PHASE SEPARATION IN THE AMORPHOUS Co76Mo16B8 ALLOY

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PHASE SEPARATION IN THE AMORPHOUS

Co76Mo16B8 ALLOY

N. Shen

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, supplément au n°12, Tome 46, décembre 1985 page C8-511

PHASE SEPARATION IN THE AMORPHOUS Co., M0n,.B0 ALLOY

/ 6 lb o N. Shen

Department of Mechanical Engineering, Zhengzhou Institute of Technology„ Henan, China

Résumé - Une séparation de phase à l'état vitreux est mise en évidence dans l'alliage Co7g MO]_g B„, obtenu par trempe du

li-quide, à partir d'observations en microscopie électronique et calorimétrie différentielle. Les compositions et les stabilités thermiques des deux phases diffèrent.

Abstract - The characteristic evidence for glass separation is shown in the liquid-quenched Co^Mo,,Bg alloy by transmission electron microscope examinations and differential calorimetry observations. Owing to the difference in the chemical composi-tion, the two glass phases are different in thermal stability.

I - INTRODUCTION

Although stable and metastable immiscibility of liquids and their sub-sequent freezing into phase-separated glass microstructures is well known in oxides glasses and is the basis of many glass ceramics, evi-dence of similar behaviour in metallic glass system is scarce. Earlier studies by Chen and others /1,2/ have described phase separation after liquid-quenching but more commonly, during the reheating of single-phase glasses in Pd-Sl, Pd-Au-Si, Pd-Cu-Si and Ni-Pd-P alloys. Predel has invastigated this phenomenon in the case of a Ni„.Pd.„Pio glass. Glass phase separation has also been suggested in as-quencned Fe-B and

Zr-Nb-Be, Zr-Ti-Be alloys /3,4,5/. In the present work, the charac-teristic evidence for glass phase separation has been found in the liquid-quenched Co7,Mo.,BR alloy by transmission electron microscope-examinations as well as differential calorimetry observations. II - EXPERIMENTAL METHODS

C8-5 12 JOURNAL DE PHYSIQUE

EDAX 9100 energy dispersive analytical system. All the samples for the

works on the microscope were electrolytically thinned in perchloric /

acetic acids. Differential thermal analyses were performed on a Dupont 900 thermal analyser and on a Perkin-Elmer DSC-2 instrument.

I11

-

The transmission electron micrograph and selected area electron diffrac-

tion pattern of the B ribbon sample in the as-quenched state

are presented in Fig. l and $i$.2. The micrograph shows an irregular light and dark pattern. THe sample was tilted through large angles without any indication of variation in image contrast, neigther spots nor sharp rings have been observed in the electron diffraction pat- terns, and the dark- field electron micrograph obtained from the first broad diffraction ring did not revealed any grains. Therefore the fea- tures noted in the micrograph are not due to any crystallity, but are believed to be the result of differences in mass caused by the parti- tioning of high and low atomic number elements, although there was n o , evidence of resolvable splitting of the broad diffuse ring in the elect- ron diffraction pattern.

Fig.1

-

A differential thermogram for the liquid-quenched Co76M016B8 alloy was

obtained at a heating rate of 20 deg K/min (Fig.3). At the temperature

of 520 deg K a small endothermic effect has been observed, which is

thought to be a glass transition temperature. This is followed by an exothermic peak which begins at 690 deg K. This temperature appears to be the first crystallization temperature. A second exothermic effect,

that is the second stage of the crystallization, begains at 1020 deg K

and gives a sharp peak. But prior to the second exothermic effect, another small endothermic effect appears at about 870 deg K, which is thought to be another glass transition temperature. This double glass transition gives clear indication of phase separation in this alloy. Because of the absence of any exothermic effect prior to the first glass transition temperature, phase separation must have happened in the as- quenched state before the start of heating.

Temperature, deg K

Fig.3 - Differential thermogram for liquid-quenched CO Mo B alloy.

76 16 8

Transmission electron microscopy and selected area electron diffraction examinations of the samples during the process of the thermoanalysis have confirmed this conclusion and in some extent revealed the role of

phase separation in the glass decomposition. In the case of samples

J O U R N A L DE PHYSIQUE

ACKNOWLEDGEMENTS

The author would like to thank professor R.E. Smallman for the provi- sion of laboratory facilities in the Department of Metallurgy and Materials, University of Birmingham. The author would also like to thank Dr. J.N. Pratt and Dr. I.P. Jones for helpful discussions.

REFERENCES

1. Chen H.S., Mat. Sci. Engng.

23,

2. Predel B., Physica [I9811 113.

3. Walter J.L. and Bartram S.F., Rapidly Quenched Metals; Ed. Cantor B.

Vol. 1, T.he Metals Society, London [I9781 307.