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Looking at metals: A lecture forschoolsJ. W. Martin aa Department of Metallurgy , Oxford UniversityPublished online: 20 Aug 2006.

To cite this article: J. W. Martin (1961) Looking at metals: A lecture for schools, ContemporaryPhysics, 3:1, 1-15, DOI: 10.1080/00107516108204442

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Looking at Metals : a Lecture for Schools by J. W. MARTIN

Department of Metallurgy, Oxford University

1. INTRODUCTION Whereas in the chemical industry

many processes are the early laboratory experiments done on a large scale, metallurgical industry has developed mainly from the older arts and crafts. I t is only in comparatively recent times that any considerable fundamental work has been carried out on metals and alloys, and there are now well established and rapidly growing branches of science which are related to metal- lurgical industry in the same way that the science of chemistry is related to chemical industry.

The science of metallurgy is a young one.

Fig. 1. Section through cast steel bar. Actual size.

About sixty-five of the chemical elements are metals, although most of them are combined with non-metal atoms, such as oxygen, in substances similar to stone or clay. The first step in making metals available for human use is freeing the metal atoms from this combined state: this is the province of the extractive metallurgist. The physical metallurgist is the one who has to know why one metal is different from another, and knows how to combine different metals in alloys in order to make them more useful. By studying their behaviour he knows how he can improve their properties by giving them special heat-treatments or by working them in rolls or under forging hammers.

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Physical metallurgy had its beginning in 1808 when Alois de Widmann- statten polished the surface of a meteorite, and upon etching it discovered a beautiful geometric pattern that now bears his name. Since this meteorite was a nickel-iron alloy, it was soon deduced that metals and alloys are actually crystalline, even though they lack the geometrical surfaces of many non- metallic crystals. Figure 1 is a photograph (actual size) of a section through a cast bar of alloy steel which has been ground and polished flat. Acid attack has revealed the individual alloy crystals, which are 1-2 mm. in diameter. Although it is possible under special conditions to grow metal crystals as large as several centimetres in cross-section, in most familiar metal objects the crystals (or ‘grains ’) are too small to be seen with the naked eye, and it is the microscopic structure of metals (or ‘ metallogruphy ’) which will be discussed in more detail below.

L/ OBJECTIVE LENS

I 1 SPECIMEN

Fig. 2. The principle of the metallurgical microscope.

2. OPTICAL METALLOGRAPHY H. C. Sorby, of Sheffield, may be regarded as the founder of metallography.

In 1864 he overcame the chief difficulty-the preparation of the metal surface. Earlier methods involved breaking the metal with a hammer blow and then looking at the fracture surfaces. As these are rough, and the depth of focus of the microscope is small, satisfactory images could not be obtained and little progress was made. The method developed by Sorby for preparing a metal for examination with the microscope is, with slight modifications, still the one most widely used today.

A specimen 1-2 cm. in its largest dimension is cut from the metal to be examined. A mirror polish is produced on one face of the specimen by grinding on an abrasive wheel, polishing on successively finer emery papers, and lapping on revolving cloth-covered wheels with fine abrasives. The polished surface is produced by the melting, flowing and resolidification of the surface layer due to heat generated from the friction between the particles of the abrasive and the asperities on the metal. The surface is thus not truly representative of the metal,

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Looking at Metals 3

and to make the underlying structure visible, the flowed layer has to be removed by ' etching '-usually with a dilute acid. The etchant also attacks various parts of the specimen at different rates and reveals the structure.

The principle of the metallurgical microscope is illustrated in fig. 2. A horizontal beam of light enters an aperture in the microscope tube, where it strikes a glass plate (G) inclined at an angle of 45". About 10 per cent of the light is reflected down through the objective lens, where it strikes the specimen surface and may be reflected back through the objective lens to form the primary image, which is then magnified further with the eyepiece.

On examining a polished and etched surface of a metal or a simple alloy under the microscope, a polygonal network of grains is observed (fig. 3), similar to that seen with the naked eye in the specimen of fig. 1. From such a structure it is concluded that a metal consists of an agglomeration of crystals fitting together as shown in the diagram of fig. 4-this is an idealised drawing of the situation of course, each of the grains is of uniform shape and size and of one of the few regular solid shapes that can be used for completely filling space.

Fig. 3. Polished and etched uranium, magnified X 200. (D. M. Davies)

Etching the polished section of fig. 3 has attacked each grain, and the sharp ' grain boundaries ' between neighbouring crystals appear clearly under the microscope. By crushing at high temperature it is sometimes possible to make a metal come apart at the grain boundaries, and a photomicrograph of separated grains of brass (60 per cent copper40 per cent zinc) appears in fig. 5. It is clear that these grains are tending towards the ideal shape (fig. 4), although they are far from uniform.

When a small amount of alloying element is added to a metal, it will usually form a solid solution whose microstructure is indistinguishable from that of a pure metal, since the crystalline arrangement is the same. When larger amounts of alloying metal are added, however, the limit of solubility may be exceeded, and crystals of a different arrangement (containing atoms of each metal) will appear in the structure (fig. 6). The first formed solid solution is

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called the primary phase, and the new crystals the secondary phase; as more and more alloying element is added to the alloy, more of the secondary phase will be produced and less of the primary phase, until eventually only the secondary phase is present, and once more the microstructure will be the same as for a pure metal. Further additions of the alloying element may produce another phase and the sequence of changes may be repeated.

Fig. 4. Idealised drawing of the grain structure of a metal.

Fig. 5. Separated grains of brass, magnified x 50.

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The mechanical properties of a metal are closely related to the micro- structure. The strength increases as the grain-size is reduced, and in two- phase alloys the strength increases as the spacing between the particles of the second phase is reduced. The changes in microstructure that occur when a metal is deformed will next be considered.

Fig. 6 . Two-phase structure of magnesium4~% cerium alloy.

Fig. 7. Deformation by ' slip '.

3. DEFORMATION OF METALS Pure metals are characteristically soft and ductile : although under slight

deformation they behave elastically and spring back to their original shape, they are readily plastically deformed-i.e. undergo a permanent change of shape.

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The study of a single metal crystal (say in the form of a wire) will show how this plastic flow commonly occurs, and fig. 7 represents such a crystal after deforma- tion by extension. This type of plastic flow is called ' slip ', since it occurs by the slipping of crystal planes over each other-the movement taking place along a certain crystallographic ' slip direction ' and usually on a crystallo- graphic ' slip plane '. Figure 8 is a photograph of a deformed single crystal wire of cadmium-the lines on the surface are due to the surface steps arising from the slip process indicated in fig. 7. These lines will only be readily visible, of course, if the original surface is very smooth, and they are usually called ' slip bands '.

Fig. 8. Slip in a single crystal of cadmium.

An ordinary piece of (polycrystalline) metal will also deform by the process of slip within each individual grain, although the grain boundaries between two differently oriented crystals are regions where slip will not readily occur. Thus a polycrystalline specimen will be harder to deform than a single crystal, and metal of fine grain-size will be stronger than coarse-grained metal. To demon- strate that deformation by slip occurs in polycrystalline metal, one would first prepare a normal metallographic section (as in fig. 3), and then plastically deform the specimen (perhaps by squeezing in a vice). Figure 9 (u) is a typical example of the resultant structure in the case of aluminium: on the surface of each grain are numerous parallel slip-bands, each crystal having produced these surface steps by a process similar to that shown in the diagram of fig. 7.

In many metals, however, a slightly more complicated slip process can occur. In the examples of figs. 8 and 9 (u ) each crystal has only slipped on one set of ' slip-planes ', thus producing only one set of parallel slip bands on the surface. Slip quite commonly occurs simultaneously on more than one set of planes, and fig. 9 (b ) shows the effect on the slip-band distribution when this ' multiple slip ' has taken place. Here part of the surface of only one grain is shown, and slip has taken place on two sets of planes.

Although the slip movement illustrated in fig. 7 resembles that of a pack of cards which has been pushed lengthwise, the actual mechanism by which slip takes place does not correspond to the sliding of cards. It is found that crystals slip under load at stresses very much below those which would be required to move two perfect crystal planes past one another; in fact metal crystals are between 1000 and 10000 times weaker than theory would predict. Nearly

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thirty years ago a theory was put forward to account for this discrepancy, but only in recent years have modern metallographic techniques brought supporting proof of the hypothesis.

Fig. 9. Slip bands on polycrystalline aluminium: x 750. (The small triangular and polygonal surface pits were produced by the etchant.)

4. DISLOCATIONS A more realistic picture of the slip process would be to visualise slip beginning

in one small area of the plane and then spreading over the rest of the plane. While this is taking place, the slip plane will be divided into a slipped area and an unslipped area, and the line of demarcation (called a dislocation line) will be

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moving across the slip plane. Professor N. F. Mott has made the amusing simile that the passage of a dislocation line across a slip-plane of a crystal is like the movement of a ruck in a carpet. It is obviously more difficult to make one carpet move bodily over another by pulling one end of it, than to make a ruck in the carpet, and to move the ruck along. The ruck separates the ‘ slipped ’ from the unslipped ’ part of the carpet in the same way that a disclocation line separates the slipped and unslipped regions of a slip plane in a crystal.

The diagram of fig. 10 (u) represents a simple form of dislocation line EE When the upper part

of the crystal is stressed as indicated by the arrow, slip takes place as shown in fig. 10 (b ) by the movement of the dislocation line to the surface at CD- producing a step there as in fig. 7.

. lying in the slip plane ABCD of a small crystal block.

Fig. 10. A simple dislocation (b)

(EE) in a crystal block.

The stresses required to make dislocation lines move are calculated to be in good agreement with the measured strengths of metal crystals-the presence of dislocations thus accounting for the weakness of such crystals. All soft metal crystals contain such lines-the average distance between them being a few thousand atoms. They will form a network within each grain-a dislocation line cannot end within a crystal, it must either run to a free surface, to a grain boundary or to another dislocation.

When a metal is plastically deformed, the yield stress for further deformation increases strongly-this effect being known as work-hardening ’, and is a valuable engineering property. This phenomenon will occur when dislocations moving on intersecting slip planes meet and obstruct one another’s motion (e.g. fig. 9 (b ) ); a high density of dislocations then builds up at such places of intersection. A grain boundary may act as a barrier in a similar way, and a traffic jam ’ of dislocations may pile up at such an interface.

The presence of dislocations can thus account for the softness of metals, and also for the fact that they become hard when deformed. The scale of the phenomena associated with dislocations is far below the resolution of the optical microscope, however, and it is the application of the electron microscope to metallography which has, in recent years, increased our knowledge in this field. Instruments are now commercially available which are capable of resolving 7 6 (i.e. of discerning two points 7 x lop8 cm apart), but before con- sidering its application to the study of dislocations in metals, the various ways in which the electron microscope may be used by the metallographer will be outlined.

The process of slip increases the dislocation content of the metal.

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5. ELECTRON METALLOGRAPHY The conventional electron microscope is of the transmission type-that is,

an electron beam passes through the specimen to be examined and is then focussed by the objective and projector ‘ lenses ’ to produce an image on a fluorescent screen. A diagram of the optical system appears in fig. 11; the entire microscope tube is under high vacuum, and the magnetic ‘lenses’ which focus the electron beam consist of carefully designed electromagnets. Contrast in the image results from differences in scattering-power when the electron beam passes through the thick and thin parts of the specimen, or the parts having differences in atomic composition.

ELECTRON GUN

SPECIMEN

OBJECTIVE L E N S

PROJECTOR LENS

FLUORESCENT SCREEN

Fig. 11. The electron microscope; a schematic diagram.

Metallographers were therefore confronted with a difficult problem : metals readily absorb electrons, and it was not easy to produce suitable thin films for examination by transmission. The problem was solved by the development

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of replica techniques. For example, an imprint of the differences in surface height on a polished and etched metal surface may be obtained by adding a dilute solution of a plastic material in a volatile solvent (rather like ' nail varnish ') to the metal surface. When the solvent evaporates a thin film of plastic is left, the upper surface of which is flat, while the lower surface follows the contours of the etched metal (fig. 12). This film is now stripped from the metal, placed on to a small copper supporting grid (fig. 13), and is then placed in the electron micro- scope. The contrast in the image is obtained by the differences in the scattering

REPLICA METAL

REPLICA 3 Fig. 12. Production of a surface replica.

Fig. 13. Electron microscope replica on copper supporting-grid: x 35.

of the electrons as they pass through the thin and thick regions of the replica which correspond to the raised and lowered parts of the metal surface. Figure 14 shows an electron micrograph of a plastic replica from a steel consisting of particles of iron carbide (white) embedded in iron (dark).

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Fig. 14. Plastic replica from a tempered steel. Electron micrograph: x 10,000.

With plastic replicas it is impossible to reproduce detail finer than 100-200 Angstrom units. The plastic material is composed of long molecular chains, and when it is irradiated with electrons in the microscope the chains appear to straighten and to combine with other chains, so producing much larger molecules which limit the resolving power. These difficulties were overcome by the

Fig. 15. Carbon replica from a steel. Electron micrograph: x 24,000.

development of the carbon replica method. The polished and etched metal specimen is placed in an evacuated chamber containing two pointed carbon rods in contact. When a current of about 50 amp is passed through the rods for a few seconds, carbon evaporates from them and recondenses on the metal surface to form a film about 50-100A thick. The film is then stripped by chemically attacking the underlying metal, and the replicas obtained give ex- cellent contrast in the electron microscope; they are very stable and almost

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structureless, and resolving powers of better than 25 A units may be readily obtained. Figure 15 shows an electron micrograph of a carbon replica from a steel consisting of' ' laths ' of iron carbide embedded in iron.

Fig. 16. Transmission electron micrograph of an aluminium foil: x 7000.

Fig. 17. Dislocations piled up against a grain boundary in slightly deformed stainless steel:

Within the last few years techniques have been perfected, and the design of electron microscopes improved, enabling thin metal foils to be examined directly by transmission in the microscope. One such method involves rolling the metal to a thin foil, which is then made the anode of an electrolytic cell. Under carefully controlled conditions the foil will get evenly thinner by dissolution-eventually dropping into holes. A specimen is then cut from the edge of a hole and mounted as in fig. 13. Figure 1 6 shows an electron micro- graph of an aluminium foil taken by transmission-the grain distribution is similar to that in figs. 1 and 3 .

One of the more fascinating discoveries has been that it is sometimes possible to render dislocation lines visible in the electron microscope, when examining

x 17,500. (Courtesy of Dr. P . B. Hirsch.)

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thin metal foils in transmission. The actual discontinuity in the crystal at a dislocation line is only about the size of an atom, and so beyond the resolution even of the electron microscope. The presence of this ' ruck ' in the crystal does, however, squeeze or stretch the surrounding part of the cI ystal (represented in fig. 10 (u) ), so that its effect can be ' felt ' perhaps over 100 A. When

Fig. 18. Distribution of dislocations in stainless steel deformed more heavily than in Fig. 17 : x 17,000. (Courtesy of Dr. P. B. Hirsch.)

the electron beam is transmitted through such material, the distortions around the dislocation lines scatter the electrons more (or less) than the perfect parts of the crystal, and so the positions of the dislocations appear clearly in the image.

I n fig. 17 a situation described in the previous section is directly observable. This foil, from a deformed steel specimen, shows a number of short dislocation lines (for they only extend from the upper to the lower surface of the foil) piling up at a grain boundary. A number of such ' traffic jams ' are clearly visible. When more severely deformed metals are studied in this way, the complex tangle of dislocations is much less easy to interpret: one such work- hardened structure appears in fig. 18.

6. OTHER MECHANICAL PROPERTIES OF METALS Many interesting mechanical properties of metals may be accounted for in

terms of dislocation movements. For example, if dislocation motion is blocked by a hard particle inside a crystal, or at a grain boundary, such a material may become vulnerable to crack formation through the simple mechanism illustrated in fig. 19. A high concentration of stress collects where the dislocation is blocked (19 b), which by the coalescence of several dislocations (19 c) may nucleate a tiny crack. In a ductile metal this will not be a serious situation, as plastic flow can greatly increase the force required to make a crack run. But this benefit is dependent upon temperature : as the temperature decreases, the

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tendency to flow also decreases, and there will be increasing danger of the crack spreading.

Some metals, such as ordinary steel, are normally resistant to cracking, but become highly susceptible to it at low temperatures. This temperature effect caused much trouble with wartime ships: in southerly waters there was no problem, but during the winter months in northern waters the hulls often

Fig. 19. Crack formation by blocking dislocations.

cracked under shocks from heavy seas. Sometimes a crack ran instantly through the plates of a ship (fig. 20); at other times a crack remained dormant and caused serious damage later. It need hardly be said that the study of the de- formation and fracture of metals forms an important part of current research in physical metallurgy.

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Two methods of eliminating any deleterious effects due to the presence of dislocations in metals may suggest themselves. Firstly, one may attempt to make them immobile through controlled precipitation of fine particles in the crystals by careful methods of alloying and heat-treatment. These will provide a series of obstacles which the dislocations must overcome; some may tend to anchor dislocations in position. Many strong alloy steels owe their hardness to this effect. Secondly, the possibility of eliminating dislocations altogether from the metal might be considered. So far, only tiny hair-like metal crystals have been prepared in a dislocation-free state. These ‘ metal whiskers ’, as they are called, are relatively tremendously strong (a tin whisker having greater relative strength than our strongest alloy steels), and it is interesting to speculate whether large-scale material will be produced in this condition in the future.

Fig. 20. Failure at sea of an American T/2 tanker.

In this survey it has only been possible to touch upon a few of the interests of the physical metallurgist. I t is a fascinating (and rapidly expanding) field of study, and there are many opportunities for a research career in it, both in Government laboratories (including those of the Atomic Energy Authority) as well as in the private laboratories of many large industrial firms. It is also hoped that increasing numbers of sixth-formers with A-level physics, chemistry and mathematics will choose to read metallurgy when they enter a university.

The Author:

he held a Fellowship from 1954-1957. and Goldsmiths’ Fellow in Metallurgy at St. Catherine’s College.

Dr. John Martin is a graduate in metallurgy of St. John’s College, Cambridge, where He is at present a University Lecturer at Oxford,

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