-

filling various purposes. Thus the facility of using polarised light often

helps in the identification of a phase. For example, particles of cuprous

oxide in copper (16.22) which, with normal illumination, appear as sky-

blue globules can be made to glow like rubies with the use of the polariser.

Microscopes equipped for dark-field illumination are very useful in the

photography of many low-contrast subjects.

10.31 Using the Microscope Most readers will be familiar with the

fact that in ordinary camera photography the depth of field reasonably in

focus at the focal plane depends upon the distance of the object from the

camera lens. Thus with a landscape view everything will be fairly sharply

in focus between about ten metres and infinity. However, if the subject is

only one metre from the lens then, assuming it is sharply in focus

itself,

zone of only a few centimetres either side of one metre will be reasonably

in focus. A microscope lens works at a distance of only a few millimetres

from the object and it follows therefore that the 'depth of field' is neglig-

ible.

Consequently, as mentioned earlier (10.20) it is essential to provide

the specimen with an absolutely flat surface and to mount the specimen so

that its surface is normal to the axis of the instrument. This is most easily

achieved by fixing the specimen to a microscope slide by means of a piece

of plasticine. Normality is assured by using a mounting ring, as shown in

Fig. 10.6. Obviously, the mounting ring must have perfectly parallel end

faces.

Mounting may not be necessary for specimens which have been set

in bakelite, since the top and bottom faces of the bakelite mount are

usually parallel, so that it can be placed directly on to the table of the

microscope.

The specimen is brought into focus by using first the coarse adjustment

and then the fine adjustment. It should be noted that the lenses supplied

are generally designed to work at a fixed tube length (usually 200 mm),

under which conditions they give optimum results. Therefore, the tube

carrying the eyepiece should be drawn out the appropriate amount (a scale

is usually engraved on the side of the tube). Slight adjustments in tube

length may then be made to suit the individual eye.

Fig.

10.6 Mounting a specimen for examination under the microscope.

MICROSCOPE SLIDE

PLASTICINE

MOUNTING

RING

SPECIMEN

PLANE SURFACE

Finally, the iris in the illumination system should be closed to a point

where illumination just begins to decrease. This will limit glare due to

internal reflections in the tube.

It is a mistake to assume that high magnifications in the region of 500

or 1000 are always the most useful. In fact, they will often give a completely

meaningless impression of the structure, since the field under observation

will be so small. Directional properties in wrought structures or dendritic

formation in cast structures are best seen using low powers of X 40 to x

100.

Even at x 40 a single crystal of, say, cast 70-30 brass may completely

fill the field of view. The dendritic pattern, however, will be clearly appar-

ent, whereas at x 500 only a small area between two dendrite arms would

fill the field of view. As a matter of routine, a low-power objective should

always be used first to gain a general impression of the structure before it

is examined at a high magnification.

10.32 The Care of the Microscope Care should be taken never to

touch the surface of optical glass with the fingers, since even the most

careful cleaning may damage the surface, particularly if it has been

'bloomed' (coated with magnesium fluoride to increase light transmission).

In normal use dust may settle on a lens, and this is best removed by

sweeping gently with a high-quality camel-hair brush.

If a lens becomes accidentally finger-marked, this is best dealt with by

wiping gently with a good-quality lens-cleaning tissue (such as Green's

No.

105) moistened with xylol. Note that the operative word is wipe and

not rub. Excess xylol must be avoided, as it may penetrate into the mount

of the lens and soften the cement holding the components together.

High-power objectives of the oil-immersion type should always be wiped

clean of cedar-wood oil before the latter has a chance to harden. If harden-

ing takes place due to the lens being left standing for some time, then the

oil will need to be removed by xylol, but the use of the latter should be

avoided when possible.

If special lens tissues are not available soft, well-washed linen may be

used to clean lenses. It is far superior to chamois leather, which is likely

to retain particles of grit, and to silk which has a tendency to scratch the

surface of soft optical glass.

10.33 The Electron Microscope Whilst much of the routine micro-

examination of metals is carried out at low magnifications in the region of

x 100, it is often necessary in metallurgical research to be able to examine

structures at very high magnifications. Unfortunately the highest magnifi-

cation possible with an ordinary optical microscope is in the region of

x 2000. Above this magnification the dimensions being dealt with are

comparable with the wavelength of light

itself.

Indeed, since blue light is

of shorter wavelength than red light, it is advantageous to view specimens

by blue light when examining very fine detail at high magnifications.

For very high-power microscopy (between X 2000 and several millions)

light rays can be replaced by a beam of electrons. The bending or refracting

of light rays in an optical microscope is achieved by using a suitable glass-

lens system. A similar effect is produced in the electron microscope by

using an electro-magnetic 'lens' to refract the electron beam. This 'lens'

consists of coil systems which produce the necessary electro-magnetic field

to focus the electron beam.

Whereas light rays are reflected to a very high degree from the surface

of a metal, electrons are transmitted. They are able to pass through a thin

metallic foil, but will be absorbed by greater thicknesses of metal. We must

therefore view the specimen by transmission instead of by reflection, and

in order to examine the actual surface of a metal the replica method is

generally used. This involves first preparing the surface of the specimen

and then coating it with some suitable plastic material, a very thin film of

which will follow the contours of the metallic surface. The thin plastic

mould, or replica of the surface, is thefi-separated from the specimen and

photographed by transmitted electrons using the electron microscope.

Following the development of the orthodox electron microscope out-

lined very briefly above, other instruments of allied design have become

available. Thus, the high-voltage electron microscope permits the use of

thicker replica specimens and at the same time offers greater magnifica-

tions.

The scanning electron microscope differs in principle from the ortho-

dox instrument in that electrons generated and reflected from the actual

surface of the specimen are used to produce the image. The image is built

up by scanning the surface in a manner similar to that employed in a

synchronously-scanned cathode ray tube. An image of very high resolution

is produced but an even more important feature is the great depth of field

available as compared with an optical instrument. This enables unprepared

surfaces, eg fractures, to be scanned at magnifications up to 50 000.

In a somewhat different category is the field-ion microscope in which the

image is formed by gas ions rather than by electrons. With this instrument

magnifications can be measured in millions enabling details on the atomic

scale such as dislocations, vacancies, grain boundary defects and very small

precipitates to be examined. Single atoms, particularly the larger ones,

come just within the range of this instrument.

A recent development is the scanning acoustic microscope

AM) which

enables the materials scientist to examine both surface and subsurface

structures. The essential feature of this rather complex instrument is a ruby

'lens'

which focuses high-frequency vibrations (of up to 1.5 GHz) on to

the region being examined. The reflected vibrations, following the same

path as the incident waves, are received by a transducer and converted

into electrical signals. By scanning in raster pattern an image can be built

up which is displayed on a TV monitor'. The main feature of SAM is that

it can examine both surfaces and subsurfaces non-destructively and with a

minimum of specimen preparation.

Macro-Examination

10.40 Useful information about the structure of a piece of metal can often

be obtained without the aid of a microscope. Such investigation is usually

referred to as 'macro-examination' and may be carried out with the naked

eye or by using a small hand magnifying lens.

Plate

10.4 10.4A A vertical section through a small ingot of

aluminium.

Normal

size.

Etched by swabbing with a 20% solution of hydrofluoric

acid.

10.4B

Section through a

'close-form'

forging.

Deep-etched

in boiling 50% hydrochloric acid for 30 minutes to reveal the flow

lines.

The

strength

of the teeth is increased by the continuous

'fibre'

produced by

forging,

x V

. (Courtesy

Forgings and Presswork Ltd, Birmingham).

The method of manufacture of a component can often be revealed by

an examination of this type (Plate

10.4B

and Fig. 6.5), whilst defects, such

as the segregation of antimony-tin cuboids in a bearing metal (Plate 18.1),

can also be effectively demonstrated. Macro-examination is also a means

of assessing crystal size, particularly in cast structures (Plate 10.4B), whilst

various forms of heterogeneity, such as is shown in the distribution of

sulphide globules in cast steel, can be similarly examined.

Usually, medium grinding is sufficient to produce a satisfactory surface

for examination, and for large components an emery belt will be found

almost indispensable. Polishing is not necessary, and for most specimens

grinding can be finished at Grade 320 paper. Bearing in mind the rougher

nature of the work, it will be realised that grinding should not be carried

out on papers which are used for the preparation of specimens for examin-

ation under the microscope. Discarded papers from micro-polishing pro-

cesses can be used for the preparation of macro-sections.

After being ground, the specimen should be washed to remove grit, but

it will not generally be necessary to degrease it in view of the fact that

macroscopic etching is often prolonged and removes more than the flowed

layer. The fibrous structure of a forged-steel component, for example, is

revealed by deep-etching the component in boiling 50% hydrochloric acid