A STUDY OF G.P. ZONES IN Al-Cu ALLOYS BY ATOM-PROBE FIM

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A STUDY OF G.P. ZONES IN Al-Cu ALLOYS BY ATOM-PROBE FIM

T. Hashizume, K. Hono, Y. Hasegawa, K. Hirano, T. Sakurai

To cite this version:

T. Hashizume, K. Hono, Y. Hasegawa, K. Hirano, T. Sakurai. A STUDY OF G.P. ZONES IN Al-Cu ALLOYS BY ATOM-PROBE FIM. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-171-C2-177.

�10.1051/jphyscol:1986225�. �jpa-00225658�

JOURNAL DE PHYSIQUE

Colloque C2, supplément au n03, Tome 47, mars 1986 page c2-171

A STUDY OF G.P. ZONES IN Al-CU ALLOYS BY ATOM-PROBE FIM

T. HASHIZUME, K. HONO*, Y. HASEGAWA, K. HIRANO' and T. SAKURAI The Institute for Solid State Physics, The University of Tokyo, Minato-ku, Tokyo, Japan

+ Department of Materials Science, Tohoku University, Sendai, Japan

Abstract - G. P. zones in an Al-1.7at% Cu alloy were investigated on an atomic scale using the high-power ToF atom-probe. The Cu concentration of the multi-layer zones was found to be in a range of 25 to 45at%, contrary to general belief that they consist of 100%pure Cu atoms.

1. Introduction

Understanding of the structures of G. P. zones in Al-Cu alloyç, such as the details of the distortions and chemical compositions, is essential in order to advance in developing theories for the

formation of the zones and their effect on the mechanical

properties. Thus the structures of G. P. zones in Al-Cu alloys have been intensively studied for over 40 years by X-ray diffraction, and X-ray and neutron small angle scattering, and more recently by high resolution electron microscopy, and EXAFS. Nevertheless, the

details are still elusive mainly because the G. P. zones are so small and extremely difficult to analyze without any prior knowledge or assumptions.

A simple model proposed by ~erold/l/ is that G. P. zones are single-layer platelets parallel to {IO01 matrix planes consisting of 100% Cu atoms and has been widely accepted as the standard model.

~oman/2/ has, however, suggested based on X-ray diffuse scattering data that the G. P. zones are multi-layer platelets with 100% Cu.

Doi/3/ applied a Fourier analytical method to Toman's data and concluded that the zones are multi-layers and the zone center consists of 100% Cu and the adjacent layers only 50% Cu. James et a1.141 suggested, using the same technique but more quantative measurement, that the Cu concentration at the zones does not exceed 57at%. ~hillips/5/ reported based on his electron microscopy

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986225

J O U R N A L DE PHYSIQUE

study that G. P. zones Vary in thickness (one to 10 layers) as well as composition (20at% to 100at% in Cu concentration). Most recently Fontaine et a1./6/ suggested making use of EXAFS that G. P. zones are single-layers with an average 50at% Cu. Auvray et a1./7/ disputed this conclusion based on their diffuse X-ray scattering and instead suggested that G. P. zones are a mixture of sigle-layers and

multi-layers and both of them consist of nearly 100at% Cu.

There, thus, exists no generally accepted mode1 for the G. P.

zones at present mainly because al1 the techniques applied so far are not best suited for this problem. Field ion microscopy and

atom-probe FIM should be in principle the most suited technique for observing the thickness of G. P. zones and determining their chemical compositions. We report here the first successful analysis of G. P.

zones in Al-Cu alloys using an atom-probe.

II. Experimental technique

The focusing-type time-of-flight atom-probe employed in this experiment has been described elsewhere/8/. The unique features closely related with the present work are: (a) the 100% detection efficiency in a mass analysis mode/9/ and ( b ) the achievement of truly ultra high vacuum Torr range) conditions at the

emitter surface area without condensation of contaminants. Al-based alloys field evaporate easily and do so even more in the presence of a small amount of hydrogen. Thus clean UHV conditions are essential to observe the stable FI image of G . P. zones in addition to

cryogenic temperatures below 20K.

The sample we studied is an Al-1.7at% Cu alloy aged at 403K for 1000 min. after solution treated at 813K. The detailed analysis of the FI images of this material has been reported/lO/. The atom-probe analysis were carried out under the following conditions: sample temperature: 30K, ambient gas: IO-^ Torr Ne gas, pulse ratio:

0.5, evaporation rate: 5 x 10-4 ions / pulse. These parameters were chosen for the best result by careful and exhaustive

examinations. The large pulse ratio (ratio between pulse voltage and dc holding voltage) of 0.5 is essential to prevent the preferential dc evaporation of Al.

III. Results

Making the best use of the atomic resolution of field ion microscopy, Hono et al. have recently shown that there are both single-layer and multi-layer G. P. 1 zones parallel to the 100 matrix planes in Al-l.7at%Cu/lO/. In the specimen many G. P. zones appeared simultaneously within a view of FI image. However atom-probe analysis has been performed using only the G. P. zones appearing at the center of the (200) planes in order to obtain the layer-by-layer compositional analysis. Such G. P. zones appear only once every couple of hundred layers in the direction of [1001.

The FIM image of a G. P. zone formed parallel to the (100) is shown in Fig. 1. The probe-hole of the atom-probe was positioned at the center of the G. P. zone lying in the (100) plane. The tom-probe analysis was carried out setting the initial evaporation voltage low enough so that unwanted field evaporation would not occur. The result is shown in Fig. 2, where the cumulative number of Cu atoms and (Al +Cu) atoms are plotted against the cumulative number of evaporation trigger pulses. Approximately 200 signals were detected by applying over 320,000 pulses, while the evaporation voltage increased £rom 6.20 to 6.28KV during this run, an increase of 1.3%.

From the FI image observed by the auxiliary chevron channelplate detector which only views the atoms within the probe-hole, there are approximately two dozen of atoms covered by the probe-hole. From the layer-by-layer evaporation data we can draw the partition lines

(vertical broken lines) separating signals of each layer £rom adjacent layers, taking into account that Cu atoms evaporate toward

Fig. 1. A FI image of the Al-1.7at% Cu alloy aged at 403K for 1000 min. showing a G. P. zone lying right in the (100) matrix plane.

Imaged using Ne gas at the temperature of approximately 20R.

the end of evaporation due to the higher evaporation field within each individual layer. Fig. 3. is such a plot, the layer-by-layer atomic concentration of Cu inside the G. P. zone. These data clearly suggest that the G. P. zone we analyzed is four-layer thick and consists of approximately 27 at% Cu.

The Cu bulk concentration can be determined £rom the data obtained by evaporation of the large segment of the bulk

(B

Z O +-+

L L O

w w m ZZ

3 .Cu

-

Fig. 2. Layer-by-layer compositional analysis of the four-layer G. P.

zone. Each layer is separated by vertical broken lines. There were 25 Cu atoms in this zone analyzed, first layer:

7, second: 5, third:

7, and fourth: 7 Cu atoms. The zone was followed by the Cu depleted region as is seen toward the right side of the figure.

O _NUMBER OF PULSES 360000

C 2 - 1 7 4 J O U R N A L D E PHYSIQUE

continuously after this G. P. zone are analyzed. In Fig. 4, the cumulative number of Cu atoms are plotted against the cumulative number of the total signals (Al + Cu). The first steep slope (A) at the beginning of the data corresponds to the G. P. zone with an average of 27at% Cu. The next section (B) has a slope of 3.8at% Cu

Fig. 3. The atomic composition of Cu at the G. P. zone. The average Cu

concentration is 27at% with a small tail of two layers.

followed by a large region (C) where the slope is only 0.75 5 0.2 at%

Cu. The last section (D) has a slope of 2.5at% Cu suggesting that the next G. P. zone is intersecting with the probe-hole. The center region in this figure corresponds to the matrix which spans approximately 70 layers in the [IO01 direction. The matrix Cu concentration obtained here, 0.75 2 0.2at%, agrees fairly well with the metastable solvus line of the G. P. zones.

Sizes of regions ( B ) and (D) are approximately 13 and 16 layers thick in the [IO01 direction. The Cu concentration there is

somewhere between those of the G. P. zone and the matrix. It is likely that the probe-hole covers both the matrix and G. P. zone.

Considering that the Cu concentration is rather close to that of the matrix, we believe that the G. P. zones covered within the probe-hole are perpendicular to the (100) plane and appear as straight lines in FI images.

3 O

I

O NUMBER OF Al+Cu ²⁴⁰⁰

Fig. 4. The cumulative number of Cu signals was plotted against the total number of signals. Region (A) corresponds to' the G.

P. zone studied.

Regions (B) and (D) are where the

probe-hole covers both the matrix and G. P.

zone. Region (C) corresponds to the matrix with 0.75at%

Cu.

A number of other G. P. zones were also analyzed using two dozen of FI specimens under various conditions. Some were thicker than the one discussed in detail here. The Cu concentration varies £rom one precipitate to another slightly but falls in the range between 25 to

45at% and averages approximately 35at%.

IV. Discussion

Our result presented above is summarized as follows:

(1) There are G. P. zones of both single-layer and multi-layer of several layers parallel to the {100 1 planes in agreement with the recent work by Hono et a1./10/. and

(2) The Cu concentration of G. P. zones of multi-layer in the Al-1.7at% Cu alloy aged at 403K for 1000 min. is in the range between 25 and 45at% with an average concentration of 35at%.

Tnis conclusion is strikingly different £rom the models proposed in the past, in particular Gerold's single-layer mode1 with pure Cu/l /.

Proposed Cu concentrations

'

distance £rom the center of the zone. 1: the present authors, 2: ~erold/l/, 3:

~homas/l2/, 4: ~oi/3/, and 5: ~oman/2/.

2 4 6 8

Number o f l a y e r s

As to the atom-probe data presented here, one may question its reliability and reproducibility. It is well known that atom-probe data strongly depend on the pulse ratio and residual ambient gases.

When the pulse ratio is not large enough, the element with the weaker bond strength evaporates preferentially-because of the lower

evaporation field, resulting in misleadingly small composition. This effect would be striking in Al - Cu alloys since the evaporation field of Al is 1.3 volt/ # and that of Cu is 3.5 volt/ 2. Indeed the apparant Cu bulk concentration registered by the atom-probe became over 3at% when 0.2 was chosen as the pulse ratio. However no preferential evaporation took place as long as the pulse ratio was kept larger than 0.3 in Our system. In the present work, we used 0.5, even higher than this value in order to assure the reliability of the data.

We also point out that a small amount of hydrogen and/or water in the system ruins the reproduciblity of the data. Our atom-probe is equipped with an additional cryo-pumping system nearby the FI specimen inside the main FIM chamber. The system was pumped at the last stage by this cryo-pump down to 3 x 10-Il Torr before cooling

~ 2 - 1 7 6 JOURNAL DE PHYSIQUE

down the specimen to approximately 20 to 30K. Thus few contaminants would be condensed at the specimen area assuring truly clean UHV conditions during atom-probe analysis.

The Cu concentration of G. P. zones determined in the present study is rather low, compared with the widely accepted values, such as by Gerold/l/. However his conclusion is based on several

simplified assumptions as pointed out by Gerold himself/l3/. More recent studies, such as of Fontaine et a1./6/, suggest that the Cu concentration is not more than 50at%.

It is known that supersaturated solid solutions of Al-Cu alloys decompose by a sequence of precipitation, such as solid solution --

G. P. 1 zones

--

--

(Cu A12). James et a1./4/ have shown that the Cu concentration of G. P. II zones is approximately 30at%. It is also known that 8' has the same 33.3at% Cu concentration as the e phase having a different crystallographic structure. Thus there is little physical basis that G. P. d zones alone should have 100at% Cu concentration. It is only natural that G. P. 1 zones studied here have a composition very similar to the rest of the precipitates, although the interelation between G. P. 1 and G. P. II are not known precisely. There is a report by Eto et a1./14/ that G. P. 1 acts as a nucleation site of G. P. II zones. Realizing these facts, Our present result that G. P.

zones have only approximately 33at% Cu, obtained with no a priori assumption, can be well accepted.

V. Conclusions

We have succeeded in analyzing G. P. zones in Al-1.7at% Cu aged at 403K for 1000 min., for the first time on an atomic scale. The chemical compositions may Vary £rom one zone to another slightly but fa11 in the range between 25at% to 45at% in Cu and averaged 35at%.

This result disagrees with Gerold's widely quoted conclusion that a G. P. zone is a single-layer of pure Cu atoms.

VI. Acknowledgements

We would like to thank Dr. A. Sakai, Miss A. Jimbo and Professor K. Osamura for fruitful discussions.

References /1/ V. Gerold, Z. Metallkd. 45, 599 (1954).

/2/ K. Toman, Acta Crystallogr. 8,587 (1955), IO, 187 (1957), and 13, 60 (1960).

/3/ K. Doi, Acta Crystallogr. l 3 , 45 (1960).

/4/ D. R. James and G. L. liedl, Acta Crystallogr. l 8 , 678 (1965).

/ 5 / V. A. Phillips, Acta Met. ZI, 219 (1973).

161 A. Fontaine, P. Lagarde, A. Naudon, D. Raoux and D. Spanjaard, Phil. Mag. 40, 17 (1979).

/ 7 / X. Auvray, P. Georgopoulos and J. B. Cohen, Acta Met. 29, 1061

1198'7 1 .

/8/ T.-sakurai, T. Hashizume and A. Jimbo, Appl. Phys. Letts. 44, 38 (1984).

/9/ T. Sakurai and T. Hashizume, Rev. Sci. Instrum. in press.

/IO/ K. Hono, T. Satoh and K. Hirano, Phil. Mag. B in press.

/Il/ M. Yamamoto, T. Hashizume and T. Sakurai, Scripta Met. l 9 , 357 (1985).

/12/ A. D. Thomas, Ph. D. Thesis, Purdue University, Lafayette, Indiana, USA ( 1 961 ) .

/ 1 3 / V. Gerold, Acta Crystallogr. 11, 230 (1958).

/14/ T. Eto, A. Sato and T. Mori, Acta Met. 26, 499 (1978).