SURFACE SEGREGATIONS IN EMBRITTLED (FexNi1-x)z (PyB1-y)1-z GLASSY RIBBONS BY AUGER ELECTRON SPECTROSCOPY

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SURFACE SEGREGATIONS IN EMBRITTLED

(FexNi1-x)z (PyB1-y)1-z GLASSY RIBBONS BY

AUGER ELECTRON SPECTROSCOPY

D. Bijaoui, A. Yavari, J. Joud

To cite this version:

30URNAL DE PHYSIQUE

Colloque C8, supplément au n°12, Tome t6, décembre 1985 page C8-437

SURFACE SEGREGATIONS IN EMBRITTLED ( F e ^ i ^ ) ^ P y B ^ y ) 1_z GLASSY

RIBBONS BY AUGER ELECTRON SPECTROSCOPY

D. Bijaoui, A.R. Yavari and J.C. Joud

Laboratoire de Thermodynamique et Physioo-Chimie Métallurgiques, Institu National Polytechnique de Grenoble, B.P. 75, Domaine Universitaire, 3840S Saint Martin d'Hères Cedex, France

Résumé : Nous avons étudié par spectrométrie Auger les ségrégations à la surface de rubans amorphes bruts de trempe et recuits de composition Fe Ni P B et (Fe, Ni, ) B .

Apres recuit de 2 heures à* i&0oC, les rubans contenant 20 et 23 % de bore

sont fragilisés, le ruban à 17 % de bore restant ductile. Tous les échantillons recuits présentent une ségrégation marquée de métalloïde, sans qu'il y ait de corrélation directe entre celle-ci et la fragilisation des rubans.

Abstract : We have searched for a correlation between surface segregation of

constituent elements and thermal embrittlement in (Fe J^i ,-)•,_ 'B p-,_ )

glassy ribbons. Surface segregations are investigated' by 'ft.E.i>. on as quenched and annealed glassy ribbons Fe Ni P B and (Fe^Ni, ) B (x = 0.17, 0.20, 0.23). Embrittlement is observed after annealing eft 573K

during 2 h on all the samples except the Fe cNi4-| cB 1 7' W e obtain large

segregation of metalloid elements on all the annealed 'samples indicating, no direct correlation with the mechanical behaviour.

I - INTRODUCTION

Many metallic glasses and especially Fe-based glassy alloys become brittle after annealing at a temperature T below their crystallisation temperature T /l/. This phenomenon is of technological importance because it occurs during the relaxation annealing of amorphous alloys with soft magnetic properties.

During inhomogeneous deformation, an amorphous alloy can deform plastically at a

stress level a (T) or undergo brittle fracture at a stress level q_(T) when cracking

occurs. Annealing at T can result in structural relaxation in the bulk /2/ with or without increasing chemical .short-range-order but it can also result in unmixing or chemical segregation of constituent elements in the bulk /3,4,5/. These evolutions

can all increase a (T) and decrease a„(T) /6/ thus contributing to the embrittlement

which is observed during deformation at ambient temperature when af(300K) < o (300K).

On the other hand, the surfaces of glassy ribbons can also play an important role in the embrittlement. In particular, certain ribbons regain their ductility after removal by polishing of a surface layer a few micrometers, in thickness. While this has been attributed to surface crystallisation /7/, we have shown that when T is well below T , the available experimental data do not support this interpretation /6/. Surface segregation of constituent elements has been observed in some embrittled ribbons /8/. Preferential structural relaxation (reduction of free volume) near the surface without any chemical segregation has also been proposed as contributing to embrittlement by rendering more difficult the nucleation and propagation of slip-bands /6/.

C8-438 JOURNAL DE PHYSIQUE

In this paper we study the embrittlement and surface segregation in glassy

(Fe Ni ) (P B ) ribbons prepared by planar flow casting /9/. In particular,

we %'tug ? ~ e ~ ~ with h ~x ~= 0.77, ~ f 0.80 ~ ~and 0.83 corresponding to hyper- ~ - ~ eutectic, eutectlc and hypoeutectic ternary alloys /lo/ where we obtain an evolution of the small-angle-X-ray scattering (SAXS) spectra with thermal embrittlement /11/. These alloys have the advantage of temperature gaps T -TB of over a hundred degrees

such that crystallisation does not occur at T

.

hypoeutectic (Fe Ni become brittle at 3 8 0 ~ after annealing for 2 hours at

5 0 ' 8 3 ~ ~ 7

.

T = 573K. All a5floys lnc udlng the hypoeutectic glass that remains ductile develop

t%e same type of surface segregation after annealing.

I1 - EXPERIMENTAL

The (Fe Ni ) B and Fe Ni P B alloys were prepared by addition of appropriate

amounts'o?&~-%i, ~ i 2 8 a,6$

%%-%

ribbons were prepared using the planar-flow-casting technique at LTPCM in a Ar or N 2 atmosphere /9/. The ribbon dimensions were lcm x 30pm x 20 m. Very ductile samples

were annealed for two hours at TB = 573K in a pyrex tube under vaccuum. Absence of

any detectable crystallisation was verified by X-ray diffraction on both sides of the ribbons before and after heat treatment. Loss of ductility after annealing at T was established by bending tests at ambient temperature. Ribbons were considereg embrittled if they broke during 180° bending. Surface compositions were determined by Auger electron spectroscopy using the cylindrical mirror type apparatus at LTPCM

which is equiped with a lock-in amplifier for synchronous detections ando an ion-beam

sputtering gun with operating beam voltage of 3kV and current of 30 mA. The etch

crater diameter was much larger than the primary electron beam diameter of a 20 pm.

After each bombardment period (using high purity Argon) derivate of electron distribution dN(E)/dE was registered as a function of electron energy E using

primary beam energy E = 3keV while the sample was under a high vaccuum of 10-

Torrs. Composition p&files were then obtained for Fe, Ni, B, P and 0 using the

evolution of elemental Auger peaks as a function of ion sputter time with a

sputtering rate of ?. 1 nm per mn. The atom fractions X. were obtained using the

relation : X. = (I. /S. ) / C . ( I . /S. ) where I. and S. aEe the peak to peak height

and sensitivity, r;spect?vefy, #'orlea&h element.'~he S. 's weren't adjusted to obtain

the exact bulk composition at maximum depth (detail; also given in

/ a / ) .

correspond to best values deduced from previous work /8/ and Palmberg reference book.

I11 - DISCUSSION AND INTERPRETATION OF RESULTS

We have performed Auger analysis on as quenched glassy ribbons and after annealing

at 573K. All annealed ribbons were brittle at room temperature except the

hypoeutectic Fe

41.5Ni41 .5B17'

We observe as a generale rule large surface composition variations in metalloid elements on annealed samples. Both sides of the ribbon exhibit similar trends with larger evolutions measured on the bright side of the samoles.

Concentration profiles measured on the bright side of various samples reveal th&

existence of an "oxide layer" at the extrem surface with thickness of roughly 100 A

on annealed ribbons and 30 A on as quenched ribbons.

Experimental results (fig. 1) obtained on the three Fe-Ni-B alloys indicate similar

surface segregation of B element after annealing. On the bright side the maximum

concentrations are approximately 4 0 k 5% at B with high level of Fe and low level of

Ni. No direct correlation can be established between surface compositions and the

embrittlement of eutectic and hypereutectic alloys after annealing. The

Fe16Ni64P1 B6 ribbon exhibits surface segregation of boron and phosphorus after

annealing ?flg. 2). More precisely, we observe at ~ x t r e m surface a segregation of B

with low levels of P and N i and below ( 50 A) a segregation of P. A semi

Application of the theoretical model of Mac Lean /13/ to surface segregation links the time variation of surface concentrations with the bulk-diffusion coefficient and the thickness of surface layer. In particular, we can calculate the quantity

(Cs I-f -Cbulk)/Cbul for B and P element. As indicated in the table 1 we observe a

quay1 atlve agreement with experimental values confirming a segregation of metalloid elements controlled by bulk diffusion.

Table 1

AC/C B P

exp 2,6 0,46

theoretical 2.7 0 3

IV - CONCLUSION

We have performed Auger analysis on as quenched and annealed glassy ribbons

Fe Ni P B and (FexNix)l-xB with x = 0.17, 0.20 and 0.23 ; all these ribbons

ex?~?bif~ei%r?ttlement at room femperature after annealing at 573K during 2 h except

the Fe41.5Ni41.5B17. In each case we have observed surface segregation of metalloid

elements on annea ed samples. The same levels of segregation have been observed on the hypo-, hyper-, eutectic Fe-Ni-B alloys indicating no direct correlation with the embrittlement.

Comparisons between experimental and theoretical surface enrichment in metalloid

elements measured on the Fe 16Ni64P14B6 confirm surface segregation controled by bulk

diffusion.

/ 1/ Chen H.S. (1981), Proc. 4th Int. Conf. on Rapidly Quenched Metals RQ4, vol.1

pp. 555-558, Jap. Ind. Metals.

/ 2/ Gerling R and Wagner R., Scripta Metallurgica, vol.

11,

/ 3/ Piller J. et Haasen P., Acta Met.

30

/

41

24,

/ 51 Yavari A.R., J. de Physique

43

/ 6/ Yavari A.R., Joud J.C., Bijaoui D., "Journ6es M6tallurgiques d'Automnel', Revue

de MGtallurgie, in press 1985.

/ 7/ Janot C., George B., Teirlinck D., Marchal G., TGte C. and Delcrois P., (1983),

Phil. Mag. A,

47,

/ 81 Bijaoui D., Yavari A.R., Wasselin J. and Joud J.C., MRS-Europe, Strasbourg,

June 1984.

/ 9/ Casanova P., Joud J.C., Senillou C., Yavari A.R., Revue de M6tallurgie. Oct.

1984, p. 553.

/lo/ Ageev N.V. (editor), Phase Diagrams of Metallic Systems, p. 155, VINITI

Publishing, Moscow 1966.

/11/ Bijaoui D., Livet F. and Yavari A.R., to be published 1985.

/12/ Bijaoui D., Joud J.C., Yavari A.R., Calphad XIV, Boston, June 1985.

JOURNAL DE PHYSIQUE fig l a I F E - N 1 ) 8 3 E 1 7 ANNEALU~I 300 C BRIGHT S l D E

l----

1

--

f i g 1c

(FE-N1177B23 ANNEALEII X r ) C BRIGHT SIDE

fig 2