ATOM-PROBE STUDY OF SURFACE SEGREGATION OF Ni-Cu ALLOYS

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ATOM-PROBE STUDY OF SURFACE SEGREGATION OF Ni-Cu ALLOYS

T. Hashizume, T. Sakurai, H. Pickering

To cite this version:

T. Hashizume, T. Sakurai, H. Pickering. ATOM-PROBE STUDY OF SURFACE SEGREGA- TION OF Ni-Cu ALLOYS. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-381-C2-387.

�10.1051/jphyscol:1986259�. �jpa-00225693�

ATOM-PROBE STUDY OF SURFACE SEGREGATION OF Ni-Cu ALLOYS T. HASHIZUME, T. SAKURAI and H.W. PICKERING'

The Institute for Solid State Physics, The University of Tokyo, Minato-ku, Tokyo, Japan

Abstract

-

A focusing-type ToF atom-probe has been designed and constructed successfully, with superb features, such as 100% detection efficiency and precise alignment of a probing area. Using this instrument, surface segregation of the Ni-Cu alloy system was investigated in detail. Experimental evidence are presented for solute Ni segregation to the surface, contrary to the general belief that Cu atoms always segregate to the surface. A new insight is needed in surface segregation to account for this observation of N i segregation in Nil-,CuX alloys where X is 0.84 to 1.

I

-

INTRODUCTION

Surface segregation of binary alloys has been intensively investigated over the last twenty years, since it became known that the catalytic activities of alloys are determined almost exclusively by their surface properties/l/. One of the most studied bimetallic catalyst systems is the Ni-Cu alloys/2/. Extensive studies have been performed using Auger electron spectroscopy (AES)/~-S/, X-ray and ultraviolet photoemission spectroscopies (XPS and UPS)/6-8/, titration method/l/, low energy ion scattering LE IS)/^/ and time-of-flight atom-probe (TOF-AP)/~O-11/. In these studies it has been conclusively shown that Cu segregates to the surface upon annealing at the temperature range of 600 K to 900 K.

Various theories have been developed based on bond

strength/l2/, atomic size, thermodynamic parameters of binary alloys/13-141, such as solidus and liquidus lines of phase diagrams/l3/, and surface energy/l4/ as well as microscopic

electronic theories/lS/, in order to account for the enrichment of copper at the surface. Some argued that the combination of bond breaking theory and strain energy theory can successfully predict almost all currently available segregation data/l6/. Apparent good

'The Department of Materials Science and Engineering, The Pennsylvania State University, The University park, Pa 16802, U. S.A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986259

JOURNAL DE PHYSIQUE

agreement between theory and experiment has helped to establish that Cu does segregate to the surface in the entire concentration range.

There is, however, considerable controversy as to the details of Cu surface segregation. For example, some people claim that the Cu enrichment extends to the first few layers15, 171, while others report an oscillatory behavior in the compositional depth profile/6,10/. A monotonic decrease in Cu composition is also suggestcd/7/. A possibility of Ni segregation was also suggested in some experiments, but with some doubts, because of poor experimental conditions/5, 181. Our detailed study has shown ,beyond any doubt, that Ni segregates to the surface when the bulk Ni concentration is in the range of 0 to 16at%: the first definitive experimental evidence of a crossover of surface segregation in the Ni-Cu solid solution alloy system.

I1

-

INSTRUMENTATION

Our focusing-type ToF atom-probe has many unique features compared to other existing atom-probes, such as the 100% detection efficiency making use of both channelplate and channeltron as a signal detector/l9/, and direct viewing capability of the probing area by a newly incorporated imaging device behind the probe-hole F . 1 These features make our instrument the most advanced among the existing atom-probes and highly suited for microscopic studies, such as surface segregation of Ni-Cu alloys.

C R Y 0 PUMP

w. Fig.1. Schematic of the focusing ToF atom-probe.

This instrument has several advanced

v. features, such as (1 ) the. 100%

ELECTROSTATIC LEN detection

efficiency and (2) precise alignment of the probing area by an additional imaging device behind the probe-hole.

TIP HOLDER TOP VIEW

I

IIT

-

EXPERI14ENTAL

Ni-Cu alloy ingots of various compositions (nominal Cu at% = 1,3,6,10,15,20,30,50180185195197,99) have been prepared from high purity Ni ( 9 9 . 9 9 9 % ) and Cu ( 9 9 . 9 9 % ) materials (Johnson-Matthey).

They were pressed into thin sheets and cut into thin rods of 0.5 mm X

0.5 mm. These rods were pulled into fine wires of 0.1 mm

diameter. A specimen tip was prepared by electrochemical etching of the wire. The tip was spot-welded on a MO wire of 0.1 mm diameter and in-situ annealing of the tip was performed by joule heating of the MO loop.

then taken to check the bulk composition and its uniformity. The tip was then annealed in vacuum (5 ^X 10-I Torr) at approximately 800 20 K for 30 sec to 10 min. in a zero field condition to equilibrate the bulk distribution. The tip was immediately cooled down to 20 K with a cooling rate of approximately 104 degree/sec, fast enough to approach freezing of the equilibrium distribution at the annealing temperature.

Gradual pulse evaporation of the tip surface was attempted starting at a low tip voltage, approximately 1/10 of the best image voltage by placing the probe-hole at the center of the desired crystallographic orientation, the (111) plane, in the present work.

The pulse repetition rate was 100/sec, the pulse to D.C. holding voltage ratio, r

,

^was > 0.75, and the rate of evaporation was

0.003 atoms/pu?.se, extremely slow to ensure accurate compositional analysis. The value of rp was kept >0.15 to assure that Cu does not preferentially field evaporate during atom-probe analysis/lO/.

Each signal, detected by a channeltron with 100% detection efficiency/l9/, is digitized in terms of flight time by a 200 MHz digital timer and is converted to mass-to-charge ratio (m/n). This value is displayed on the screen with appropriate identification of the chemical species and is simultaneously plotted on the second screen in the form of compositional depth profiles of the individual elements.

I11

-

I

.

CU SEGREGATION

When a tip of a Ni-5at% Cu alloy was analyzed, we obtained a uniform bulk composition of 5.4 0.2at% Cu over several hundred layers in the direction of the < I l l > axis. Upon annealing at 900 K for 30 sec, the atom-probe analysis of the surface region (over 20 layers) has shown strong Cu enrichment, 56 -+ 4at% at the first layer, in agreement with the results obtained by others/5,9,10/. Cu enrichment at the surface was observed for all the alloy specimens in the range of O< Cu at% < 84.

111

-

^2. Ni SEGREGATION

However, when a tip of Ni-99.5at% Cu was studied, Ni enrichment was found at the surface region. A typical example is shown in Fig.

Fig.2. Atom-probe

N i-99.5at.%Cu

data upon annealing

a Ni-99.5 at% Cu

920K. 30sec.

tip for 30 sec at

920K in vacuum of 5

i n

vac.

^X 10-l1 Torr.

Cumulative numbers of Cu and Ni signals coming from the (111) plane are plotted against cumulative number of evaporation trigger pulses.

NUMBER OF PULSES

3 . 8 x 1 0 6

JOURNAL DE PHYSIQUE

concentration at the (111) plane c

of Ni-96.5at% Cu at two different temperatures, 870 K and 970 K, show a slight difference but are essentially the same;

approximately 40atb Ni at the first layer and decreasing slowly to the bulk value over 4 layers.

2, where the cumulative numbers of Cu and Ni signals from the (111) plane are plotted against the cumulative number of evaporation trigger pulses, from the beginning of evaporation immediately after annealing the field evaporated clean tip at 920 K for 30 sec in vacuum ( < 5 x 10-I Torr). Field evaporation of the top 4 layers are shown here, which was achieved by administrating over 2 million pulses. Since the information of atomic composition of each layer is well separated from the adjacent layers in our atom-probe anlysis, it is possible to obtain the compositional depth profile at the surface region with single atomic layer resolution without any ambiguity. (vertical broken lines in Pig. 2) Inspite of the fact that the bulk Ni concentration is only 0.5at%, 91 out of 270 signals at the first layer were Ni atoms, which corresponds to 33.7at$ Ni.

Fig. 3 shows the compositional

2

\ P

- -

-

- -

- - -

- - -

[ ' / ^6.2

0 0 2 4 6 8

#

o f

l a y e r s

Fig. 3. Depth profile.

depth profiles for Ni-6.2, 41.8, 56.5, 96.5 and 99.5at% Cu alloy tips. From this figure, one can immediately notice that (1) in the case of Ni enrichment, namely,

96.5 and 99.5 at% Cu alloys, its

80

segregation extends three to four layers and decays gradually, while Cu surface segregation is

layer and decays rapidly in the .-I

c, " '

essentially limited to the topmost

60.

case of Cu enrichment (6.2, 41.8, (d

and 56.5at% alloys), and (2) the L

4 0

depth profile in the case of Cu

segregation exhibits an _(U oscillatory behavior, first noted

2

by Ng in the case of Ni-Sat% Cu

alloy/lO/. We also point out

S 20

that the depth profiles of Ni

.-.

I11

-

^{3. SUMMARY}

Using the alloy specimens of various compositions, the atom-probe analyses were performed over the entire composition

range. The results are shown in Fig. 4 in the form of Cu enrichment at the outermost layer against Cu bulk composition, together with data obtained by others/3,5,9,10,17/.

In the case of alloys in which Cu is the solute, strong Cu enrichment has been well documented, although significant scatter exists in the data. Nowever, there are only few data points

available in the Ni solute range. Our atom-probe data clearly show that Ni segregates toward the surface when the Ni concentration is less than 16at%. At 84at% Cu, the crossover point, neither Cu nor Ni segregates to the surface upon annealing at 820K.

---,---

N i s e g r .

$, ^%

l 1, A I I A',

i

^{I \ 1 l A A}

I \ l A\ A l '

I A

"

l b

I

I I A

Cu

v\

IV

-

DISCUSSION

.- 99. 5

'

96.5

56.5 41.8 B u l k cu a t $

This is the first case where solute enrichment at both ends of alloy composition is documented beyond any doubt in surface

segregation of the well-studied Ni-Cu binary alloy system. The present result disagrees with all the existing theories, including

advanced microscopic electronic theories since they predict surface enrichment of only one of the elements of a binary alloy/15,20,21/.

Thus, we cannot find an adequate theoretical description of this phenomenon, although a large number of theoretical works have been devoted to this alloy system. Electronic theories of surface segregation give quite controversial results: the first calculation by Kerker et a1./15/ showed a significant crossover, but successive more advanced calculations/20,21/ predicted no Ni segregation when higher order effects, such as off-diagonal disorder, intra-atomic correlation, and the conservation of the total number of electrons, were taken into consideration/20/. A strain energy due to the size misfit of alloy atoms is a well-known factor to promote the minority segregation/l2,22/. In the case of Cu-Ni alloys, the contribution of strain energy, however, is 2OmeV/atom, very small (the difference of the radii is only 3%) and amounts to only approximately 4% of the surface energy difference (0.46 e~/atom), which is the main driving force of the Cu segregation. Another possibility is the gain of magnetic energy by forming the ferromagnetie Ni overlayer. Although such an effect is interesting in connection with the surface

magnetism of Cu-Ni alloys, it is unlikely to contribute to Ni segregation in the present case because the annealing temperature (-

900 K) is much higher than the surface Curie temperature of the Ni crystal (Tcs -630 K)/23/.

It has been pointed out by some authors/15,24/ that there always exists a driving force of minority segregation in alloys if their regular solution parameter is positive: w = E A ~

-

^1/2(EAA

+

EBB) > 0, where EAB, EAA, and EBB represent the bond energies of.AB, AA, and BB atoms, respectively. The value of W of Cu-Ni alloys is known to be positive. Unfortunately, its magnitude (

-

¹⁰

meV)/11/ and the change with alloy concentration are not large enough to compensate for the surface energy difference.

One possible mechanism of the observed crossover is the increase of the regular solution parameter at the surface ( us> W ) due to the bond relaxation/25/. In that case, a large value of W, may lead to phase separation in the surface layer/26,27/ which

qualitatively explains the observed segregation of Ni. According to the simple Bragg-Williams type approach, however, the critical

temperature of the surface phase separation is given by Tcs = 112

( ws/w )~,~/26/ for the ( 1 1 1 ) surface of fcc crystal and a bulk

-

3

^100-

a r

,,rlo-5w~

the surface layer as a

U function of bullc Cu

S

3 composition determined by

2 80

_\ various workers. Also

X2 ^{I \}

0

:.

^{A , ,} plotted is the catalytic

I $ activity of cyclohexane

4.l

2

_J

60'. .

^{A I .}

_{I O - ~}

" dehydrogenation against bulk cu composition by

c

Q)

2 40.

o

U

q ²⁰

-+

S-

B

A (9

.

a

.

^,/"

' ^X

-

Singelt et al.(-- a - - ) . E 0 : present data,

A : AES by Natanabe et

2

al, A : AES by Takasu et

;

^{a , 0 : LEIS by}

-2 Brongersma et al,

-lo-' <

^{m :} atom-probe by Ng et

"

3 ^{4 al, X :} AES by Quinto et

20 40 60 80 100 al,

4

: AESIXPS by Webber et al.

Bulk c o n c e t r a t ion(at%Cu)

C2-386 JOURNAL DE PHYSIQUE

Tcb of Cu-Mi alloys is around 400 to 700 ~ / 2 8 / . Thus unreasonably large enhancement of 60% to 400% is required in order to raise Tcs above the annealing temperture used in the present work. We have thus no satisfactory explanation at present for unexpected enrichment of solute Mi at the surface.

An interesting observation in connection with our observation is the report by Sinfelt et al. on catalytic activity of Ni-Cu alloys in cyclohexane dehydrogenation into benzene/2/. When they measured the reaction rate as a function of alloy composition, they found that it increases rapidly as Cu bulk concentration increases 0 to 59, reaches a plateau in the Cu concentration range of 10 to 80 at% and again decreases as Cu bulk concentration further increases close to 100% pure Cu. They explained that the initial rapid increase in the catalytic activity is due to Cu enrichment at the surface but had difficulties in comprehending the steep drop in the Ni solute range and had to invoke another rate-determining process to account for it.

One, however, notices excellent agreement between our present surface segregation data and the catalytic activity ~neasured by Sinfelt et al. which is reproduced in Fig. 4. If our result of the solute Ni segregation is taken into consideration, then the simple idea that the activity of cyclohexane dehydrogenation is simply a function of surface enrichment of Cu can account for the observation made by Sinfelt et a1./2/.

ACKNOWLEDGEMENTS

We acknovrledge fruitful discussions with Ms. A. Jimbo, Dr. A.

Salcai, Dr. I<. Terakura and Dr. M. Cole. This work was partially supported by the National Science Foundation, Metallurgy Program, Division of Materials Research, Contract No.DMR-83-14168.

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