Appearance of dislocations in metal crystals on evaporation: Twinning dislocations and their relation to annealing twins in copper

APPEARANCE OF DISLOCATIONS TWINNING DISLOCATIONS AND E. VOTAVA

IN METAL CRYSTALS ON EVAPORATION: THEIR RELATION TO ANNEALING TWINS IN COPPER* and A. BERGHEZANt

Evaporation of copper surfaces produces a step-structure which is related to the orientation of the crystals. On non-coherent twin boundaries this step-structure takes up a characteristic aspect in which two cases can be distinguished, namely: (a) simple or multiple evaporation spirals, or (b) evaporation channels. An interpretation in terms of twinning dislocations is given. Furthermore, the sense of the spirals is determined by the direction of the displacement of the non-coherent twin boundary. This is due to the twin dislocation loops which are confined in the (111) planes and have opposite signs at the two emergence points. FORMATION DE DISLOCATIONS DANS LES CRISTAUX DISLOCATIONS DE MACLAGE ET LEUR RELATION

METALLIQUES OBTENUS PAR EVAPORATION: AVEC LES MACLES DE RECUIT DU CUIVRE

L’evaporation de surfaces de cuivre produit une structure a gradins qui est en relation avec l’orientation des cristaux. Sur des frontieres de maoles non coherentes, cette structure rev& un aspect caraoteristique ou deux cas peuvent 6tre distingues: (a) des spirales d’evaporation simples ou multiples; (b) des oanaux d’evaporation. Les auteurs proposent une interpretation baa% sur les dislocations de maclage. En outre, le sens des spirales est determine par la direction de deplacement de la frontiere de ma& non coherente. Ceci est dii aux boucles de dislocation de macle qui sont confinees dans les plans (111) et qui ont des signes opposes a leurs deux points d’emergence. ERSCHEINEN DER VERSETZUNGEN VON METALLKRISTALLEN BEIM VERDAMPFEN: ZWILLINGSVERSETZUNGEN UND IHRE BEZIEHUNG ZU REKRISTALLISATIONSZWILLINGEN BE1 KUPFER Bei der Verdampfung von Kupferoberflachen entsteht eine Stufenstruktur, die von der Orientierung der Kristalle abkilngt. An nichtkoharenten Zwillingsgrenzen nimmt diese Stufenstruktur charakteristische Formen an, von denen zwei Fiille unterschieden werden konnen, namlich: (a) einfache oder mehrfache Verdampfungsspiralen oder (b) Verdampfungskanille. Es wird eine Deutung anhand von Zwillingsversetzungen gegeben. Weiterhin wird der Sinn der Spiralen durch die Verschiebungsrichtung der nichtkohiirenten Zwillingsgrenze bestimmt. Dies rtihrt davon her, dass die Ringe der Zwillingsversetzungen auf die (111 }-Ebenen beschrankt sind und an den beiden Austrittspunkten entgegengesetzte Vorzeichen haben.

INTRODUCTION

* This work is part of a project sponsored by Union Carbide Corporation, New York. Received August 7, 1958. t European Research Associates, s.a. 95, rue Gatti de Gamond, Brussels, Belgium. ACTA

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coherent twin boundaries with the non-coherent boundaries. The latter can naturally join the “normal” grain boundaries where they produce a small deviation. In contrast to the frequency of recrystallization twins, mechanical twins in f.c.c. metals are produced only under special conditions. Thus Blewitt et al.@) have found mechanical twins in copper, gold and silver deformed at 4.2”K and 77.3’K, Haasencs) in nickel at 4.2”K and 20°K and Suzuki and Barrett(4) in silver-gold alloys at temperatures under 0°C. These authors explain the deformation of these f.c.c. metals by twinning in terms of the movement of partial dislocations (twinning dislocations) which are bounded on the { 1111 planes. Their stress induced movement leaves behind stacking faults, which form elementary twins. In the following sections, some experimental results on recrystallization twins in copper are reported and it will be shown that here too, the twinning dislocations play an important role. the

Copper, like most f.c.c. metals and unlike hexagonal and b.c.c. metals, forms annealing twins. These twins can easily be recognized under the microscope by their lath shaped form and their rectilinear boundaries. These rectilinear boundaries represent on the surface the trace of the twinning plane, the mirror plane of the two twinned crystals. The atoms of the twinning plane are shared by the two neighbouring crystals, and matching across this interface is perfect. For this reason these boundaries are called “coherent twin boundaries ” .(1) Along the coherent twin boundaries there are very often cross-d&continuities in the twinning plane which are called “non-coherent twin boundaries” and exact matching of neighbouring atoms across them is no longer possible. It is also important to stress the very frequent association of

1959

392

VOTAVA

THE

AK~

TWINNING

DISLOCATIONS

IN

COPPER

393

EVIDENCE OF DISLOCATIONS IN COPPER CRYSTALS

It is now generally crystals

BERGHEZAN:

accepted

that the growth

at small supersaturations

of

is made possible,

as predicted by Frank (j), bv the presence of screwdislocations and proceeds b;r the well-known spiral mechanism.(6>7)

The

has a spiral form. valuable

resulting

surface

proof of the presence

For hhe evaporation reasonable

configuration

In turn, the spirals represent

a

of strew-dislocations.

of crystals it seems therefore

to accept the reverse spiral mechanism,(8)

which has already been confirmed for ionic crystals by one of US.(~)In this case the screw-dislocations become visible bg evaporation spirals. This technique has now been extended as an example

to metals,(iO) especially

of a f.c.c.

Cold rolled

to copper

metal.

bars of electrolytic

copper

used, which were heated under vacuum

have been (l-4

x lo-”

mm Hg) in a quartz tube, either by high frequency in a resistance

furnace

at between 900” and 1000°C.

The heating by high frequency 2-5 hr of annealing resistance furnace.

gave as good results in

its did 2-4 days heating

During heating a considerable deposited this

with a

amount of copper is

on the inner wall of the quartz tube.

process

the

recrystallization on etching.

different become

Microscopic

characteristic using

or

crystals

visible

beam

By

during

in the same way as

observation

shows

structure

is formed,

evaporation

multiple

formed

interferometry

this

that

a

and

has been

shown to be a step-structure. The e~aporatioll result

of several

of crystal surfaces is nat~~r~ll~ the processes:

(a) The release

of the

atoms from the lnt’tice and partly direct evaporation (b) Surface diffusion some evaporation. portant

r6le of surface

transformation development oxygen, dissolved

of the atoms and then aga,in (c) MenzeP)

diffusion

the very

which

produces

of the surface configuration. of the step-structure

either

in the remaining

in the

metal,

EIam(12), Menzel’ll),

seems

Chalmers

and BBnard(lQ have shown. experiments

showed

ima

For the

a small amount atmosphere

of

across the twin plane.

At the normal grain boundaries

the lines meet only by chance. (c) ~c)n-~oherexlt

twin

boundaries

a deep channel. clearly visible. developed

In the centre there is an especially well

evaporat~ion spiral which has its origin on

a non-coherent

twin

boundary.

corner a second twin boundary graph;

In the upper traverses

on both sides, the symmetry

lines shows that this boundary plane which surface.

is almost

Removal

and surface

perpendicular

diffusion

has brought

rise to a roof-like

3

twin boundaries,

way the exact

matching

the surface

configur~~tion.

the non-coherent

boundary

of

single spiral at

a micro-twin

(left’ hand

side of the micrograph). Whenever

several

successive

non-coherent

twin

bo~llldaries displaced in the same direction are present, each one of them is associated w&h a spiral having the s;tme screw sense (Fig. 3).

or as

are

revealing

of the atoms

into

Fig. 2 shows a,not,her well developed

change every time a grain boundary is crossed. (b) Evaporation line figures of the two twinned crystals meet at coherent

to the crystal

the low index planes such izs (111) and (100) giving

important : (a) The morphology of the evaporation figures is related to the orientation of the crystals as the figures

in a striking

of the evaporation

of successive layers by evaporation

It is certain that in our

step-structure

left

the photo-

is the trace of a twin

to be necessary as et ~1.“~) and Moreau

of this

spirals or with

In Fig. 1 some of these aspect,s are

the oxygen fixation on the steps helps to

show up this structure. The following aspects

are associated

either with one or more well developed

FIG. 2.

;+,1247

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FIG. 5. Fm. 3.

Figs. 4, 5 and 6 show other examples of the various aspects of single or multiple spirals starting at noncoherent twin boundaries. As the evaporation velocity is related to the step height, the different spirals will have different evaporation velocities, which will be greater for smaller step heights. For this reason, separated multiple spirals can meet together during their development by evaporation (Fig. 4). It is interesting to note that the step-height of the spirals seems to be related to the width of the non-coherent twin boundaries. Even without exact measurements, this is clearly visible on the micrographs (Fig. 4). We must mention here, that the exact measurement of the step-height of the different spirals was not possible because of the rooflike surface configuration on these twin boundaries and the high magnification used. Another feature, extremely important for the present st,udy, is the correlation between the direction of displacement of the twin boundary and the sense of the spiral developed. This is shown in Fig. 7 where left and right handed evaporation spirals are present

FIQ. 4.

x1008

x 1061

x 1352

FIG. 6.

FIQ. 7.

x787

x956

VOTSVA

along

the

same

non-coherent ments. the

AND

twin

boundary

twin boundaries

boundaries

the

DISLOCATIOXS

IX

COPPER

two

displace-

between

the

together.

non-coherent

On this micrograph,

coherent twin boundaries become

two

twin

is much smaller here the two spirals meet

and develop

perceptible

twin boundary and

because

have opposite

TWINNING

The same feature can be seen in Fig. 8, but as

distance

small.

BERGHEZAN:

the non-

are no longer visible;

only by a slight bending

and accordingly

they of the

the step height is very

Finally, Fig. 9 shows the same for a micro-twin so

proves

in

a general

way

this

relation-

ship. Apart

from

boundaries

the spiral figures,

deep channels.

a spiral structure

In another

is visible.

FIG. 8.

x 1029

FIG. 9.

.r:693

crystal

This micrograph

represents also a good general example structure

twin

are marked by

Fig. 10 shows this clearly in the crystal

at the centre of the photograph. above

non-coherent

in some other orientations

of the surface

after evaporation.

(d) Inside the crystals spirals have only rarely been found

and are not related to the non-coherent

boundaries,

twin

as can be seen on the right hand side of

Fig. 2. However,

on single crystals a greater frequency

of spirals has been found as can be seen in Fig. 11. Here,

the evaporation

points

of total

spirals mark

the emergence

dislocations.

Fra. 10.

x1305

395

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When a recrystallization crystal

discontinuities partial The

twin boundary,

are the emergence

dislocations

width

surrounding

these

points of Shockley the stacking

of the non-coherent

therefore directly partial

twin terminates inside the

or has a non-coherent

twin

faults.

boundary

is

related to the number of Shockley

or twin dislocations.

Alternatively

one may

cutt’ing a solid block non-coherent

approach

this matter

by

so as to give a twin having

boundary

in the surface.

a

The block of

the Fig. 12a is cut in the plane ECHG and then the parts tl and /3 are pushed in a direction EG parallel to FIG. 11.

x 722

this plane until the two angles y and y’ are equal. twin is formed

DISCUSSION

The interesting

of Fig.

A

12b.

AB and CD are then the traces of (111) twin planes

fact of this study is the association

of the non-coherent

with the configuration

twin boundaries

either with spiral

and BC the non-coherent the twin

dislocation

twin boundary,

with

screw

created by

character.

So the

figures or with deep channels, both of which represent

step BCFE will wind up in a spiral during evaporation,

the

because it is fixed in C by the twin dislocation.

emergence

surface. total

points

of

dislocation

dislocations

gives

a full

explanation

observed figures, at the non-coherent the total

lines on the

Whereas inside the crystals the emergence of

dislocations

twin relation. Thompson

cannot

produce

the necessary

in studying

and growth of mechanical

the formation

twins have suggested that

some kind of partial dislocations the twinning.

the

Frank(15), Frank and Van der Merveo6),

and Millardo’)

called ‘twin

of

twin boundaries

dislocations,”

of the Shockley type,

can satisfactorily

model explains non-coherent between

boundary,

at the

but also the relation

and width

of the non-coherent

which are both related to the number

of twin dislocations.

A similar approach

twin-dislocations

with

can be used

edge

character.

Here,

that only

a channel

can be

however,

it is evident

expected

after evaporation

(Fig. 13a, 1)).

explain

We think that this explanation

also

/

applies to annealing twins. A twin dislocation

twin

step-height

twin boundary, for

This

not only the spiral formation

/

/

/

is the contour of a stacking fault.

The normal stacking structure of the (111) planes in a f.c.c. metal is: ABC ABC ABC etc. stacking see that

can be written: this fault

A fault in this

ABC& CABC.

represents

which is, inside the crystal, surrounded partial dislocation.(l*) type

on neighbouring

structure

of the form:

twin relationship

It is easy to

an elementary

(111 } planes will build up a A B C A C B A where the

case of two parallel twinning

FIG. 13

by a Shockley

Several successive faults of this

is clearly visible.

(b)

(a)

twin,

A more frequent

planes is:

A B C A B

The

twin

confined

dislocations

to

dislocation

the

or

twinning

Shockley

planes

loops inside the crystal.

of the dislocation

partials

(1 ll}

and

are form

As the character

changes along the dislocation

when the loop arrives at the surface,

line

its emergence

points can have a screw, an edge or mixed character

CACBACBABCABC. -

depending

upon the orientation.

A dislocation emergence

A

position

loop coming

points

which

but of opposite

such loops

coming

to the surface has two

are usually

in the

same

sign. Figs. 7, 8 and 9 show

to the surface in two emergence

points, both of which are screw but of opposite sign a.s the spirals have opposite senses. Fig. 14 shows schematically

(b) FIQ. 12

pla,ne. The relation

these loops

confined

to the twinning

of the recrystallization

twinning-dislocations

is

well

twins to the

demonstrated.

This

VOTAVA

AND BERGHEZAN:

TWINNING

DISLOCATIONS

IN

with different velocities. that

when

boundary of twin

twin dislocation

an

COPPER

The zig-zag boundary

initial

straight

dislocations,

these

to determine

movement traction Fig.

14.

stopping

can occur In

boundary

loops

a recrystallized

This

or the con-

as can be seen in

matrix,

force seems to lie in the tendency

because

dislocations.

by the expansion

to reduce the enclosed surface.

move

a definite crystallo-

of twin

of the dislocation

can

As a consequence,

graphic plane of the non-coherent of the chance

twin

a large number

dislocations

in smaller numbers.

it seems impossible

shows

non-coherent

is large enough to contain

independently

397

the

driving

of dislocation

However,

loops

this cannot

explain the large expansion of these loops in a deformed matrix.‘24) While this movement the

movement

of

the

grain

is very similar t,o

boundaries,(25)

in

a

deformed matrix the loops move away from the centre of curvature,

that is to say in a recrystallized

matrix

the loops usually contract and in the deformed matrix expand. FIG. 14

relation

leads to some consequences

properties

of the twin dislocations.

tant is due to the confinement dislocations

imposed

by the

The most impor-

of the Shockley partial

in the (111) planes so that they can move

only in directions

parallel to these planes.

Thus, the formation

of recrystallization

be related to the octahedral or disappearance

twins must

planes and their growth

must be parallel to the same planes.

In the literature, there areseveral indications,‘1g’20~21) that the growth of recrystallization the

(111)

plane.

observations a double of

technique

to follow

boundaries,

the

twins is parallel to

tried way.

to

verify

these

For this purpose

previously

used by one

displacement

of the grain

was applied to follow the growth and the

disappearance

of annealing

Jacquet’s

formation

have

in a systematic

colouring

US,

used

We

of

twins.

method,(23) thin

surface exhibiting

For colouring

which

sulphide

layers

interference colours.

is made, the layer is dissolved, and after annealing

coloured

we

consists

of

on

copper

the

the

The colouring

the sample annealed

again.

By this method

Fra. 15.

The coherent

main

conclusion

twin boundaries

of twin dislocations. preferential

shows

and to follow

a micrograph

their movement.

in which

the initial

Fig.

15

and final

positions of the twin boundaries are clearly visible. The final position is represented by the zig-zag boundary between the light and dark regions of the photograph. The zig-zag boundary indicates clearly that the movement of the twin boundaries is parallel to the twin plane (1 ll} and that the initial straight non-coherent boundaries move in sections, possibly

of this work

is that non-

are the emergence

points

This casts a new light on the

etching of these boundaries

in contrast to

coherent boundaries,(26,27,2s) and the preferential cipitation at their sites.(2g,22,30)

it is possible to see the initial and final positions of the boundaries

x473

pre-

ACKNOWLEDGMENT

We are indebted

to Dr. R. H. Gillette

for continued

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398

ACTA

METALLURGICA,

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