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CONTRIBUTION TO S.A.N.S. OF THE SURFACE
STATE OF Pd80Si20 AMORPHOUS ALLOYS
B. Rodmacq, Ph. Mangin, A. Chamberod
To cite this version:
CONTRIBUTION TO S.A.N.S. OF THE SURFACE STATE OF Pd Si AMORPHOUS
B O 2,\J
ALLOYS
. *+
B. Rodmacq, Ph. Mangin and A. Chamberod
DRF-Serviae de Physique, Metallurgie Physique, CEN Grenoble, 85 X, 38041 Grenoble Cedex, France
*Institut Laue-Langevin, 156 X, 38042 Grenoble Cedex, France
Résumé. Des expériences de diffraction de neutrons à petits angles ont été réalisées sur des alliages amorphes PdgnS^o- Le signal à petits angles varie fortement en fonction du traitement de surface (échantillon brut de trempe, poli ou attaqué). Ceci indique que l'état de surface est à l'origine d'une grande partie de l'intensité diffusée. Ce résultat est confirmé en plaçant les échantillons dans des mélanges CH3OH-CD3OD. Dans chaque cas l'intensité diffusée passe par un minimum en fonction du rapport H/D du liquide, et suit la variation théorique de la différence de longueurs de diffusion à l'interface. Les résultats sont analysés en termes de loi de Porod dans le cas de diffraction par une surface. Les résultats obtenus par diffraction de neutrons sont confirmés par l'étude en microscopie électronique à balayage.
Abstract. Small-angle neutron scattering (SANS) experiments have been performed on amorphous PdQoSi2o alloys. The small-angle signal varies strongly with surface treatment ( a s - q u e n c h e d , polished or etched sample). This indicates that the surface state Is responsible for a large part of the scattered Intensity. This is confirmed by placing the samples In various CH3OH-CD3OD mixtures, in every case the small-angle intensity goes through a minimum as a function of the H/D ratio of the liquid, and follows the theoretical variation of the scattering length difference at the interface. Results are analyzed in terms of Porod law for the case of surface scattering. Scanning electron microscope measurements confirm the neutron scattering results.
I. INTRODUCTION
Small-angle scattering experiments often lead to the observation of a signal which increases strongly as q tends towards zero. This indicates the presence of heterogeneities in the sample, the scattered intensity being related to the contrast between these heterogeneities and the matrix. The study of the modification of this signal as a function of various parameters can be used to follow structural or chemical changes of the sample. For example, such heterogeneities can arise from grain boundaries or dislocations in the case of polycrystalline materials / 1 . 2 / . The same qualitative behaviour has been often observed in small-angle neutron or X-ray scattering studies of metallic glasses / 3 - 6 / . in this case, the contrast originates from fluctuations of density or chemical composition of the sample, and models have been built up to fit the observed scattered intensity. At this point. It has to be recalled that surface heterogeneities have already been considered as a possible origin of the observed small-angle signal 11-91. In particular it has been shown / 9 / that surface scattering can represent a non-negligible part of the total scattering even in thick samples of polycrystalline materials.
This rises the question of the contribution of surface scattering in metallic glasses. If one considers that the thickness of amorphous alloys is always between about 10 to 100 times smaller than that of polycrystalline metals or alloys, it seems therefore very important to "•"Permanent address: Laboratoire de Physique du Sollde (CNRS. LA155). Universite de Nancy I. BP 239. 54506 Vandoeuvre. France
C8-5 00 JOURNAL
DE
PHYSIQUEconsider surface scatterlng as a p?ssibie origin of the observed low-angle signal. This problem Is certainly less crucial If one considers only the evolution of a given sample as a function of an external parameter (temperature for example). In thls case. It is possible to study only the differential scattering curve. if one supposes that. in a first approximation, surface scattering is independent of temperature.
In this paper we present evidence of the important role played by the surface state of an amorphous Pdg0SigO alloy in the observed small-angle neutron scattering signal. Firstly it is shown that this signal depends strongly on the surface treatment. at is it the case for polycrystalline samples. Secondly. the fact that this signal can be almost completely suppressed by changing the contrast between the sample and the outside is a definite proof of the importance of surface scattering
/ l o / .
This is confirmed by scanning electron microscope measurements on samples with various surface states.II. EXPERIMENTAL
Experiments were carried out on amorphous Pdg0SieO alloys prepared by melt spinning in the form of ribbons 50 pm thick and 6mm wide. The influence of surface effects was checked by uslng samples with three different surface states : as-quenched (sample A ) .
mechanically polished (sample 0 ) . and chemically etched in a HN03-HCI solutlon (sample C). Neutron scattering measurements were performed at lnstitut Laue-Langevin on the
0 1 7 diffractometer. The experimental conditions have been previously reported / l o / . Scanning electron microscope measurements were performed at LETI/CRM (Grenoble).
ill. RESULTS
Figure 1 shows the small-angle scattered intensity ( i n cmz/cms) corresponding to PdgoSipO samples with different surface states. It can be easily observed that the smali- angle signal varies over more than one decade between the as-quenched (fig. l a ) and chemically etched sample (fig. I c ) . This is a first indication of the large contribution of surface scattering to the total slgnai.
Flg. 1. Small-angle neutron scattering Fig. 2. Small-angle neutron scattering
curves of amorphous PdgoSigo curves of chemlcaliy etched PdgoSieO
samples (a) as-quenched. ( b ) polished. sample in various CH3OH-CD30D
and ( c ) chemically etched. mlxtures (a) 30% ( b) 60% ( c ) 100% CD30D
(this will depend on the wettlng properties of the liquid). such a procedure will permit to vary the scattering length dlfference. and even to cancel It by use of an appropriate liquid. Organic solvents are well suited for such an experlment. because of their good wetting properties. Moreover the posslbllity of hydrogen-deuterlum substitution wlll result in large variations of the mean scattering length per unit volume.
Such a procedure has been used for our PdgoS120 samples. They were placed in a quartz cell containlng mixtures of deuterated and hydrogenated methanol at dlfferent concentrations. Taking into account the coherent scatterlng length of hydrogen ( b ~ =
-
0.374 x 10-l2 cm) and deuterium ( b g = + 0.667 x 10-l2 cm) it was thus posslble to var continuously the coherent scatterin length per unit volume of the liquid from 5. 985 xloro
cm/cm3 (CD OD) to-
0 3 7 8 xloB0
cm/cm3 (CH30H), that of amorphous Pd80Si20 (bo = 3.874 x 10b cm/cm3) corresponding to a liquid composition of 68 9b CD3OD. Figure 2 presents the modiflcations of the scattering curves of sampie C (chemically etched) for three CH30H-CD30D mxitures. Each scattering curve can be decomposed into a coherent contribution and an lncoherent one. The lncoherent part increases as the proportion of CH30H increases, as a result of the large incoherent scattering cross- section of hydrogen (0inc = 79.9 x 10-24 cm2). The most important point concerns the variation of the coherent contribution which goes through a minimum as a function of the H/D ratlo. As the scattered lntensity is directly proportional to (Ab)z where Ab is the scattering length difference at the interface (whatever thls interface Is). the ofliy explanation of such a behavlour is that almost all this scattered intensity arises from the surface of the sampies. Taking Into account the vaiues of the coherent scattering lenpth given above for CH30H. CDQOD and amorphous P d g ~ S i ~ ~ . the theoretical scattered intensity should first decrease. then go through a minimum and Increase as a function of CD30D concentration. which is totally in agreement with the observed variation.The same behaviour has been observed with the two other P d g ~ S l ~ ~ samples and, for each CH30H/CD30D composition. the scattered intensity has been fltted to the well-known Porod law which describes the asymptotic behaviour of the scattering curve in the case of a two-phase system with a well-deflned interface / 9 / . In thls case the scattered Intensity can be written
27TS(Ab)Z
I t s ) =
9
,
+
Iits)
(1)with q = 4n/A sine. Ab is the contrast varlatlon at this interface of total area S.
I
~ ( q ) Is a constant term whlch takes into account the incoherent scatterlng of the CH30H-CO30D mixture.For each of the three sampies studied, a plot of q4 I ( q ) versus q4 was used In order to determine the vaiues of K = 2nStAb) 2 and
I
i ( q ) . A good agreement was found between theobserved and calculated contributions of the lncoherent scattering for the hydrogen-rlch CH30H-CD30D mixtures. In this case it can be reasonably assumed that the incoherent scattering is the ieadlng process / 1 l/.
The varlatlon of the obtalned vaiues of K as a function of CD30D concentratlon is shown in flgure 3 for the three samples studled. One observes in each case the presence of a mlnlmum corresponding to the concentration of CDQOD at which the scattering length dlfference between the sample and the liquid Is the smallest. In the same way the scattering length of the llquid Is zero at 6 % CD30D. At this concentratlon, the value of
K must thus be equal to that of the samples measured in air (figure 7 ) . These values are also shown In flgure 3 and agree well wlth the other data polnts.
C8-5 02 JOURNAL
DE
PHYSIQUEFig. 3 Variation of K=2rrS(Ab) 2 wlth CD30D
concentration for amorphous PdgoSI2o ailoys ( a ) as-quenched ( b ) pollshed. and ( c ) chemicaliy etched. Dotted lines correspond to a fit to the theoretical parabolic variation.
Fig. 4 SEM Images of amorphous Pd80Si20 ailoys (a) as- quenched ( b ) polished. and
( c ) chemically etched. The bar is 1 ~ .
where bo is the scattering iength at the surface of the sample. If bo is unique for a given sample. the values of the total interface area S and of the scattering iength bo can thus be determined from a fit of the variation of K as a functlon of x. The dotted lines In figure 3 show the result of such a fit for the three samples and the values of bo and S (normalized to the total surface of the samples So) are presented in Table
1.
Table I. experimental values of the scattering lerqth bo and of the normalized surface S/So. IV. DISCUSSION s/So 0.06 0.16 0.64 1 PdgOSi20 sample as-quenched polished etched
From the results presented in the preceedlng section. it can be concluded unambiguously that surface scatterlng is a predominant contribution to the total small-angle scattering Of amorphous ailoys (we are qulte sure that the case of PdSi ailoys 1s only an example among many other systems. and this has been confirmed recently In the case of amorphous FeCrPC alloys /12/). bo l0l0 cm/cm3 2.84 2.77 3.75
corresponds to a value of the scattering length ver close to that of the bulk sample ( 3 . 75
r
xlo1'
cm/cm3 instead of 3.874 x 1 0 l 0 cm/cm ).
A good fit to Eq. (1) is also obtained for the as-quenched sample (figure 3a). The minimum is not found at the theoretical value. but corresponds to a mean diffusion length of the surface bo = 2. 84 x 1 0 l 0 cm/cm3. A possible explanation Is that an oxide or hydroxide layer has formed on the surface. leading to a different dlffusion length as compared to that of the bulk.
The case of the polished sample is more complex. Figure 3b shows that the corresponding minimum is rather flat and exends from about 30 % up to 70 % CD3OD. A possible explanation of this behaviour can be the presence on the surface of regions ( a t least two) with different scattering lengths. The resulting variation will thus be the sum of several contributions arising from these different regions. This important result shows that even if one tries to eliminate such a surface scattering by immersing a given sample into liquids of various scattering lengths. one cannot be sure that any resldual small- angle scattering arises from the bulk of the sample. This residual scattering can be simply a proof of the non-unicity of the scattering length of the surface, due to the presence of oxidized regions for example. In other words, it is more easy to demonstrate the absence than the presence of any bulk small-angle scattering in these materials. As it has been demonstrated in the case of polycrystalline samples / 9 / . the values of the ratio S/So reported in Table
f
can be compared to those obtained by supposing a sinusoidal surface with an amplitude A and a wavelength ho :This ratio is thus a measure of the rugosity of the surface. From Tablef one sees that this ratio is always smaller than one for the three Pd80Sl20 alloys and is the largest for the chemically etched sample. as it could be expected. One can also notice that this ratio is larger for the polished sample than for the as-quenched one. This is due to the presence of numerous fine scratches introduced during the polishing process.
Flgure 4 shows the scanning electron microscope (SEM) images obtained from the surface of the samples. These Images confirm the values of Table
1.
and show that the surface of the chemically etched sample is highly dlsturbed as compared to that of two other samples. Numerous holes can be identified with slzes as small as 200 to 500A.
This means that these inhomogeneities will give rise to a scattered Intensity for values of q smaller than about 5 xA - l ,
which corresponds well to the experimental observations. ACKNOWLEGDMENTSWe wish to thank S. Marthon and M. Dupuy (LETI/CRM Grenoble) for the SEM measurements.
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