Viscous flow, thermal expansion and phase separation of Fe40Ni40Si6B14 amorphous alloy

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Materials Science and Engineering, A133 (1991) 532-534

Viscous flow, thermal expansion and phase separation of

Fe40Nia0Si6B14amorphous alloy Krassimir Russew and Liljana Stojanova Institute for Metal Science and Technology, Bulgarian Academy of Sciences, 1574 Softa (Bulgaria)

Antal Lovas and Gesa Konzcos Central Research Institute for Physics, Hungarian Academy of Sciences, P.O.B. 49, H-1525 Budapest 114 (Hungary)

Abstract The thermal behaviour (viscous flow, thermal expansion, glass transition(s), phase separation and crystallization) of FeaoNi40Si6B14 glassy alloy is studied with the aid of thermomechanical analysis and DSC. Viscous flow and thermal expansion measurements revealed two glass transition temperatures (amorphous phase separation) in the glassy alloy studied. The possibility of using these alloys as sensitive structural probes for detecting possible phase separation processes in amorphous metallic alloys is shown.

1. Introduction

Amorphous alloys are thermodynamically unstable. Under heating they undergo structural relaxation [1]. A special kind of relaxation is the separation of the initial one-phase glass after heating it above the glass temperature into two supercooled melts [2]. Many glassy alloys decompose in this way prior to crystallization [2-9]. Depending on the chemical composition of the glassy alloys, different experimental methods, e.g. DSC [2, 9], Mrssbauer spectroscopy [10], atom probe analysis [11] or X-ray diffraction [12], are suitable for the detection of amorphous phase separation. The aim of this study was to examine the thermal behaviour (viscous flow, glass transition, thermal expansion, possible phase separation, ~ crystallization, etc.) of Fe40Ni40Si6B14 amorphous alloy. As far as we know this behaviour has not been studied until now. The methods of thermomechanical analysis for viscous flow and thermal expansion measurements were used in order to determine the temperature dependence of the viscosity r/ and of the coefficient of thermal expansion a, of the alloy studied. By these measurements an anomalous behaviour of r/and a, at temperatures close to, but lower than, the crystallization temperature Tx was established, which was considered as evidence for 0921-5093/91/$3.50

amorphous phase separation. The possibility of using viscous flow and thermal expansion measurements as a sensitive probe for detecting p o s s i b l e amorphous phase separation phenomena in glassy alloys is discussed. 2. Experimental

Narrow (produced by Chill Block Melt Spinning) and wide (produced by Planar Flow Casting) Fe40Ni40Si6B14 amorphous ribbons were used in this study for viscous flow and thermal expansion measurements, respectively. The viscous flow measurements were carried out under continuous heating conditions (heating rate 20 K min- ~) with the aid of a Perkin Elmer TMS-2 thermomechanical analyser using a specially designed silica glass three-point bending assembly [1]. The deflection Ah(t) of the ribbon under an applied load of 0.04 kg at its middle point as a function of the temperature is shown in Fig. 1. Similar curves were used to calculate the temperature dependence of the viscosity r/of the alloy studied. More details about the experimental techniques and the viscosity calculations are given in ref. 1. Thermal expansion measurements were performed on wide ribbons under the same continuous heating conditions and with the aid of the same TMS-2 © Elsevier Sequoia/Printed in The Netherlands

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TEM~aATm~ (K) Fig. 2. DSC trace obtained from as-quenched Fe40Ni40Si6B]4 amorphous ribbon under a heating rate of 20 K min- 1. The temperatures of the first glass transition, the second glass transition, and the onset temperature of crystallization of the glassy alloy studied are represented by Tlg, T2g and Tx, respectively.

device. DSC measurements at a heating rate of 20 K min- 1 were performed with the aid of a Perkin Elmer DSC-2C device (Fig. 2). 3. Experimental results and discussion

The temperature dependence of the viscosity ~/ in coordinates In(r/)/1000 T K-1 is shown in Fig. 3, together with the temperature dependence of the thermal expansion coefficient a, in coordinates a,/1000 T K -1. As can be seen, up to 684 K ( Tlg) the temperature dependence is virtually linear with an apparent viscous flow activation energy of 38.4 kJ mo1-1. At temperatures higher than the first glass transition temperature

Tlg, a second curved portion of the q-temperature dependence up to 714 K (T2g) is observed. The initial slope of this curve corresponds to an apparent viscous flow activation energy of 483.6 kJ mol- ~. This slope gradually decreases between Tlg and T2gin a way similar to the decrease in the slope of the viscosity temperature dependence [ 1] at temperatures higher than the onset of crystallization temperature Tx, due to the increasing volume fraction of crystalline particles in the amorphous matrix. As is seen from Fig. 2, however, the onset of crystalfization temperature TX of the alloy studied is 721 K and no apparent heat evolution occurs in the temperature range Tlg-T2g, The conclusion could be drawn that in this temperature range amorphous phase separation occurs, probably with precipitation of more dense regions of the (Fe, Ni)3B amorphous phase [3] suspended in a supercooled melt with higher fluidity. In addition to increasing the temperature up to 714 K the glass transition temperature T2g of the (Fe, N!)3B amorphous phase is reached, manifested by an additional rapid decrease in viscosity (Fig. 3). At temperatures higher than Tx a rapid increase in the viscosity is observed due to the increasing volume fraction of the crystalline particles. The above considerations are also supported by the temperature dependence of the thermal expansion coefficient a, of the alloy studied (Fig. 3). Only a slight decrease in a, in the temperature range 620 K-Tlg along with increasing the temperature is observed, which is most probably due to the annealing out of the excess free volume. On reaching Tlg loosening of the

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amorphous structure due to the first glass transition is manifested by a sudden increase in a,. The passage of a, over a maximum and its decrease in the temperature rarige Tlg-T2 g again could be explained by the possible precipitation of the more dense amorphous (Fe, Ni)3 regions, this process not being connected with apparent heat evolution (see Fig. 2 ). An attempt was also made to obtain direct structural evidence for the amorphous phase separation of the alloy studied with the aid of X-ray diffraction. For this purpose the X-ray spectra of as-quenched as well as of specimens heated to temperatures between Tlg and T2g and consequently cooled to room temperature, were analyzed. In both cases no subhalos or any traces of crystallinity were observed. As expected according to the Ehrenfest relation [12], the split X-ray first halo of a given glassy alloy could be seen only if the major constituents of this glassy alloy have significantly different atomic diameters. 4. Conclusions The results obtained in this study suggest that by heating the Fe40NiaoSi6B14 glassy alloy, amorphous phase separation prior to crystallization occurs, probably connected with precipitation of more dense (Fe, Ni)3B amorphous regions. Viscous flow and thermal expansion measurements could be used as a sensitive method for detection of possible phase separation events in amorphous metallic alloys.

Acknowledgments We thank the Alexander von HumboldtStiftung, R.F.G., for donating the Perkin Elmer thermoanalytical equipment, without which it would have been impossible to perform this study.

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