Mechanism of structural relaxation in a metallic glass

Journal of Non-Crystalline North-Holland, Amsterdam

Solids 92 (1987)

341-353

MECHANISM OF STFWCTURAL IN A METALLIC GLASS G.P. TIWARl

‘, P.K.K.

NAIR

341

RELAXATION

‘, G.L. GOSWAMI

b and P. RAJ ’

“ Physsicol Merolhqy Dioision. h RadiomerollurgV Division and ’ Chemisrry Bhobho Alomic Research Cenrre, Trombay. Bombay - 400 085, India

Received 29 September 1986 Revised manuscript received 23 January

Dioision,

1987

In the present paper, the relaxation phenomena in a multicomponent metallic glass, has been investigated through electrical resistance. acoustic emission Ni,,Fe,Co,,Cr,,Mo,B,,. and MGssbauer spectroscopy IO understand the mechanism of structural relaxation in the amorphous matrix. Isothermal electrical resistance measurements clearly show that the structural relaxation occurs discontinuously through a number of discrete steps. There exists a good deal of correspondence between the variations of the acoustic emissions and the electrical resistance with time and temperawe. It is shown that the stepwise change in the electrical resistance arises due 10 the viscous flow in the matrix which is associated with the acoustic emissions. Fourier deconvolution analysis of the Miissbauer spectra recorded at room temperature was carried out for the as-received sample as well as those relaxed at various annealing temperatures. The QS-distribulions and QS-IS correlation, so obtained, clearly show that the structural relaxation induces a small but definite change in the short-range order. Present results indicate that the viscoelastic flow is the primary unit process responsible for structural relaxations in the matrix.

I. Introduction In the as-quenched state, the metallic glass is metastable with respect to the crystalline as well as the equilibrium amorphous state. The latter condition is a fictive state which represents the structure of the liquid at the ambient temperature [l]. This state can be realised through bypassing of crystallisation in such a way that the equilibrium is always maintained. The annealing of the metallic glass below the crystallisation temperature causes structural changes and the matrix relaxes into a stabler configuration [2]. The structural relaxation is accompanied by physical property changes such as density [3,4], viscosity [5], elastic constants [3,6], electric [7-91, and magnetic properties [lO,ll], etc. Such a relaxation, which precurses the transformation into the crystallinity, is an important consideration in any industrial application of the material. The relaxation behaviour can be studied by following the associated property changes. The investigations reported here are concerned with the mechanism of structural relaxation in a multicomponent metallic glass employing electrical resistivity, Miissbauer spectroscopy and acoustic emission technique. The 0022-3093/87/%03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division) ,

B.V.

342

G. P. Tiwori

er al. / Sf~crural

relaxation

in (I merollic

glass

experiments are aimed at establishing the mechanism of relaxation in the amorphous matrix. It has been concluded that structural relaxation in this material takes place through viscous flow in the matrix.

2. Experimental

procedure

The present studies have been carried out on meltspun tapes, manufactured by Allied Chemical USA, having a nominal composition of Ni,,Fe,Co,,Cr,, Mo,B,,. The X-ray diffraction showed a pattern characteristic of amorphous matrix. The tapes have a cross-section of 3 mm X 40 pm. The measurement of the electrical resistance on the tapes was carried out with a four-point probe and Kelvin double bridge having a sensitivity of 2 x lo-’ L?.The bridge was operated at 0.25 A dc to minimise heating of the specimen. Isothermal measurements were carried out at 415, 421, 446, 457 and 482. K. The specimens were introduced in the furnace maintained at preset temperature, the temperature control being fl K. As the aim of the experiment was to study the structural relaxations in the amorphous state, it was essential to determine the crystallization temperature of the material. Hence, several runs on a differential scanning calorimeter, Perkin Elmer DSC-2 model, were carried out at two different heating rates, 10 and 20 K/mm. The crystallization temperature was found to be around 700 K. The monitoring of the acoustic emissions was carried out during isothermal heating in the temperature range below the crystallisation temperature using a microprocessor-controlled two-channel acoustic emission set-up. The metallic glass samples were mechanically coupled to a long thick stainless steel strip which acted as a wave guide. Two PZT sensors were attached to this stainless strip after the application of the couplant. The coupling was tested for the acoustic response of the specimen before introduction into the furnace and the sensors were found to pick up the emission satisfactorily. The dummy heating of the assembly without the specimen did not yield any acoustic signal. The signals monitored here were emitted from the specimen during heating and were recorded at 175 and 375 kHz resonant frequencies. Each set of acoustic emission data was collected for a total period of 3840 s. Proper band-pass filters were used to cut the undesired signals. Gain as well as the threshold amplitude were selected so as to record even weak signals emitted during heating. The signals generated during heat treatment were analysed by the microprocessor to evaluate various acoustic parameters. Mijssbauer spectra were recorded at room temperature using a 57Co(Rh) source coupled to a constant acceleration drive and a conventional multi-channel analyser. The samples were vacuum annealed for about 150 rnin at different temperatures (T’) and quenched to room temperature. It is believed that within this time, the samples get relaxed to the configuration characteristic of TA, see e.g. ref. [ll]. The Mossbauer absorbers consisted of a single layer of these relaxed metallic glass tapes closely held together by a sticking tape.

G.P. Tiwari

et al. / Structural

relaxation

in a nwtollic

glass

343

3. Results and discussions

3.1. Resistioity measurement As the aim of the present experiment was to study the structural relaxations in the amorphous state, the temperatures selected for isothermal annealing experiments are below the crystallization temperature. The resistance vs. time plots obtained isothermally at 421,446,452 and 482 K are shown in fig. 1. The significant feature of these plots is that the changes in the electrical resistance are taking place in a discontinuous manner. Every change in resistivity is followed by a pause during which it remains constant. Each subsequent change is usually of smaller magnitude and occurs after a longer duration of pause. To the authors’ knowledge, this type of resistance variation with time

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resistance with time during isothermal annealing variation with time is observed at all temperatures.

at various

344

G.P. Tiwari

er al. / S~rucrural

rehorion

in LI merollic

glass

has not been reported earlier for a metallic glass. It is believed that these changes in the electrical resistivity are a manifestation of structural relaxations in the matrix. The important question, in the present context, is to identify the unit process responsible for the discontinuous change in resistivity at a fixed temperature. The possibility that this is a diffusion controlled phenomenon is contradicted by the following observations: (1) Diffusion-controlled processes generally have an incubation time. Here the resistivity changes begin almost immediately with the start of heating. (2) A diffusion-controlled reaction does not proceed in a discontinuous manner as seen in fig. 1. (3) For diffusion controlled processes, the progress of reaction, when plotted against time, gives either sigmoidal (e.g. recovery of electrical resistance after quenching) or C-type (T-T-T diagrams for steels) plots. This is not observed here. In view of the above considerations, it is suggested that the discontinuous changes in the electrical resistivity shown in fig. 1 occur through the viscous flow taking place in the matrix. The viscous flow takes place in a step-by-step manner. The height of the step as well as the duration betwen two successive steps is controlled by the magnitude of the driving force. If this is large, the step would be of greater height and the duration between them would be smaller. As the annealing progresses, the matrix relaxes into a stabler configuration and hence the driving force for the structural relaxation is diminished. Consequently, the height of the step decreases and the interstep duration also increases with the progress of annealing. The variation of resistivity with time shown in fig. 1 conforms to this pattern. Thus the nature of the resistance change can be accounted by only the viscous flow in the matrix. The viscous flow in a metallic glass is associated with acoustic emissions [12]. Hence, the possibility of a viscous flow can be checked by monitoring acoustic emission during isothermal annealing. These experiments are reported in the next section. 3.2. Acoustic emission The experiments were carried out isothermally at temperatures of 446, 482, 533 and 553 K. The acoustic events emitted during isothermal annealing at 446 and 482 K are shown in figs. 2 and 3. The salient features of the acoustic emissions are as follows: (1) The acoustic emission takes place in discrete steps and not in a continuous manner. (2) To start with, the time interval between two emissions is small and as time proceeds, the intervals become longer. This is also evidenced by the fact that the slope of total acoustic events vs. time plot in fig. 2 is high in the beginning and falls off with time. It is obvious from figs. 1-3 that the time scale of the acoustic activity and the resistance changes are the same. Since the temperature selected for

G.P. Tiwari

er al. / Structural

OU

36.4

see ( IO21

Fig.

2. PIOI of total

acoustic

events

relma~ion

in (I merollic

--I O 9.6

glass

345

19.2 27.0 sac ( IO2 I

against rime at 466 K; (a) 175 kHz; time = 3840 s.

(b) 375 kHz.

38.4

Total

recording of acoustic emission and the resistivity measurements are the same, it is concluded that these two processes are intimately related to each other as they occur under similar time and temperature conditions of the material. There also exists a great deal of similarity between the resistance vs. time plot of fig. 1 and the acoustic activity output given in figs. 2 and 3. Therefore, the same unit process is responsible for resistivity changes as well as acoustic emissions. In this case, the process, in all likelihood, is the viscous flow of the atoms in the glassy matrix. An analysis of the acoustic signal emitted from the metallic glass specimens provides a deeper insight into the nature of the viscous flow process occurring in the matrix. The important parameters which characterise an acoustic emission are the ring down count and the event duration. The ring down count represents the total number of individual acoustic signals emitted within the event duration. The event duration is the time period during which the acoustic signals are emitted. The distribution of events on the basis of event duration and ring down counts, obtained as microprocessor output, for the temperatures 446 and 482 K are shown in figs. 4 and 5. As each acoustic event

9.6 set Fig. 3. The variation

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I IO21 (b) 375 kHz. Total

346

G. P. Tiwori

relaxorion

it, a metallic

glass

446K 175KHz

a IOOO5 z

et al. / Structural

446 K. 175 KHz

000-

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Y 6

12

I I I I0 24 30 RING DOWN

I I I 36 42 46 COUNTS

Fig. 4. Distribution of events on the basis of ring down counts.

0

100200 EVENT

400 600 DURATION ( ps 1

Fig. 5. Distribution of events event duration.

on the basis of

is associated with a single or identical combination of unit processes, the distribution of events is an indication of the number of different microscopic processes going on simultaneously within the matrix. It is observed that the event duration as well as the ring down count remain unchanged for most of the acoustic events. This shows that the same process is repeated again and again and a single unit process is responsible for the structural relaxation. 3.3. Effect of load on acoustic emission A critical test of the proposal regarding the viscous flow can be performed by investigating the effect of externally applied load on the acoustic emission. If the acoustic emissions originate from any flow process taking place within the matrix, the rate of emission must go up with the application of load. Further, the influence of load on the parameters characterising the acoustic signal can provide additional insight into the nature of the operating flow process. Hence, an experiment was carried out as follows: A ribbon of the metallic glass approximately 25 cm long was passed through a furnace. One end of this ribbon was rigidly fixed to a stainless steel strip. The other end carrying a weight amounting to a stress of 0.142 kg/mm’ passed over a pulley which was firmly mounted on a heavy frame. The two PZT sensors were attached directly to the ribbon at the two ends. A 5 cm long constant temperature zone at the centre of the furnace was maintained at the desired temperature. Thus there was a temperature gradient in the ribbon, the temperature being higher at the centre and falling off toward the ends. The part of the ribbons outside the furnace carrying the PZT sensors and the load were at room temperature. The presence of the temperature gradient might alter the viscous flow and corresponding acoustic emissions. However, this does not affect our conclusions because the temperature conditions before and after the application of load were kept exactly identical. The acoustic emissions with and without load were recorded for a total period of 3840 s. Disturbances caused during the loading of ribbon give rise to a sudden enhancement of acoustic activity. These signals die down very soon

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events with lime at 446 K at (a) 175 kHz the lime when the load was applied.

and are disregarded in the analysis of the overall acoustic pattern. The total events emitted at 175 kHz and 375 kHz are shown in fig. 6 and the exact time of application of load is also indicated there. The slope in total events vs. time in fig. 6(a) registers a distinct rise after the application of load, indicating a rise in the rate of acoustic activity and thus confirming that these signals emanate from a flow process in the matrix. This will be evident by a comparison between figs. 2(a) and 6(a). However, the events recorded at 375 kHz are practically unaffected (fig. 6(b)). There is no change in slope before and after the application of stress. This particular mode is perhaps actuated by internal stress only and hence the external constraints do not have any influence. The number of events recorded at this frequency constitutes less than 2% of the total events. Hence, the signals recorded at 375 kHz are not very important for assessing the effect of load on the acoustic emission. However, it is significant that both frequencies of the acoustic signals are not altered by the application of load. The fact that the frequencies are unaltered by the load application suggests that the nature of flow processes does not undergo a change with the load.

446K 175KHz

446 K 175 KHz

0 0

6

12 I.5 24 30 36 42 RING DOWN COUNTS

46

54

Fig. 7. Distribution of events on the basis of ring down counts. Here an external stress of 0.142 kg/mm’ was applied after nearly 30 min (see fig. 4).

0

100 200 300 400 500 600 700 800 SJO EVENT DURATION (JIS I

Fig. 8. Distribution of events on the basis of event duration. Here an external stress of 0.142 kg/mm’ was applied after nearly 30 min (see fig. 5).

01

I 9.6

I II 19.2 27.0 sac ( IO21

Fig. 9. A plot of voltage

38.4

Cl1 9.6

level (pv) against time at 446 K. The voltage release during the acoustic emission.

19.2 27.0 see ( IO2 I

level is a measure

36.4

of energy

This is further substantiated by analysis of the ring down counts (RDC) and event duration (ED) shown in figs. 7 and 8. Along with the frequency, the values of RDC and ED fully characterise an acoustic event. The average values for the RDC and ED are 6 and 100 ps under unloaded conditions. These values remain unchanged after the application load except a few new modes are activated. Moreover, in either case, more than 90% of the signals have the same values for RDC and ED. The observation that these parameters remain unaltered strongly suggests that the viscous flow process, giving rise to acoustic emission, remains unchanged except that load causes an enhancement in the rate of emissions. This is reflected in the higher value of events recorded under load (see figs. 4 and 7). Energy release during the acoustic emission is another important characteristic of the flow process. This parameter is recorded for both unloaded as well as loaded conditions in figs. 9 and 10. In the presence of load, there is only a marginal increase in energy emission. This again shows that the load only enhances the rate without altering the nature as well as the characteristics of the process itself.

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after

nearly

30

G.P. Tiwari

3.4. Wssbauer

et al. / Swucrural

relaxation

in (I merdic

glass

349

spectroscopy

Any changes in structural arrangement arising from the change in the short-range order (SRO) are manifested in the EFG tensor components. The resulting quadrupole interaction changes are not only sensitive to the number of nearest neighbours and their separation from the probe nucleus but also depend on their angular distribution. Mijssbauer studies were carried out with a view to monitor the possible changes in the SRO which could be taking place during isothermal treatments mentioned earlier. Figure 11 shows the “Fe transmission Mbssbauer spectra, recorded at room temperature on the as-received sample as well as those annealed in vacuum for 150 min at temperatures indicated in the figure. In order to obtain the distribution of quadrupole splitting (QS) from these spectra, the analysis was carried out using a model-independent Fourier deconvolution method [13], without using the least-squares fitting procedure. In this method of analysis, an even part of the Fourier deconvoluted spectrum gives an even part of the distribution of dominant hyperfine interaction, which is QS in this case. The odd part of the deconvoluted spectrum determines the correlation of quadrupole splitting with

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temperatures.

350

G. P. Tiwori

et al. / S~~ctural

OS.

Fig. 12. Probability

of QS dislribution

relaxorion

m.m

/set

for MGssbauer

in ~1merollic

glass

-

spectra

shown in fig. 11

the chemical isomeric shift (IS), i.e. d(IS)/d(QS) is estimated. Figure 12 shows the QS-distribution obtained from this analysis. The goodness factor obtained from the analysis of each of these spectra (fig. 12) was found to be better than 0.9, which is quite satisfactory [13]. The value of d(IS)/d(QS) was found to be 0.02 k 0.01. This rather low value of d(IS)/d(QS) justifies our assumption in this deconvolution analysis that the quadrupole interaction is the dominant interaction. It also shows that QS is much more sensitive to the structural arrangements within the amorphous system than IS. These results suggest that, at least up to 630 K, the sample remains amorphous. This is in conformity with our DSC results. The sample heated to 700 K in the DSC run was also found to be partially crystallized by X-ray diffraction and showed a distinctly different Mijssbauer spectrum. The QS-distribution (fig. 12) shows the the environments around the “Fe probe undergo a small but definite change on relaxation. It is reported by Czjzek et al. [14] that for a completely random distribution of atoms, the probability of QS-distribution, P(QS), for values of QS close to zero, is proportional to I&. Non-zero values of P(QS) near zero QS for all spectra mean a definite deviation from complete randomness, i.e. a selective near-neighbour environment is preferred. If zero (QS) is taken to be the

criterion for the randomness, it follows that the as-quenched sample is most random having the highest fictive temperature and the one annealed at 478 K is least random. In conclusion, the annealing of this metallic glass at least up to about 638 K does not lead to any gross compositional changes as seen by the iron atoms. This may be interpreted to mean that long-range diffusion, which precedes the formation of a new phase, does not take place here. QS-distribution analysis, however, suggests that small but definite changes in the SRO take place as a result of annealing. It is not, however, possible to attribute these changes either to topological or chemical SRO. 3.5. Mechanistic

considerations

The burden of experimental evidence presented here brings out two facts. The structural relaxations - as reflected through the resistivity changes - take place in a discontinuous manner. Moreover, this relaxation is accompanied by the acoustic emissions. The acoustic emissions are a consequence of the propagation of the stress waves in the matrix. In the present context, this is possible only through the relative movement, on a microscopic scale, of the different regions of the amorphous matrix through a viscous flow mechanism. It should be pointed out here that viscous flow in the metallic glasses may occur under externally applied stress as well as during the transition of the amorphous matrix into the crystalline state [12]. However, the viscous flow is identified here, for the first time, as a unit process responsible for structural relaxations in the metallic glass arising from thermal annealing. The presence of l-5% of the microcrystalline material cannot be ruled out in the material studied here. A crystalline matrix can give rise to acoustic signals through dislocation movement, twinning and/or fracture taking place during phase transition [15] or plastic deformation [16,17]. In the present studies Miissbauer and DSC runs have failed to indicate the presence of any phase change. Secondly, the acoustic signals are given out during isothermal annealing without the application of any stress. Hence, movement of dislocation or twinning can be ruled out as a possible source for acoustic signals. The amorphous nature of the matrix after the measurement of resistivity and recording of acoustic signals was repeatedly confirmed through X-ray diffraction. It is evident, therefore, that the amorphous matrix is the only source of acoustic emissions recorded here. During viscous flow, part of the material moves as a whole over other regions. In such a process, the preferential movement of a particular species to form a cluster or segregation is ruled out. However, at the end of viscous flow, minor local re-arrangements must take place to acquire a new configuration of lower energy. This is precisely what is seen in the Mossbauer spectra taken after annealing. It has been suggested that such flow units could be quasimolecular defects having multicentered lengthy configuration [18]. The activation volume associated with the flow process has been determined (to be

352 Table 1 Homogeneous

strain

components

in a metallic

glass

Sample No.

Name of the flow process

Recoverable/ permanenl

Time dependence

stress dependence

1 2 3 4

ideal elaslicity anelaslicily visco-elaslicily inslanlaneous

recoverable recoverable permanent permanent

inslanlaneous time dependent lime dependenl inslanlaneous

linear linear linear non-linear

plaslicily

reported later) using load relaxation experiments, the value obtained being lo-l3 cm3. Thus about 10’“-lO” atoms are involved in a single flow event. Whenever a material is subjected to external loading, the resulting strain can be classified either as homogeneous or heterogeneous. Heterogeneous strain arises due to a rise in the level of internal stress at a particular cross-section in the matrix which ultimately leads to fracture. This is, however, not of interest here. According to Nowick and Berry [19], the homogeneous strain in a glassy metallic alloy comprises the four components which are listed in table 1. Viscous flow observed here in the metallic glass during isothermal structural relaxation must belong to one of the four flow processes mentioned in table 1. Isothermal annealing causes permanent changes in physical properties (resistivity, ductility, density, viscosity, etc.). Similarly in the present case, the room temperature value of the electrical resistivity did not return to the starting value after annealing. Hence, the associated strain should also be permanent [9]. Secondly, the process is not instantaneous but occurs over a period of time as seen in figs. l-3. Thus it is clearly time-dependent. Again at the flow stress employed here, the flow processes in metallic glasses generally obey Newtonian behaviour so that flow is linearly dependent on stress. From these considerations, and as shown in table 1, it is concluded that the flow process observed here can be characterised as viscoelastic flow. Structural relaxation in metallic glasses precedes the crystallisation. Hence, the mechanism of structural relaxation has a very crucial role in the stability of such materials. The important implication of the preceding results and discussion is that the viscous flow is the primary unit process in the structural relaxation of the metallic glass studied here. The diffusional processes seem to have a relatively insignificant role in the initial relaxation. The material studied here has several constituents. It is likely that in simpler binary metallic glasses, the interstitial movement of the metalloid atoms may have some role in structural relaxation even in the initial stages of annealing. In any case, experimental evidence suggests that the formation of a stabler amorphous structure is initiated through a kind of flow process. The interface where such flow units meet may form the centre of crystallisation nuclei at subsequent stages. Similar studies in different metallic glasses are in progress to establish the generality of the phenomena reported here.

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relaxarion

in a metallic

glass

353

References [l] J.H. Gibbs and E.A. Dimarzio. .I. Chem. Phys. 28 (1958) 373. [2] H.S. Chen, Rep. Prog. Phys. 43 (1980) 353. [3] A. Kurusomovic and M.G. Scolt. Appl. Phys. Letl. 37 (1980) 620. [4] A. Kurusomovic. E. Girt. E. Babic, B. Leontic and N. Njuhovic, J. Non-Cryst. Solids 44 (1981) 57. [5] A.I. Taub and F. Spaepen, Acta Metall. 28 (1980) 1781. [6] K. Bothe and H. Nashaser. J. Non-Cryst. Solids 56 (1983) 279. [7] M. Balanzat. Scripta Metall. 14 (1980) 173. [S] J.R. Cost and J.T. Stanley, Proc. 4th Int. Conf. on Rapidly Quenched Metals (1981) p. 491. [9] M.G. Scott and A. Kurusomovic, Acta Metall. 30 (1981) 853. [lo] H.H. Liebraman, CD. Graham Jr. and P.J. Flanders, IEEE Trans. Msg. MAG 13 (1971) 1541. [ll] C.D. Graham Jr and T. Egami. J. Magn. Magn. Mat. 15 (1980) 1325. [12] A.E. Lord, Jr and P.M. Anderson III, Lett. Appl. Eng. Sci. 5 (1978) 335. [13] 1. Vincze. Nucl. Inst. and Meth. 199 (1982) 247. 1141 G. Czjzek, J. Fink, F. Gotz, H. Schmidt, J.M.D. Coey, J.P. Rebouilhat and L. Lienard. Phys. Rev. B 23 (1981) 2513. 1151 C.S. Barett and O.R. Tantz. Trans. AIME 175 (1948) 579. [16] W.P. Mason, Eng. Fract. Mech. (EFMEA) 8 (1976) 89. [17] W.W. Gerberich and C.E. Hartbower. Int. J. Fract. Mech. 3 (1967) 185. [I81 S.A. Demborsky and E.A. Chechetkina, J. Non-Cryst. Solids 64 (1984) 95. [19] A.S. Nowick and B.S. Berry. in: Anelastic Relaxation in Crystalline Solids (Academic Press, New York, 1972) ch. 1.