_The Nature of Crystals: Metals_
THE use of metals has been one of the great factors in the development
of the activities of the human race. The beginning of the story is
so far back in the ages that we can only make guesses as to how men
first made metal tools and weapons. Perhaps copper was picked up in
its native state, and its weight suggested its usefulness in a fight.
Copper is too soft to take a cutting edge, and it may not have been
very long before it was found that there was an alloy of copper and tin
which was far harder and more serviceable than copper alone. Perhaps
there was tin in the stones of which the copper smelting furnace was
built, perhaps copper and tin occurred together in the same mineral.
And so the age of bronze set in. Iron came later, of course. From that
time to this there have been workers of metal--important members of the
human community. We have but to think of the magnitude of the metal
industry in this country alone, to realize how great a part the metals
play in the life of the world.
In all these thousands of years a vast body of experience has been
gained. Some of it is in books, some of it is still a tradition handed
down by the skilled workman to the apprentice. There is even a sense of
the nature, or condition, or property of a metal which cannot be put
into words, and is only taught by example to such as have the power
to understand. Nor is this any trifling matter: the whole movement of
trade and the welfare of a nation may rest upon it.
On the other hand, the properties of the metals must depend, in the
first place, on the properties of the individual atoms, and, in the
second place, on the atomic arrangement, which is in effect the state
of crystallization. In the very centers of the metal industries it has
been realized of recent years that the scientific observer with his
microscope can bring some system into the mass of disordered knowledge,
and can improve the quality of the manufacture and the certainty of its
production.
[Illustration: PLATE XXII.
Two photographs of aluminium: the surface has been prepared so as to
show the different crystals. The crystals scatter the incident light
differently because they are set in different ways, and the surfaces
exposed by treatment are therefore different in nature, in one figure
the scale has been reduced somewhat from the natural size, in the other
considerably enlarged.
(From a paper by Carpenter and Elam, read before the Institute of
Metals, Sept. 1920.)]
Yet, as I have said already, the microscope can only go to a certain
length: it stops far short of the point which we must reach if we
are to understand how the atoms are acting so as to give the various
materials their specific properties. It can show the existence of the
separate crystals in the metal, but not the arrangement of the atoms
in the crystals (Pls. XXII, XXIII). In the X-rays we find a new hope;
indeed, it is more than a hope. We may be sure that the intimate
knowledge which they give us will in the end throw a flood of light
upon the inner meaning and purpose of all the complex properties of
metals. It may be a long time before the new movement will become great
and obvious. The experience of thousands of years has to be caught
up with and explained. It is quite otherwise with such a subject as
electrical engineering, or wireless telephony, which is a branch of it.
Here the whole process is rooted in the work of the physics laboratory,
and development has been directed by knowledge and anticipation. The
worker in metals has been guided through the ages by trial and error,
by experiment with little knowledge to guide it. It is a very slow
process; but it has been going on a long time, and its findings
command respect. They must be studied very carefully in the new light
which the X-rays give us.
Already we begin to find explanations, as we may call them, of some
of the properties of the metals. They depend upon the crystalline
structure, as we might have expected. Sometimes the crystals are to be
seen by the naked eye; sometimes they become obvious when the surface
is properly prepared and placed under the microscope. But the easiest
and most complete way of discovering them is by means of the X-rays,
with their fineness of vision.
The structures of almost all the metal crystals have been determined
by the X-rays, and it appears that they are usually very simple.
For instance, the atoms of gold, silver, copper, and aluminium are
put together like the piles of round shot that used to stand beside
the guns of a hundred years ago. It is worth while to look a little
carefully into this arrangement, although we are really repeating the
comparison (p. 189) between the structures of ice and diamond.
[Illustration: PLATE XXIII.
(By courtesy of the University of London Press, Ltd.)
The photograph shows the irregular outline of the crystal grains in a
sample of steel, and, in addition, a cross-crossing of lines within
each grain, known as “Widmanstätten lines.” These last are due to the
fact that the polishing tool has rut across the “atom layers,” in much
the same way as polishing cuts across layers of mother of pearl.
(From Belaiew’s “Crystallisation of Metals.”)]
Suppose we put together a number of balls into the triangular
arrangement of Fig. 53, and surround them by a triangular guard, as
shown, just as balls are packed together for playing pool on the
billiard table.
[Illustration: Fig. 53.--A close-packed arrangement of balls in one
layer.]
We lay on these another layer, forming a triangle a little smaller
than the first, and again other layers until the triangular pyramid is
finished (Plate XXIV b). Obviously there can be no closer method of
packing round balls together. Now if we look into the arrangement of
the layers one above the other, we find that the balls in any layer are
exactly over the balls in the next layer but _two_. In the absence of a
model this effect may be realized by the help of Fig. 54. The crosses
represent the centers of the balls in a certain layer, the circles
the centers in the next layer, and the black spots the centers in the
third. The centers in the fourth will be over the crosses, in the fifth
over the circles, and in the sixth over the spots, and so on.
When balls are arranged in this way, it is possible to cut cubes out of
the assembly, as in Plate XXV a. It is always a surprise when
this fact is first realized, but it is well to understand the cause of
it, because so many crystals are made up of atoms piled together in
this way, and they so often grow as cubes or in some way show their
close connection with the cubic form.
Now if, when we have laid down two layers and come to the arrangement
of the third, we place it so that each ball is exactly over a ball in
the first layer, which arrangement is the only alternative to the one
we chose before, we have another way of packing the balls which is as
compact as the other. In this case, the balls in any layer are exactly
over the balls in the _next layer but one_, and Fig. 54 a will be
replaced by Fig. 54 b, and the arrangement of Plate XXV a by that of
XXV b. This arrangement will not stand up now without containing walls,
if we are to have a reasonable number of balls in the model; we must
pin them together in some way. When we look down on this model from
above, we see six-sided tunnels running through it, and we do not sec
any arrangement of this kind when we look in any other direction.
[Illustration: PLATE XXIV.
A. Small groups of shot are in close packing, and there are irregular
gaps between the groups.
B. A pyramid, built by the super-position of layers like that of Fig.
53.]
[Illustration: Fig. 54.--In A is shown the arrangement of the layers as
seen by an observer looking along a diagonal of the cube of Plate XXV
a. The black spot represents the ball at the corner. The small
circles represent the six balls in the next layer, and the crosses
the fifteen in the next layer. In B the arrangement is as seen by an
observer looking down on Plate XXV b from above. The nineteen
black spots represent the balls in the top layer, and the twelve small
circles the balls in the next layer. The third layer is like the first,
the fourth like the second, and so on. The repetition is after every
second layer; in A it is after every third.]
The model has a single axis in the vertical direction, and round that
axis the arrangement is such that a crystal built on this plan would
naturally form hexagonal columns.
In the case of the cube there are four ways of thinking of the
arrangement of the layers; there is a layer perpendicular to each
diagonal of the cube, and, as a cube has four diagonals, there are four
sets of layers. This does not mean that the atoms in any one layer are
specially tied together in that layer; merely that one can sort out the
atoms of the crystal into this kind of layer in four different ways.
Now it turns out that these layers are of very great importance
in respect to the properties of the metal crystals built on the
close-packed cubic plan. Gold, silver, copper, aluminium, and other
metals like them can be drawn into wires, rolled into sheets, and
beaten into various shapes. They are, as we say, ductile, and their
ductility is one of the characteristics that make them so useful. They
can be bent and pulled into all sorts of convenient forms. It seems
possible to make a metal flow like treacle. Gold can be hammered into
leaves so thin that the metal in a sovereign will cover a large field;
the others can be beaten nearly as thin.
[Illustration: PLATE XXV.
A. Cubic packing. B. Hexagonal packing.
(From Pope’s “Modern Aspects of the Molecular Theory.”)
A shows how balls are packed together to form a cube. It is exactly the
same packing as in Plate XXIX B. The close-packed layers of Fig. 53
are horizontal in Plate XXIV B, and in A they are perpendicular to a
diagonal of the cube.
B shows the other form of close packing. Each horizontal layer is a
close-packed layer of Fig. 53.]
Cups and vessels of all sorts, chains and ornaments, and innumerable
useful things are made by taking advantage of this singular property of
ductility. The first thing that we should like the X-ray to explain for
us more clearly, if we can make them do so, is that feature in their
structure which accounts for this most valuable property. We should
also like to understand the inner meaning of the hardening and other
changes that are due to “cold-working,” as it is called--that is to
say, to hammering or straining the metal when it is cold. And what is
annealing, the softening and relief from strain that heat brings about?
Why are all these things so obvious in the case of a metal, while they
do not appear in, for instance, diamond or rock salt or quartz?
Already we begin to see some little way into these difficult questions;
and in particular we have found out something about the way in which
the metal yields to a pull or any other strain, and have learned that
it has to do with the layers of which I have spoken.
A metal is rarely one whole crystal: it is, in general, an assemblage
of crystals, pointing in all directions. Sometimes these crystals can
be seen easily; sometimes the microscope is required to show them. Very
often they are too small even for the microscope, and the X-rays alone
can make them clear.
If we put a number of shot on a tray and let them all run together by
tilting the tray slightly (Plate XXIV a), we observe that
there is a tendency for the shot to arrange themselves like the balls
in Fig. 53. It will not often happen that all the shot will form one
arrangement: there will be groups, each properly arranged in itself,
but not correctly aligned with its neighbors. In just the same way
there will be local arrangements among the atoms of a metal--in other
words, there will be crystallization in groups, larger or smaller, the
connection between the groups being somewhat irregular. We may observe
at once that the connection between group and group is not necessarily
any weaker than the connection between the atoms in any one group. Why
this is so, it is difficult to say. We need not be surprised at it,
because the ties between atoms are complicated things, imperfectly
known to us, and we cannot predict accurately what will happen in every
case.
[Illustration: Fig. 55.--The two blocks are stuck together, but can
slide over each other. When pulled they change in relative position
from (_a_) to (_b_).]
It is said that when gold is at a high temperature a fracture cuts
through the crystals, but when the gold is cold it goes round them; and
this will illustrate the complexity of the effect.
A block of one of these metals may reasonably be expected, therefore,
to consist of a mass of crystals, large and small; and this is exactly
what the X-rays show to be the case, even when the microscope fails
because the crystals are too small for it to see.
Now when we take a single crystal and try to bend it or distort it, we
find always that it gives finally through a slip along a plane: all
that is on one side of the plane slipping with respect to all that is
on the other. These planes are the planes we spoke of before--those
that contain atoms arranged as in Fig. 53. A single metal crystal does
not give way exactly in the direction in which it is pulled. If we had
two blocks of glass, let us say, held together by grease as in Fig. 55,
and pulled them, they would give along the plane between them.
[Illustration: Fig. 56.--This figure represents in the form of a
diagram the slipping on one another of the layers of the aluminium
crystal.]
Naturally they will slide over one another on this plane rather than
themselves be torn to pieces. In the case of a metal there is not
merely one plane, but many planes, and many of them will be planes of
sliding either together or one after another. We might represent the
crystal by a set of lines as in Fig. 56 (_a_), which, if pulled
in the direction of the arrows, would yield as in Fig. 56 (_b_).
Often when a single metallic crystal has been stretched, we can see the
marks on its surface which show the lines along which slip has taken
place.
Professor Carpenter and Miss Elam have shown recently some beautiful
examples of this kind of effect in the case of large crystals of
aluminium. An ordinary piece of the metal consists of a multitude of
crystals pointing in all ways, as we have already understood to be the
case. By a somewhat complicated process of heating and stretching, the
many small crystals can all be made to line up and form a small number
of large crystals, just as we might imagine that by shaking or tapping
the tray of shot shown in Plate XXIV a in some way, to be
found out by experience, we could get all the minor regular groupings
merged into one large one.
The pieces chosen for experiment were of a form often used for pieces
to be tested for their resistance to pull; the form is shown in Plate
XXVI; the original length of each piece is eight inches. The broad
ends are intended to be gripped by the jaws of the machine that is to
stretch the piece; the narrower part is that which is to give way, and
to show by the way in which it does so, and the pull that is exerted,
the capacity of the metal to resist the forces that would strain it.
When one of the test pieces so treated is put into the testing machine
and pulled, it gives way in a curious fashion, which differs for
different specimens (Plate XXVI, 1 to 4). Sometimes the width of the
piece remains the same, and it thins out gradually as the test piece
lengthens under the pull: it may grow longer by several inches before
it gives way. Sometimes the thickness remains the same, and the piece
shrinks sideways, developing a waist which finally is the place of
breaking. At other times, again, there are more curious changes still.
These pieces have been examined by the X-rays, and it turns out that
the nature of the yield depends entirely on the way in which the large
crystals are set toward the line of pull. The metal gives way along the
plane of slip. If, for example, the crystal is so set--and there is no
telling during the heat and strain treatment how the forming crystal
will lie--that the layers of which we have spoken are as in Fig. 56,
then stretching will make the piece draw in sideways.
Sometimes the direction in which the crystal gives way depends on a
more complicated use of two sets of slip planes alternately. When the
crystal might slip on more than one set of planes, it is apt to choose
the one which is more nearly perpendicular to the line of pull. We can
imagine that this is so because a slip means a riding of one set of
atoms over another, and the motion would be helped by a force tending
to pull one layer away from the other. If we had a solid body made up
of a row of balls like the top layer in Fig. 57, and it rested on a
similar row like the bottom layer, it might be easier to drag the top
layer over the bottom if the line of pull were along _P_ rather
than along _Q_.
[Illustration: Fig. 57.--The top layer, as a unit, might be more easily
pulled through from position _A_, through _B_, to _C_, if the pull were
along _P_, than if it were along _Q_.]
Since (see Fig. 55) the pull always tends to bring the plane of slip
more nearly into its own line, there arises a sort of seesaw action:
the crystal slips along one set of planes until the set comes too
nearly into line with the pull, and then along another. In the end
the line of pull bisects the angle between the two sets. The balance
is often shown in the shape of the broken ends; in Plate XXVI, 3, for
example, there is a kind of knife-edge at the point of rupture. The
two sides of the edge are parallel to two different sets of slip
planes, and are equally inclined to the line of the pull which finally
tore the metal in two.
In the case of the aluminium, the yield is so easy that a sheet of some
thickness, when composed of a single crystal, can be bent quite easily
by one’s fingers. An ordinary piece of aluminium sheet is quite stiff,
however, and the explanation of the difference is that, when there are
crystals pointing in all directions, there are some ready to take and
bear the strain, no matter from what direction it comes. The strength
of a chain is that of its weakest link, and the weak part of a crystal
is its slip plane. This is a point of extraordinary importance in the
manufacture of metal, though it is often linked up with so many others
that its special effect is difficult to sort out from the rest. Many
factors go to the design of steel, let us say, for some given purpose;
but one of them is certainly the degree of fineness of the crystal
grains of which it is composed. Fineness and uniformity of size both
contribute to the toughness of steel and its quality generally.
[Illustration: PLATE XXVI.
These photographs an due to Professor Carpenter and Miss Elam
(_Proceedings of the Royal Society, A._, 100, p. 346). They show
the yielding of aluminium under strain, the part finally giving way
being a single large crystal in the narrower portion of the test piece.
In (1) and (2) the slip planes are disposed--to the observer--like the
layers of Fig. 56. The test piece has contracted sideways, and finally
developed a waist, as the picture shows. There is no thinning from back
to front. In (3) and (4) the reverse has taken place before the break:
the width has remained the same, but the material has grown thinner
(this cannot be seen in the photograph).]
When the single crystal of aluminium gives way along a layer, we may
suppose there is a moment when the one set of atoms is riding over
the other, followed by a drop into place again: _A_ goes through
_B_ into _C_ (Fig. 57). When the latter case is reached, the
close-packed arrangement is resumed. The metal is still a crystal. Now,
as we know, the regular crystalline arrangement is the natural one, and
so the substance slips easily from one natural arrangement to another,
adjusting itself to the pull or other strain by doing so. No doubt this
is one of the causes, and a very important cause, of ductility.
But why does the metal often become harder when it is beaten? And what
happens to it when it is annealed? Perhaps we are guided toward an
answer by considering what happens to gold when it is beaten into leaf
and subsequently heated. Gold leaf is very thin, as we have seen. It
is even transparent, but it absorbs part of the spectrum of light that
passes through it, allowing a greenish light to filter through. It is
yellow when viewed by reflected light, as we know. It is very curious
that when it has been heated to a dull red heat it becomes permanently
transparent, and white by reflected light. Faraday was very interested
in this fact; he suggested as a partial explanation that the thin
layer of gold broke up, the metal gathering itself together in little
heaps, and that the light went through the holes that were left. Sir
George Beilby has made many experiments, and added considerably to
the information we have in regard to the behavior of this and other
substances when heated in the same way. If there are holes in the
heated leaf, they are exceedingly small, he says, beyond the power of
the microscope to see. Now the X-rays have something to say on this
point. When gold leaf is examined by their aid, it is found that it
consists of masses of cubic crystals of gold all lying with faces
parallel to the leaf. They are not necessarily cubes, of course. They
consist, like ordinary salt, which is cubic, of rectangular blocks of
all sizes. They must be exceedingly thin blocks, and no doubt their
thickness is far less than their width or length. When the leaf has
been heated, the blocks are piled up anyhow, perhaps gathered together
to some extent in heaps, as Faraday supposed, even if they are too
small to be seen by the microscope; and perhaps this is the reason
why gold and silver leaf become transparent when heated. Why gold
should be green when looked through is a mystery. But we do see that
the beating of the gold has spread out the crystal blocks so that they
all lie with one face in the leaf surface, and that heat has destroyed
this amount of regularity of arrangement. When the heated gold leaf is
pressed with a body having a smooth, hard surface, such as an agate,
it goes back to the other condition, as we might expect, since the
pressure would force the blocks once more into the flat. In both cases
the metal is crystalline, but there is more arrangement in the usual
than in the annealed form of the gold leaf. The same effect is found
with silver. To show it in the case of copper it would be necessary to
carry out the experiment under such circumstances that the air could
not act on the metal. When copper is heated in the open, a film of
copper oxide quickly forms all over it, an action which also can be
followed by the X-rays. We often see this tarnish form slowly on copper
even when no heat is applied. But it is easy to show by X-rays that
in copper foil there is the same arrangement of the crystal block as
in the case of the gold leaf. On the other hand, a block of ordinary
copper shows no such arrangement; the crystals are arranged anyhow.
The hardening of these metals by cold working is, therefore, due in
some way to the fact that they are put into a state of strain by the
rearrangement of the crystals which the X-rays show; annealing is the
release of this strain and the destruction of the arrangement. As to
why this is so, we are still very ignorant: we can simply be satisfied
that we have made one step forward.
It is worth noting that, in general, when a metal has been thrown into
a state of strain in this way, it is more readily subject to the action
of chemicals, as we might expect. It is not so well settled into what
we may call a comfortable condition.
We ought now to go on to the consideration of other peculiarities
possessed by metals, since we may expect them all to be due to more or
less the same causes and we must study them all together. Two of their
most remarkable properties lie in their powers of conducting heat and
electricity. We all know how quickly heat spreads through a metal:
we might be inclined to say that a metal could be identified by its
possession of that property. We all know, too, how metals, especially
copper, are used as conductors of electric current.
Going back to our first consideration of the nature of the atoms,
and of the differences between the various atoms, we find at once a
feature which on the whole seems capable of giving us a satisfactory
explanation of their conducting powers; no doubt, too, it has much to
do with their crystalline structure and their ductility. The atoms of
the metals always have one or more electrons which are lightly held.
For instance, sodium has eleven electrons; two of these are very close
to the nucleus, eight more form a very strongly held system round the
first two. The odd electron belongs to an outer system altogether,
which becomes filled up as we go from sodium to magnesium with two
in the outermost system, aluminium with three, and so on. This odd
electron is not held tightly. When it is stripped off for any reason,
the atom is outwardly reduced to the form of the unsociable atom
“neon,” except that as a whole it carries a positive electric charge
due to the want of balance on loss of an electron. An aluminium
crystal is an assemblage of spheres like neon, all in close packing,
as explained, and all the odd electrons are more or less free to move
about in the structure. It must be said, however, that this picture
is doubtless much too crude to be the whole truth; there must be much
more in the design of which as yet we know nothing. Yet it must be
right to a certain extent. We see at once why metals are conductors
of electricity: it is because the electrons, the fundamental charges
of negative electricity, can move about so easily. When a current of
electricity runs along a metal wire, it is the electrons that make the
flowing stream. It is curious that they must move, being negatively
charged, in the opposite direction to that in which the so-called
current of electricity is always imagined to flow. It was always a
matter of words, this talk of a flowing current of electricity. It is
quite a new discovery that anything moves at all, and we need not be
surprised that the real direction of flow is opposite to that which had
been supposed.
So we must think of the battery or the dynamo, not as manufacturing
electricity, but as sending round a circuit a stream of the electrons
that are already there and are more or less free to move.
[Illustration: PLATE XXVII.
(By courtesy of the University of London Press, Ltd.)
Damascus blades.
(From Belaiew’s “Crystallisation of Metals.”)]
Just so the engine in a factory makes a leather belt continually travel
round a certain circuit; but the engine does not manufacture leather.
When a metal is heated, the contained electrons dance more quickly
to and fro, and may break away into the open. Electrons are pouring
in a continuous stream from the hot wire in the “valve” of wireless
telephony, and the outpouring is necessary to the action of the valve.
The electrons do not move so easily in a metal when it is hot as when
it is cold. Here again it is easy to imagine how this may be. We can
see that the electrons will have more difficulty in threading their way
among the atoms of the metal if through heat the latter are moving to
and fro and getting in their road. It is much more difficult to explain
the strange fact, discovered at Leiden by Kamerlingh Onnes, that
some substances when their temperature has been lowered to a certain
very low point--a point which differs for different metals--offer
no resistance at all to the movement of the electrons, so that a
current once started will keep on running for days before it finally
fades away, the metal being kept continuously at this extremely low
temperature.
The electrons must to some extent contribute to the capacity of a metal
for conveying heat as well as electricity, because the electrons at the
hot end of a metal bar must pass on some of their excessive energy to
the electrons at the cold.
Thus the presence of electrons in the metal, able to move with some
freedom among the atoms of the structure, gives a very good reason why
the metal conducts both heat and electricity. Of course it is only a
rough picture that we have drawn; many details require to be filled in,
and no doubt many really important facts have been left out altogether
because of our ignorance.
Let us turn back to the question of the ductility of metals, and
consider whether the presence of the electrons helps us here also.
We now see our atoms as spheres, all of them charged with positive
electricity and packed closely; and we may perhaps be right in thinking
that the electrons hold them together like a cement.
[Illustration: PLATE XXVIII.
(By courtesy of the University of London Press, Ltd.)
The long needle-shaped bodies are cementite crystals forming part
of the general mass of steel. (From Belaiew’s “Crystallisation of
Metals.”)]
But the most important point is that the atoms are not tied together by
sharing electrons as in the diamond: they must rather repel one another
than otherwise, being all charged with positive electricity. They are
free to roll or slide over one another, because they are not attached
to one another at definite points, as often occurs in other crystals.
These things seem helpful when we consider the slipping of one plane
over another.
So far we have been considering the crystalline structure and
properties of a few of the metals in the pure state. Now in practice
we meet with alloys far more often than with pure metals, and that
for the reason that alloys have properties of their own of the
greatest value. Alloys are, in fact, extraordinarily interesting in
their immense variety and in the wide range of their usefulness. New
forms are constantly being discovered. No matter what peculiar virtue
may be required for some special purpose, an alloy of some sort is
forthcoming, which satisfies the demand more or less completely. We
must try to see some reason for these things, if we can, in what we
have recently discovered. Of course, we know so little as yet, and
there is so much to explain, that in a few years’ time we may think
very little of our present attempts, but we must make a beginning.
It very often happens that the addition to a metal of quite a small
quantity of a second metal, or even a non-metal, causes a notable
improvement in hardness. Pure metals are generally very soft, because
their slip planes are so ready to give. The first of all the great
alloys was bronze, a mixture of copper and tin, which is far harder
than either metal alone. The mixture of copper and zinc produces the
serviceable brass, of which there are varying qualities, depending
on the proportions of the mixture. Steel is formed by the addition
of a small percentage of carbon to pure iron. There are alloys of
copper and aluminium, which are very tough and do not corrode, but are
difficult to work in the shaping machine. There is an alloy of copper
and nickel, which does not corrode and is easily molded; it is used
for the coverings of bullets. German silver is a white, ductile alloy,
non-corroding, which is used in the manufacture of such articles as
spoons and forks, which are afterward coated with silver in the process
known as electroplating. An alloy of nickel with chromium stands very
great heat, and is used for the wiring of electric furnaces.
[Illustration: PLATE XXIX.
(By courtesy of the University of London Press, Ltd.)
(By courtesy of Sir George Beilby.)
A. Section magnified a thousand times and showing the cementite needles
in process of being broken up and rounded off.
(From Belaiew’s “Crystallisation of Metals.”)
B. The dark band is a scratch made by a very fine needle in a polished
piece of speculum (mirror) metal, highly magnified. The fine vertical
scratches are made by emery powder in polishing. Many small particles
have been torn up and deposited in the trough made by the needle.
(From Beilby’s “Aggregation and Flow of Solids.”)]
Chromium, cobalt, and tungsten combined in definite proportions make
stellite, an extraordinarily hard, non-corroding substance; some of the
standard weights at the National Physical Laboratory are being made of
it. There are alloys for the making of bells, very soft alloys for type
metal, and a great variety of solders. There is the aluminium bronze,
which is used for cheap jewelry and consists of aluminium with a small
percentage of copper. And so on to a long list, if it were necessary to
make one.
Let us take one of the simpler cases--for example, that in which
the addition of a small quantity of aluminium to copper produces an
increase in hardness. The X-rays show that the structure of the copper
crystals remains the same, except that here and there an aluminium
atom takes the place of a copper atom. Now the weakness of the crystal
lies, as we have seen, in the fact that one part slides on another
part along a certain plane. This plane is now no longer even: there
is a scattering of aluminium atoms in it, and we can readily suppose
that the slipping has become more difficult, and that we have here
the cause of hardening.[5] There is a remarkable effect which makes us
think we are right in supposing so. The atoms of aluminium must strain
the structure of the copper crystal, because the copper will not take
up more than a certain number. If an alloy is made containing more than
about 10 per cent of aluminium, the X-rays tell us that the copper
crystals are broken up altogether, and a new structure is formed.[6]
The aluminium atoms must be distorting the copper crystal, and this
fits in very well with the fact that it hardens the copper. On the
other hand, when nickel is added to copper the atoms of the former
replace the atoms of the latter to any extent: evidently they can
slip into the places of the copper atoms without straining the copper
crystals at all. And in this case there is no hardening effect, which
is just what we should expect. It is only when we push in atoms which
really strain the copper crystal and make its planes uneven that the
hardening is brought about. We have jammed the sliding planes.
[Illustration: PLATE XXX.
In A a piece of speculum metal, after being rubbed with fine emery, has
been polished with rouged leather. The metal has been dragged over the
emery scratches; there is a reminiscence of butter spread on bread.
In B the polishing with rouge has been carried further; the emery
scratches have disappeared, but the outlines of the grains in the metal
begin to appear.
(From Sir George Beilby’s “Aggregation and Flow of Solids,” by courtesy
of the author.)]
In the case of steel the action is of the same kind, but here the
carbon atoms that are the cause of the hardening do not replace the
iron atoms, but are forced into the empty spaces between them. We
can easily see that this may distort the iron crystal, and as before
prevent the movement along a plane of slip. Once again there is a limit
to the amount of the alloying substance: only a small percentage of
carbon can be introduced into the iron without breaking up its ordinary
simple structure.
The problems of iron and steel contain, however, many more
complications than this. We have only to ask what happens when more
carbon is put in than the iron structure can carry, and we find we have
a new problem. Among other things, a new crystal appears, formed of
molecules, each containing three atoms of iron and one of carbon; it
is known as cementite. The new crystals are very hard and unyielding,
and in form are like needles (Plate XXVIII). Their presence hardens the
iron very greatly and makes it difficult to work. A beautiful example
of its effect on steel is to be found in the old swords that once made
their way from India through Damascus into Europe. Damascus steel was
greatly valued for the excellence of its qualities. Fine specimens are
to be seen in the Wallace Collection; they show the characteristic
wavy pattern (Plate XXVII) which has always been looked on as evidence
of genuineness. When examined under the microscope the lines of the
pattern are seen to consist of multitudes of dots, forming a sort of
Milky Way in the steel. These dots are the tiny crystals of cementite.
As Colonel Belaiew tells us, the steel when it was first made was most
difficult to work. The smith, with his little furnace, would heat the
steel red hot, but after he had struck but a few blows and made a
slight impression on the steel, the momentary softening had gone. The
hardness due to the cementite crystals had only been removed for an
instant. More heating, a few more blows, and slowly the steel became
less rigid. In fact, the cementite crystals were changing their form.
They were becoming less like needles, gathering themselves together
into more rounded shapes, and as they did so the steel became more
pliable (Plate XXIX a). At last the fine Damascus steel was
reached, so strong and yet so elastic.
[Illustration: PLATE XXXI.
In A the metal has been etched with acid: the “flowed” parts have been
readily attacked and removed, the grains now show up very clearly. In B
polishing has begun again.
(From Sir George Beilby’s “Aggregation and Flow of Solids,” by courtesy
of the author.)]
It is very likely that much of the keen edge that these swords would
take was due to the presence of the very hard particles embodied and
held in the softer iron. The edge would be like a saw with extremely
fine teeth. In the trial of skill between Saladin and King Richard
which Walter Scott describes in _The Talisman_, the former threw
a gossamer veil into the air and severed it by drawing his scimitar
across it, a fine test of keenness and of skill. Richard, on the other
hand, used his sword like an ax, and clove in two an iron bar, the mace
of one of his knights. This also was a test requiring great qualities
in the steel, but on the part of the man the skill lay more in the
power to strike a terrific blow than in delicacy of touch.
Grinding, sharpening, and polishing are really very interesting
operations. When we put a knife on the grindstone we let the hard
crystals in the stone cut minute furrows in the steel, actually
removing the material. This is one stage of the sharpening process.
But the polishing on the oil stone or the strop is a different thing
altogether. Here we actually make the steel to flow, smoothing down the
furrow; sometimes, as Sir George Beilby has shown, actually drawing
a skin of metal over the deeper hollows. The metal seems to remain
crystalline all the time; the X-rays show readily the crystals in a
razor blade. Probably the action is the same as that which took place
in the gold leaves when they were heated. The oil that we use helps
in the smoothing process. The metal is strained by the flow; in time
the strain tends to come undone, and heat especially can take away the
keenness of the edge (Plates XXIX b, XXX, XXXI).
An alloy is generally a much worse conductor of electricity than a pure
metal. It may well be that when the stranger atoms are forced into the
structure of the pure metal, and the planes of atoms are made uneven,
the electrons are more hampered in their passage through the metal.
More energy is required to force them along, and the metal becomes
hotter through the passage of the current than if it were pure. In the
case of a pure metal, as I have already said, the resistance to the
movement of the electrons becomes greater if the temperature is raised.
We can imagine that the electrons find it harder to get past the atoms
when the latter are more active: heat makes them move to and fro more
quickly about their proper positions.
[Illustration: PLATE XXXII.
A. The resistance of the pure metal coil in the same circuit as the
shining lamp has been reduced by surrounding it with a freezing mixture.
B. The iron wire is stretched by a hanging weight; its expansion and
contraction are magnified by the lever arrangement. The wire is heated
by passing an electric current along it.]
But heat does not make so much difference in the case of alloys,
because the passage of the electrons is already so difficult that heat
does not make much change. We can show this by a simple experiment:
A battery sends a current round a circuit which has two branches, as
in the figure (Plate XXXII a). One of them contains a coil
of copper wire, M, and a lamp, L{1}, the other a coil of an
alloy such as German silver, for example, and a lamp, L{2}.
The coils are so adjusted in respect to the resistance which they offer
to the passage of the electric current through them that the lamps
both burn dimly. A vessel containing liquid air is brought up so as
to include the coil, M, and the lamp, L{1}, at once burns
brightly. The cooling of the copper wire has lowered its resistance to
the passage of electrons, and more current flows through the lamp. But
when the alloy is, in its turn, immersed in liquid air, no change is
made.
Sometimes metals crystallize in more than one way. Iron furnishes one
of the simplest and most striking examples. At ordinary temperatures
the iron atoms are arranged so that each atom has eight neighbors. The
latter are at the corners of a tiny cube, of which the former atom
occupies the center. This is not the closest form of packing, as will
readily be found on trial. The packing of the pile of shot of which
I spoke before gives the closest packing, and in that each shot has
twelve neighbors, six touching it round an equator, and three more
round a line of latitude in each hemisphere: or, as we may put it,
six in its own layer and three in each of the next layers. It is the
packing of gold, silver, copper, and aluminium. It is very curious that
when iron is heated to a cherry red the atoms change their arrangement
and pack in the tightest form, that of the pile of shot. The effect
is easiest to see when an iron wire is heated beyond this point and
allowed to cool. When it comes to the critical temperature, the atoms
suddenly adopt the looser packing and the wire stretches a little: the
increase of length is easily observed by the use of some magnifying
device. It is very curious, too, that when the old form changes to the
new, some energy is set free and the iron suddenly brightens up again.
The stretching and brightening have long been matters of observation,
but it is only quite recently that we have discovered that the packing
of the atoms into two different crystalline forms is at the bottom of
what we have seen (Plate XXXII b).
These very few instances of the relation between the properties of a
metal and its crystal structure are drawn from an immense subject,
most of it still waiting exploration with our new helpers, the X-rays.
We cannot say beforehand what will be found out. We can be very sure,
however, that the better we understand our materials the better use we
can make of them.
FOOTNOTES:
[Footnote 1: See “The Legacy of Rome” (Oxford University Press), p.
270--an article by Dr. Singer.]
[Footnote 2: From a Friday Evening Discourse before the Royal
Institution of Great Britain, March 4th, 1881.]
[Footnote 3: For lecture purposes the working of the apparatus is
illustrated by a kinematograph film which has been made for the
purpose. It shows a series of successive expansions, each forming a new
set of lines like those shown in Plate III.]
[Footnote 4: C. V. Boys, “Soap Bubbles and the Forces Which Mold Them.”]
[Footnote 5: Rosenhain, “The Inner Structure of Alloys,” Institute of
Metals, May 2, 1923.]
[Footnote 6: Jette. Phragmen, and Westgren, Institute of Metals, March,
1921.]
NOTE
AFTER trials of many ways of making models of atomic structure and of
many substances I find that two have real merits:--
Balls representing the atoms may be made of hard dentists’ wax, which
softens in boiling water and can then be pressed into proper shape in
metal molds made for the purpose, just as we used to remake our golf
balls in the old days. The spherical mold is made in two halves; and
it is convenient to mount them in the lathe, one on the head and one
on the back center. Small balls harden at once, and can be made very
quickly: larger balls must be left a little while in the mold. The hard
wax can be drilled without becoming softened and deformed by the heat
generated in drilling. The models made of the wax are very finished in
appearance, and will stand all ordinary temperatures. The wax is rather
costly.
Gramophone needles make good connectors, the balls, wax or wood, being
drilled to receive them. The holes should be drilled true and in
correct position. Convenient little contrivances can be made to be used
for this purpose on the lathe.
THE END
=TRANSCRIBER’S NOTES=
A list of figures has been included to enhance readability.
Parentheses were applied to chemical compounds to prevent confusion
with italicized text in the transcribed version.