SURFACE SEGREGATION OF Cu-Ni ALLOYS

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SURFACE SEGREGATION OF Cu-Ni ALLOYS

T. Hashizume, A. Jimbo, T. Sakurai

To cite this version:

T. Hashizume, A. Jimbo, T. Sakurai. SURFACE SEGREGATION OF Cu-Ni ALLOYS. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-435-C9-439. �10.1051/jphyscol:1984972�. �jpa-00224460�

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Colloque C9, supplément au n012, Tome 45, décembre 1984 page C9-435

S U R F A C E SEGREGATION O F Cu-Ni ALLOYS

T. Hashizume, A. Jimbo and T. Sakurai

The I n s t i t u t e for S o l i d S t a t e Physics, The University o f Tokyo, Roppongi Minato-ku, Tokyo 106, Japan

Résumé - Nous avons entrepris une étude de la ségrégation de surface d'alliages Cu-Ni dans tout le domaine de composition, en utilisant notre sonde à atomes à temps de vol. La grande efficacité de détection de cet instrument le rend particulièrement adapté aux études de ségrégation. Les alliages pré- parés sont Cu-x% pds Ni : x=3,6,9,15,20,30,45,80,90 et 95. Nos résultats sont

1) Les éléments en solution tendent à subir une ségrégation vers la surface lors du recuit dans une gamme de températures comprise entre 300 et 600°C.

Typiquement, les atomes de Ni présentent une ségrégation dans les alliages riches en cuivre. 2) Le cuivre tend à s'évaporer d'abord, laissant intacts les atomes de Ni de la même couche. Ceci rend possible l'imagerie de la distribution des atomes Ni dans une couche mélangée. Enfin 3) Cu tend à former des multicouches pures sur les trois plans principaux (IOO)., (110) et ( 1 1 1 ) quand la pointe est recuite même à des températures faibles, voisines de 300°c.

Abstract - We attemped to inverstigate surface segregation of Cu-Ni alloys in the entire composition range using our ToF atom-probe whose superior detection efficiency makes Our machine most suited for segregation study.

Alloys prepared are Cu-x wt% Ni: x = 3,6,9,15,20,30,45,80,90 and 95. Our findings are: (1) Solute elements tend to segregate to the surface upon annealing in the temperature range of 300°C to 600°C. Namely Ni atoms segregate in Cu rich alloys. (2) Cu tends to evaporate first, leaving Ni atoms on the same layer intact. This makes it possible to image the distribution of Ni atoms on a mixed layer. And (3) Cu tends to form pure multilayer on the three principal planes, (100), (110) and (111) when the tip is annealed even at low temperature of approximasely 300°C.

Surface segregation of Cu-Ni alloys has been investi ated extensively by various techniques such as AES(~), 1~S(2)and a t ~ m - ~ ~ o b e f i ~ ) (4). Watanabe et al.

in particular had studied this alloy system over the entire composition range by AES(~). By utilizing the difference in excape length of Auger electrons with differnet kinetic energies, they concluded that the first layer shows significant Cu enrichment over the entire composition range (Fig.1). However their high energy data shows slight Ni enrichment in the case of Cu-10 wt% Ni alloy. Even low energy data at this dilute Ni alloy shows significant deviation from the normal Cu enrichment tendency. Furthermore, AES is by no means an ideal analytical technique to study the compositional depth profile of binary alloys because of the uncertainty of the escape length of the Auger electron. Y.Kuk et al. had indeed observed evidence of Ni enrichment in a Cu-10% Ni alloy using a ToF atom-probe at Penn State ( 4 ) . However the experimental conditions then were.not favorable enough to reach the unambiguous conclusion on solute segregation. It is thus Our aim here upon constructing the superior ToF atom-probe to re-investigate Cu-Ni alloy system and to settle this unsettling question.

Indeed Ire have observed Ni enrichment for Cu-3,6 and 9 wt% Ni alloys.

Cu segregation was observed in the Cu concentration of 5 to 70 wt%. Fig.2 shows the data during the sequential evaporation at the (111) plane in the Cu-3.5 at % Ni (nominal bulk concentration: 5 wt % ) . This run was taken after annealing at 600°C fior 60 sec in vacuum and quenching down to 50K. Here one

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984972

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C9-436 JOURNAL DE PHYSIQUE

can see the segregation of Ni and P unexpectedly. Also layer-by-layer evaporation can be seen and if one utilize the experimental fact that Cu atoms field evaporate first at the mixed plane and the remaining Ni atoms tend to evaporate at the final stage of layer evaporation. Each layer probed by the atom-probe is marked by arrow. The size of the each layer being probed is ~ 5 0 atoms although the first few layers contain more atoms (Q60 atoms), possibly due to Ni and P segregation. Fig.3 is the enlargement of the initial stage of sequential field evaporation, Fig.2. Eleven Cu atoms are field evaporated and detected first which is followed by phosphorus and nickel atoms. Toward the end of field evaporation of Ni, Cu atoms are detected and the field evaporation of the first layer appeared to be completed. Phosphorus segregation is limited to the outermost layer only. At the second layer, again Cu atoms evaporate first exclusively and Ni atoms were detected only toward the end of the layer evaporation. This data can be reduced to map out the depth profile of Ni and P at annealing temperatures of 870K and 970K. Ni is approximately 32% and 14% at the first and second layer, respectively and returns to the bulk value thereafter.

P abundance at the first layer is ~ 3 5 % for both 870 and 970K and decays rapidly to zero. Another Cu-3.5 at % Ni wire was tested and similar Ni segregation data was obtained but without phosphorus this time. The result (Fig.5) shows a slightly higher degree of Ni segregation, 40% and 34% at the first layer and at 970K and 870K respectively. Ni segregation decreases gradually and reaches the bulk value at the 6th or 7th layer. It appeares that P and Ni compete for

surface sites for segregation and that P suppresses segregation of Ni in this case.

Copper abundance upon annealing at the outermost layer is plotted against the bulk concentration for various alloy compositions and compared with other data(Fig.6).

It is striking to see that Ni segregates by a large amount in the solute Ni concentration range of 3 to 10.1 at %. In the Cu concentration of 5 to 70 at %, Our result shows Cu segregation, in a qualitative agreement with the data reported by others. These results would suggest that there must be an alloy of certain composition vhich does not produce any surface segregation. We have indeed observed this phenomenon with a Cu-15 at % Ni alloy. This alloy-have s h o w no surface segregation at al1 upon annealing at 870K.

Since a simple theory on surface segregation of Ni-Cu alloys does not produce this kind of segregation diagram obtained here, we have no immediate answer to this result. But solute segregation of Ni is conclusively established.

References

(1) K. Watanabe etal., Surf. Sci. 2, 721 (1977).

(2) H. H. Brongersma etal., Surf. Sci. 71, 657 (1978).

(3) Y. S. Ng etal., Phys. Rev. Lett. 588 (;979).

(4) Y. Kuk, Ph. D. Thesis, The Pennsylvania State University 1981.

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Fig.2 Our atom-probe data 200 in the case of Cu-3.5 at% Ni alloy shows enrichment of Ni at the surface. Q

W

Accidentally we also found =;

P segregates to the surface.

n W

However at 90% Cu, the experimental data deviates from its trend.

Bulk composition ( % Cu)

L I

NUMBER OF PULSES 24000

IP trm

--

^<-

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C9-438 JOURNAL DE PHYSIQUE

Fig.4 Depth profile of Ni and P after annealing at 870 and 970K. Enrichment of Ni is evident over the first 2 to 3 layers.

Fig.3 Sequential plot of individual elements shows that eleven Cu atoms evaporate first and 18 P's and 9 Ni's follow to complete the evaporation of the outermost layer.

OIP iim mrr t -.<ai

CU-3.5atXNi(l l l )

Annealed f o r 60sec i n vacuum

0 N i ( 8 7 0 K i

A P (870Ki O Ni(970K) P (970K)

1 l

O 2 4 6 8 1 O

# OF LAYERS FROM THE SURFACE

S e g r e g a t i o n o f N i t o t h e

X

50

-

( I I I ) + u r f a c e o f Cu-3.5atXNi upon anneal i n g i n vacuum

6

⁴⁰

-

⁰870K.15sec

f--

30 O 970K.lilsec

+- Z W

2

20

O O . , I O

Z

O

O 2 4 6 8

# OF LAYERS FROM THE SURFACE

Fig.5 In the case of a P free sample Ni segregation is even more pronounced, up to 40% at the outermost layer, and decays gradually.

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L E I S

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9 A + A

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enrichment in the Cu bulk concentration up to 70 at%.

However beyond that Ni does segregate to the surface as much as 40%.

B u l k concentrat ion(at%Cu>