Al-Mn METASTABLE PHASES PREPARED BY SOLID STATE INTERDIFFUSION AND OBSERVED BY TEM AND OPTICAL MEASUREMENTS

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Al-Mn METASTABLE PHASES PREPARED BY SOLID STATE INTERDIFFUSION AND OBSERVED

BY TEM AND OPTICAL MEASUREMENTS

J.-M. Frigerio, J. Rivory

To cite this version:

J.-M. Frigerio, J. Rivory. Al-Mn METASTABLE PHASES PREPARED BY SOLID STATE INTER-

DIFFUSION AND OBSERVED BY TEM AND OPTICAL MEASUREMENTS. Journal de Physique

Colloques, 1990, 51 (C4), pp.C4-163-C4-168. �10.1051/jphyscol:1990419�. �jpa-00230779�

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COLLOQUE DE PHYSIQUE

Colloque C4, suppl6ment au n 0 1 4 , Tome 51, 15 j u i l l e t 1990

A1-Mn METASTABLE PHASES PREPARED BY SOLID STATE INTERDIFFUSION AND OBSERVED BY TEM AND OPTICAL MEASUREMENTS

J.-M. FRIGERIO and J. RIVORY

Laboratoire d'optique des Solides. URA CNRS 781, Universite Pierre et Marie Curie, 4, Place Jussieu, F-75252 Paris Cedex 05, France

RQumt

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Des tchantillons de couches alterntes Aluminium et Manganese sont prtparts par tvaporation thermique sur des substrats de verre et de NaCl. Les traitements thermiques sont rtalists dans des conditions d'ultra-vide pour tviter la formation d'une couche d'oxyde. Les tchantillons sont chauffts P une vitesse constante de 2K par minute, puis maintenus deux heures B la tempkrature de recuit fixte. Les modifications induites par la rCaction en phase solide sont suivies h chaque ttape du recuit par microscopie Clectronique en transmission et par mesure de la rtflectivitt optique. L'inter-diffusion commence P 220°C; une phase amorphe All-,Mn, apparait B partir de 225°C; une phase cristalline, probablement A16Mn, est observte P une temptrature de recuit de 230°C, elle croit au detriment de la phase amorphe. Ainsi dans nos conditions exptrimentales (concentration de Mn, epaisseur des couches) nous ne sommes pas parvenus B produire des tchantillons monophasCs amorphes ou quasi-cristallins par rtaction P l'ttat solide, contrairement au mixage ionique.

Abstract

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Alternate layer samples of aluminium and manganese are produced by vapor deposition on glass and NaCl substrates. Heat treatments are then performed in ultra high vacuum conditions in order to prevent the formation of an oxide layer. The samples are heated up at a constant rate of 2K per minute, then maintained during two hours at the fixed annealing temperature. The modifications induced by solid-state reaction are followed at each annealing step by transmission electron microscopy and optical reflectivity measurements. The interdiffusion begins at 220°C; an amorphous AI1-,Mnx phase starts growing at 225°C; a crystalline phase, presumably A16Mn, is observed to grow at an annealing temperature of 230°C at the expense of the amorphous phase. Therefore in our experimental conditions (Mn concentration, thicknesses of the layers) we were not able to produce either amorphous or quasicrystalline single phase samples by solid state reaction in contrast with ion beam mixing.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990419

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COLLOQUE DE PHYSIQUE

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INTRODUCTION

Several methods are used to prepare metastable metallic phases. Rapid quenching from the melt (10+6 K/s) was widely used to produce metallic glasses. Intermediate quenching rate are appropriate to elaborate quasi-crystalline phases /l/. Recently new methods have been developed for the preparation of metastable phases, involving solid state reaction under thermal or mechanical constraints /2,3/.

The aim of this paper was to examine the possibility to produce amorphous or quasicrystalline phases in the binary Al-Mn system by solid state interdiffusion and to connect these results with those obtained on the same starting material by diffusion under irradiation (i-e, ion beam mixing by Xe ions

141).

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EXPERIMENT

Alternate layers of aluminium and manganese are vapor deposited in ultra high vacuum conditions on various substrates: optically polished glass for optical measurements, NaCl for transmission electron microscopy (TEM) characterization, carbon for determination of composition by Rutherford Backscattering analysis (R.B.S.). The stacks consist in two AI/Mn bilayers, with an additional Al layer on the top, in order to prevent oxygen contamination through the stack after breaking the vacuum. The whole thickness is about 1200

A.

Each period is 430

A

⁽330

A

of A1 , 100

A

of Mn). The thicknesses are controlled using a quartz monitor and adjusted to obtain the desired composition. The Mn concentrations are ranging from 14 to 23 at%. They are checked by R.B.S. after deposition.

The annealing cycles are performed in a special chamber consisting in a silica tube pumped down to a pressure of 10-9 Torr by a turbomolecular pump; the silica tube containing the samples is heated by a ring furnace, the length of which is large enough to give a temperature homogeneity of

+/-

1" over 10 cm at its center. The heating rate is constant : 2 K per minute; at each step, the annealing temperature is maintained during 2 hours and the cooling down to room temperature is performed after removing the furnace out of the tube.

After each annealing step, the reflectivity of the sample is measured at normal incidence between 0.5 and 6.2 eV by a Cary 17 spectrophotometer and the atomic structure is checked by TEM on a piece of the sample collected on a grid which has the same thermal history as the sample used for optical measurements.

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RESULTS AND DISCUSSION

We report here on the modifications induced by heat treatments performed on AI/Mn multilayer stacks having a global Mn concentration of 22.7 at%. Figure l shows several, electron diffraction patterns corresponding to the same sample: as deposited (la), after two annealings at 220°C and a subsequent annealing at 225°C during 2 hours (lb), after subsequent annealing at 230°C (lc), and after the final annealing at 250°C (Id). On figure la, we recognize the Al lines (111,200,220,311, 122, starting from the center) and the more intense Manganese line located between the (111) and (200) A1 lines. The relative intensities of the AI lines show a preferential orientation in the Al films, with the (111) planes (dense packing planes) parallel to the substrate. The evolution of the patterns due to solid state reaction can be detected after maintaining the sample during 2 hours at 220°C

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(not reported in figure 1). It is more apparent after a further 2 hours annealing at 22S°C; we indeed observe (fig lb) a decrease in intensity of the (111) and (200) AI lines indicating a consumption of this elemental component, and a corresponding growing of diffuse halos, the first one at the position of the main Mn line , the second in the vicinity of the (220) Al line and the third near the (311) Al line. These halos correspond probably to the formation of amorphous All,Mnx, as it has been observed in vapor quenched /S/ and ion beam mixed films

141.

Some spots are present, corresponding to the early formation of a crystalline phase. A further decrease in intensity of the (111) and (200)

A I

lines occurs after a 2 hours annealing at 23S°C. The diffuse halos are always present, but the crystalline phase is growing (fig lc). After annealing at 25OoC, the pattern (Id) exhibits essentially rings which can be mainly attributed to crystalline A16Mn. The great amount of reflections makes the identification of the phase rather uncertain.

FIGURE 1

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Electron image and diffraction patterns of an AlMn sample with 22.7 at.% Mn.

Al/Mn multilayer as deposited;

]

Al/Mn multilayer after two hours at 225OC;

Al/Mn multilayer after two hours at 230°C;

\

Al/Mn multilayer after two hours at 2S0°C

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CQ-166 COLLOQUE DE PHYSIQUE

The conclusion of this study is that we do not obtain for this concentration and this bilayer thickness a fully amorphous sample, due to the nucleation of the equilibrium intermetallic compound A16Mn. We do not observe either the formation of a quasicrystalline phase as indicated by Knapp and Follstaedt /2/. In fact, the diffraction patterns that these authors obtained by diffusion of multilayer stacks (period 125

A)

in the microscope exhibit broad rings at the position of the known icosahedral reflections, but the main doublet is not resolved; annealing at higher temperature does not produce an increase of the grain size.

It is worth comparing our results on solid state reacted samples with already reported results /4,5/

obtained by ion beam mixing with 800 keV Xe+ ions of the same multilayer stacks. Indeed we were able to obtain single phase samples consisting either in the amorphous or in the quasicrystalline phase by adjusting the dose and the temperature at which the irradiation was performed (typically 7% for the amorphous phase and 450K for the quasicrystalline one). The reason for this difference could be that the intermetallic compound would be unstable against irradiation at the temperature under consideration.

We also followed the structural evolution by measuring the reflectivity of the as deposited stack and after each annealing step, as reported on figure 2, where the thermal history of the scans is noted in the insert. We notice the strong similarity between the optical reflectivity of the as-deposited stack

Energy ( e . V . )

FIGURE 2

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Reflectivity versus energy for an AlMn sample with 22.7 at.% Mn. and for bulk Al.

Al/Mn multilayer after two hours at 250°C

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and the one of bulk Al. The reason is not that the light falls on the Al top layer, because its penetration depth is large enough to see also the first Mn layer, but the optical reflectivity of pure Mn as function of energy is very smooth, so that the reflectivity of the layered sample is dominated by the feature characteristic of pure Al : i.e. the deep minimum in reflegivity at 1.5 eV due to the presence of interband transitions between parallel conduction bands /6/. Therefore we will comment on the modifications brought by the solid state reaction essentially in the low energy part of the reflectivity. We emphasize here that the optical reflectivity is a very sensitive measurement.

Indeed, after two hours annealing at 220°C (step B), we can hardly detect a change in the electron diffraction pattern; by contrast the reflectivity of the sample is decreasing with respect to the one of the starting stack in the visible and ultra-violet energy range without changing the position of the minimum. The loss in reflectivity is due to the decrease of the A1 thickness, indicating an early stage of interdiffusion. At the step C (diffusion pattern lb) we observe a change in the curvature at low energy, corresponding probably to the growth of the amorphous phase. The reflectivity is strongly modified after subsequent annealing at 230°C (step D) (diffraction pattern not reported here); one observes a decrease in the reflected intensity over the whole energy range due to the disappearance of the main part of Al. At the step E (diffraction pattern lc), AI is fully reacted, the minimum at 1.5

Energy ( e . V . )

FIGURE 3

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First derivative of reflectivity versus energy for an AlMn sample with 22.7 at.% Mn.

$1

M/Mn multilayer after two hours at 250°C

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C4-168 COLLOQUE DE _PHYSIQUE

eV has disappeared; a new minimum is observed at 1 eV, which has to be attributed to the formation of a crystalline phase as indicated by the diffraction pattern. Further annealing produces only minor modifications of the spectrum E.

In order to enhance the modifications induced by the heat treatments, we found useful to represent the first derivative of the reflectivity in the low energy part of the spectrum (figure 3). The minimum in reflectivity (zero value of the derivative) moves slightly towards lower energy before the total consumption of A1 (curves A to D). Curve E corresponds to a new phase.

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CONCLUSION

In this paper we have shown that the optical reflectivity is very complementary to transmission electron microscopy or X ray diffraction in order to follow the structural evolution of a multilayer stack submitted to heat treatments. Both techniques applied to the AI-Mn couple give a direct evidence of the consumption of one elemental component, i.e. Al. Unfortunately the evolution of the unreacted fraction of Mn cannot be evaluated.

We failed in producing single phase amorphous or quasicrystalline samples in our experimental conditions. But it is known /7/ that the extent of the reacted interface is strongly dependent on the deposition method and the bilayer thickness for a given diffusion couple. We mention that, in fact, we obtained results very similar to those of Knapp and Follstaedt for a sputter-deposited multilayer stack (18 at% Mn) with a 250

A

period /8/, confirming the existence of a critical thickness of the disordered phase above which crystallisation occurs.

REFERENCES

/l/ D. Shechtman, I. Blech, D. Gratias and J.W. Cahn, Phys Rev Lett

53

(1984) 185.

/ 2 / D.M. Follstaedt and J.A. Knapp, Phys Rev Lett 56 (1986) 1827.

/3/ J. Eckert, L. Shultz, E. Hellstern and K. Urban, J. Appl. Phys. @ (1988) 3224.

/4/ J. Rivory, J.M. Frigerio, A. Meddour, A. Perez, M.G. Blanchin, J.C. Plenet and J.P. Dupin, Mat Sci and Eng

99

(1988) 361.

/S/ A. Meddour, ThCse d'Ctat, Paris 1988.

/6/ N.W. Ashcroft, Phys Rev B, 19 (1979) 4906.

/7/ B.M. Clemens and R Sinclair, MRS Bulletin,

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(1990) 19

/8/ J.M. Frigerio and J. Rivory, Proceedings of 7th Conf. on Liquid and Amorphous Metals, Kyoto 1989 (in press)