Observation of monolayer Guinier-Preston zones in Al-at 1.7 % Cu

(1)

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Observation of monolayer Guinier-Preston zones in Al-at 1.7 % Cu

Bernard Jouffrey, Dominique Dorignac

To cite this version:

Bernard Jouffrey, Dominique Dorignac. Observation of monolayer Guinier-Preston zones in Al-at 1.7 % Cu. Journal de Physique I, EDP Sciences, 1992, 2 (6), pp.1067-1074. �10.1051/jp1:1992196�.

�jpa-00246588�

(2)

J. Phys. I France 2

(1992)

1067-lo74 JUNE 1992, PAGE lo67

Classification Physics Abstracts

81.40 07.80 81.30M

Observation of monolayer Guinier-Preston

_zones

in Al-at 1.7 % Cu

Bernard

Jouffrey(~

and

Dominique Dorignac(~)

(~ Ecole Centrale Paris, LMSS-Mat, Grande Voie des Vignes, 92295 Chitenay-Malabry, France (~) ^CEMES-LOE CNRS, B-P. 4347, 31055 Toulouse Cedex, France

(Received

21 February 1992, accepted in final form 6 April

1992)

Rdsumd. La prdsence de _zones de Guinier-Preston

(zones GP)

aprbs trempe et recuit

(ici

20 heures h

130°C),

explique le durcissement des alliages 16ger d'aluminium-cuivre, qui est I

l'origine de leur intdrdt pratique. Nous prdsentons dans l'article qui suit, des observations mendes

en microscopie h haute rdsolution

(400 kV),

qui permettent ^d'allirmer _que les _zones observdes

(GPI)

sont des monocouches atomiques, plus riches _en cuivre _que la matrice. Une structure ordonnde peut dire observde I l'intdrieur de certaines

zones.

Abstract The formation ofGuinier-Preston _zones

(GP zones),

after quenching then followed by annealing

(in

^this _paper 20 hours _at

130°C),

explains the hardening ^of aluminum-copper

light

alloys, which is at the origin of their usefulness in metallurgy. In this _paper, recent results in transmission electron microscopy

(400 kV)

_are presented. Atomic resolution observations show

that _many GP _zones _appear _as monolayers

(GPI).

Contrast simulations show that the observed

contrasts _can be interpreted _as due to zones rich in _copper. Ordering _can be observed inside

some of them.

These last _years, electron

microscopy

^has ^made

quite spectacular improvements.

In _par-

ticular,

transmission electron

microscopy

has _now _a

resolving

_power at the level of atomic dimensions. This

technique

is

capable

of

giving

structure

images

and _so to detect atomic columns _even in metals which need _a

point

to

point

resolution of 0.2 _nm _or better.

Moreover,

it is

capable

^of ^elemental

analysis

at a level of1 nanometer _or less in

specific

_cases. So it is

interesting

to look

again

at some old

problems

^which are not

completely

solved and _are still the

subject

of controversy. One amongst ^these

questions

^is ^related to GP

zones in aluminum

alloys,

and

specially

in

aluminum-copper alloys,

which present a

hardening following quenching

and

subsequent annealing.

This

hardening

makes the _use of this category ^of

light alloys (with

more than two

components),

_very _common. Since the

early

works _on GP _zones and the model

independently proposed by

Preston

[ii

^and Guinier [2] on the first steps ^of

precipitation

in

supersaturated

solid solution of aluminum

containing

_a few percent of copper,

important

work

(3)

1068 JOURNAL DE PHYSIQUE I N°6

has been carried out.

The scheme which is

generally

admitted _can be drawn from

a review and _a work due to

Phillips

_[3].

Solidsolution

- GPzones

- 0"

- 0'

-

0(CuA12)

(GPI) (GP2)

At

150°C, typical

values of

annealing

time _are in table I:

Table 1.

0 to10 hours GPI

10 to 60 hours GP2 0"

quadratic

> 60 hours 0'

quadratic

The parameters ^which are

given

in the literature _are in table II:

Table II.

a

(nm)

_c

(nm)

_c

la

0"

quadratic

0.405 0.763 1.884 0'

quadratic

0.573 0.581 1.014 0

quadratic

0.6066 0.4874 0.803

In their

original model,

Guinier and Preston

analysed

_a GP _zone _as

composed

of _a

single (100) copper-rich plane

surrounded

by

aluminum atomic

planes

with _a

slightly

shorter distance

from the

original plane

than in the solid solution.

From

X-ray

measurements [4] ^it ^has ^also been

proposed

that GPI _zones _were _not

only

copper

monolayer

zones.

They

could be _up to _a few atomic

planes

thick. Different models

were

proposed by

Guinier [5], ^Gerold [6], ^Toman [7].

Following

the X-ray

work,

electron

microscopists

_[3,

8-14]

^tried to confirm and

complete

information

provided by

_means of X- rays. References _can be found in

[15]. Using

EXAFS

technics,

but also

X-rays,

controversial

proposals

have been also made

recently

[16, 17].

Following

the formation of

GPI, by increasing

the

annealing

time

(Tab. I),

or the temperature, _a _new distribution of copper ^atoms is

observed,

which has been called either f"

by people

who considered that it is _a _new

phase

_or GP2

by people

who think it is not

really

_a

phase

but rather _a kind of

ordering. X-ray

diffraction sho,ved

that GP2

platelets

have _a maximum thickness of10 _nm. The maximum diameter _was

thought

to be 150 _nm. It is

generally

admitted that GP2 _are

essentially

constituted of tivo copper rich

layers separated by

three Al

lay,ers

_[18]. Guinier admitted that the 0" _zones _are coherent ivith the matrfi~ both _on their

(100)

^Al ^habit

plane

and in the <100>

perpendicular

directions with

a 4il misfit.

The 0'

phase

is

quadratic

with _a lattice parameter identical to the _one of the FCC aluminum mat.rix. This

phase

is semi-coherent with the matrix. The 0'

(100)

face is flat and coherent with the matrix. The

equilibrium phase, 0,

is incoherent.

It has been claimed that both kinds of GP _zones have been observed

by

_means of

high

resolution electron

microscopy,

but these

micrographs

have been taken in

using

tivo _or many beam lattice reflections ivith tilted illumination [11]. ^In ^these

conditions,

the

interpretation

of the

image

is

unfortunately

difficult. Another

study

has been carried out _a feiv _years ago [19]. ^The

required

condition to observe _a lattice

image

^is to be in _a

symmetric

situation _as it is shown _on the diffraction pattern ^of

figure

1. It _means that the diffraction pattern ^has to be very well oriented what is done in

figure ((001)

_zone

axis)

but not in

figure

2

((110)

_zone

(4)

N°6 MONOLAYER GP ZONES IN Al-at 1.7% Cu lo69

Fig. 1. Fig. 2.

Fig. 1. Diffraction pattern corresponding to _a <loo> _zone axis. The streaks related to the _presence

of GP _zones _are clearly visible.

Fig. 2. Diffraction pattern corresponding to _a < ii o> _zone axis. The streaks related _to the presence of GP

zones are visible in _one preferential direction due to the _presence of only _one family of GP _zones

in this orientation. The sample, here, is not perfectly _zone axis oriented.

axis).

With _very thin

samples,

the orientation is _a little _more difficult _to be obtained

compared

to thicker

samples

for which Kikuchi lines enable _a _very

high precision.

We tried about ten years ago ^to confirm the _presence of

copper-rich monolayer

_zones. In _our first

result, using

the 500 kV HAREM of the

University

of

Kyoto,

_we succeeded in

observing

zones

composed

of two

layers

rich in copper [20]. ^More

recently

_[8,

21],

we were

able, using

the 400 kV CENG

microscope

to observe GPI

monolayer

_zones. The

samples

_were Al-at 1.7il Cu

(Al-wt 4ilCu). They

_were

quenched

from _a temperature close to the

melting point

and

subsequently

annealed for 10 hours at 100°C and for 20 hours at 130°C. We present in this paper results

concerning

the second

annealing.

. DiiLraction Two orientations _were

privileged

for the

observations, (100)

and

(110).

As it is well

known,

streaks _appear, due _to the small lateral extension of GP _zones

(Fourier transform).

Both orientations have been used with _zone axis

oriented,

in such _a way that

they

were available for

high

resolution

(see above).

That _means also that the thickness of the

samples

had to be very

small,

of the order of10 to 15 _nm. In the

(100) orientation,

streaks

appear in two

perpendicular

orientations.

They correspond

to the _two

(010)

and

(001)

GP

zones

families,

the _zone axis

perpendicular

to the foil

being

[100]

(Fig. I).

The third

family,

that

lying

in

(100) planes,

also

produces

streaks

parallel

to 100 in the

reciprocal

_space, which is

body

centered

cubic,

from each fundamental reflection. The section of these streaks

by

the Ewald

sphere,

here the

plane

of the diffraction pattern ^of ^the [100] zone axis

gives

rise to the dots located at

(011).

With the other

orientation, (110),

the streaks _appear in _one direction

only (Fig. 2),

the _one,

[002],

^which ^is

perpendicular

to the

(002) planes,

^after ^rotation around

(5)

lo70 JOURNAL DE PHYSIQUE I N°6

the normal to this

plane

from [100] zone axis.

These orientations _are favourable _to the

study

of GP _zones, in

particular

for

high

resolution atomic

imaging.

^These diffraction patterns have been taken in thicker _areas but close to the

ones used for

high

resolution

imaging.

Fig. 3. <loo> _zone axis orientation. Atomic columns _are visible. Only the _zones which _appear _as series of bright doted lines _can be conveniently interpreted. They _are extended from the top to the bottom of the sample. The other _ones end inside the sample _or present some surface relaxation due to _an oxyde layer for instance.

.

High

resolution The two

foils,

were

privileged

for atomic

imag- ing.

The

positions

of substitutional Cu atoms _can be

explored through

^these two orientations.

We have shown that the

complementary

informations obtained from these two orientations

are

quite

useful. It

is,

in particular,

interesting

to observe that the

(l10)

orientation is

quite

favourable to the observation of

copper-rich monolayer

_zones. This orientation

corresponds

to the most dense columns of _atoms.

Figures

3 and 4 show two

micrographs corresponding

to both orientations. It

clearly

_appears that the observation of

single

_copper ^rich atomic _monc-

layers

is much easier with the orientation of

figure

4. This

point

is not very

surprising,

_as it is known in other materials where the most dense orientation is favourable to the observation of atomic columns. As it _can be observed _on

it,

^GPI _zones _appear as atomic

monolayers.

This

micrograph

_seems to present also artefacts

(moird patterns)

due to

deposition

of _copper

during

the

polishing,

_as _we showed

by

_means of local

X-ray

elemental

analysis,

and _as it _can be also

thought

^from the diffraction pattern of

figure

2. It could be

thought,

that this moird is

originated

in the _presence of inclined GPI _zones, because the

density

of moir4

patterns

are

important

and not,

here,

in contradiction with the

density

of GP _zones in the

sample.

The

origin

of these contrasts is not

completely

understood and could be related for instance to the

change

of parameter ^due to _a local misfit between the _zones and the matrix.

However,

it _was

(6)

N°6 MONOLAYER GP ZONES IN Al-at 1.7% Cu lo71

Fig.

4. <llo> _zone axis orientation. This orientation _seems the best _one to studying ^GP zones.

It corresponds to the _more dense columns of atoms. GP _zones _appear clearly _as monolayers. ^Moird patterns _seems to be artefacts due to impurities deposited _on the surface during polishing.

never observed under the

(100)

orientation, and is not

reproducible. Recently

Karlik

[22],

was

able to prepare cleaner

samples.

It is clear that extreme

precautions

have to be taken to be _sure about the

interpretation

of these

monolayer

_zones. In

particular,

the _zones _can also _appear _as

multilayer

_zones for different focus. Simulations confirm the

experimental

results.

Figure

5

presents

a zone which have been observed under

a

(100) orientation,

with _a correct focus _to avoid

misinterpretation.

Another condition for

observing

the _zones

correctly

^is to select _zones extended

through

the thickness of the

sample,

from the top to the bottom.

Figure

5 shows also the

corresponding

simulation.

Figures

^{6 and} 7 show also

simulations,

for the _same

sample

and thickness, but different

focussings.

It _appears that _some

misinterpretations

could be deduced from _a _too

rapid analysis,

since in the different _cases which _are

presented here,

the contrasts _are

corresponding

to the _same _zone. The thickness of the

sample

is about 10

nm. This thickness has been determined from the best

agreement

^between simulations and

experiments.

The simulations have been carried out with 100 it of substitutional _copper atoms

in the _zones. It is _now clear that this situation is not

always

the _case. At the

opposite

of _our

first work _on the

subject,

_we did not introduced here _any

change

in the parameter ^around ^and

in the _zone. In _our _case,

here,

it would not

change

the essential of the

results,

_as these

changes

are very small.

Unambiguously single plane

_zones have been observed _very often in both orientations. Their size is

variable,

of the order of 4 to 10 _nm in diameter.

.

Ordering

^One

interesting point

comes out from _a careful observation of

figure

4. One

zone has been

magnified

_as shown in

figure

8. It

clearly

_appears that _some columns of atoms, inside the _zone presents a

periodical

contrast. Every two

columns,

the

intensity

of _an atomic

(7)

GUIWER.PRESTON ZONE: COPPER MONOLAYER IN Al 1.7 at% Cu

0,2048nm

Al

_

p~oj~ci~D poi~~Tj~~

Fig. 5. - <lo0> orientation. _Micrograph and

correspondingmultislice

simulation. The

the ample 10 nm. The focus is indicated. No eformation of the

the contrast C is _given by the ratio C =

(Imax - Imjn) _/Imean, where Imean is

the total

ZONE: COPPER

0,2048nm m

~

AI

= =

i

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Z

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Ml_ _

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Fig. 6. - imulation corresponding to a different focus.

(8)

N°6 MONOLAYER GP ZONES IN Al-at 1.7% cu lo73

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Fig. 7. This simulation in the _case of another focus, shows that the monolayer _zone _can _appear _as

more complex _even if it is not. This experimental condition doesn't give the structure image.

column is the _same _as the column in the matrix appear. This is

observable,

at least _on _one part ^{of the} zone. This

point

_means that _some columns of atoms are rich in _copper and others

are not. There is present an

ordering

inside the

monolayer

_zone. In

addition,

it _means that copper ^atoms are not in this _case 100 lt

forming

the _zone. One

goal

is _now

understanding

the

precision

which _can be obtained _on the determination of the percentage of copper ^atoms in _a column. In other

words,

what is the influence of the number of substitutional _copper atoms _on the contrast. When this

point

is

clearly understood,

it is

possible

to

give

^and answer on the

percentage of _copper atoms in _a GPI _zone, what is not

really

the _case until _now.

Fig.

8. This

enlargement

of figure 4 shows that

a zone presents ordering at least

on one part ^{of it.}

Columns of atoms appear successively _as rich in copper, rich in aluminum etc.

(9)

. Elemental

analysis

An

approach

to the determination of the elemental

composition

of

GPI _zones has been carried _out

through

another way.

Obviously

the

problem

is not

simple.

We made _some

experiments by

_means of characteristic

X-ray

emission and EELS. The

only

result _we obtained until _now is the evidence of _an increase of Cu content at the level of _a _zone.

Our present ^work ^is now to

approach

the

composition

of the _zones

through

local direct elemental

analysis, compared

to simulations and atomic resolution

experiments.

Acknowledgements.

The authors _are indebted to A. Bourret and J. Thibault for

a kind _access _to the 400 kV CENG

microscope, spending

time to

explain

the tricks for _a

good

_use of this

microscope

and

helpful

discussions.

References

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