J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 3 ( 1 9 8 4 ) 1 4 9 - - 1 5 2

Pressure induced structural relaxation in amorphous solids-example of Fe4oNi4oP14B6

MOHAMMAD YOUSUF, K. G O V I N D A RAJAN Reactor Research Centre, Ka/pakkarn 603 102, India

We report the first observations of pressure induced structural relaxation in the amorphous system F%oNi4oP14B6. This alloy was found to undergo at room temperature two stages of structural relaxation at around 40 and 60 kbar, respectively, Electrical resistivity and X-ray diffraction were used to monitor the relaxation, and the results seem to point out that pressure stabilizes the liquid-like features in a rapidly quenched amorph- ous alloy.

From many detailed studies on alloys rapidly quenched from the liquid state, the following picture emerges: (a) these alloys are never fully stabilized and (b) subsequent thermal annealing, which does not cause crystallization, induces changes in the structure which results in densi- fication. The effect of this kind of structural relaxation (SR) on a number of physical and mechanical properties in liquid quenched metals has been widely studied [1-6] . The nature of the influence of SR on the physical properties is believed to be connected to the increased topo- logical short range order in the amorphous system [7-9] .

Recently Egami et al. [2] and Chen e t al. [3] have attempted to interpret the structural relax- ation as arising out of the "structural defects" in amorphous alloys and their behaviour during a thermal treatment. The structural defects in amorphous solids are defined in terms of the distribution of the internal stresses at the atomic level and of the symmetry of the environment of individual atoms. Thus, it appears interesting to consider the effect of an external pressure to an amorphous system especially from the view point of a structural relaxation (similar to that induced thermally). This is because, the structural defects in the model by Egami e t al. [2] and Chen et al. [3] can interact with an external pressure, and, if these defects are truly diffuse, collective structural disorders, should lead at room temperature to

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structural relaxation occurring sharply at high pressure.

The system chosen for study was the amorph- ous alloy F%0Ni4oP14B6, supplied by Allied Chemicals, USA. The samples were in the form of ribbons 14mm wide and 50micron thick. Discs, 3 mm diameter, were cut and loaded in a Bridgman anvil high pressure resistivity cell. The cell used here was similar to that described in [10], and the hydrostatic pressure transmitter was 1:4 ethanol: methanol. This was done to ensure that the pressure was truly hydrostatic. The voltage output from the Metglas sample was fed to a multipen x - t recorder, and the time evolution of the elec- trical resistance was monitored after every small pressure step was applied, and the sample was held at each pressure for an hour. The steady state electrical resistance (R~) as a function of pressure was observed to be as shown in Fig. 1. Pressure inside the cell was gauged by simultaneously monitoring the electrical resistance of a pressure marker (bismuth or tin). The time variation of t he electrical resistance of the amorphous alloy was analysed using the standard relaxation equation (with the usual notation):

R~ = R0 p + (R p --RoP)[1 -- exp (-- t/r)].

The value of r so obtained for various pressures are as shown in Fig. 1. It is seen that r shows two peaks, one at ~ 40 kbar and the other correspond- ing to ~ 60 kbar. This can be interpreted in terms of pressure induced relaxation occurring sharply at these pressures.

An elegant method of following a relaxation process, which is suspected to be structural in origin, is to perform X-ray diffraction measure- ments. The diffracted intensity I (q ) as a function of q(= 4zr sin 0/X) leads itself through the inter- ference function analysis to a detailed structural information [6, 7]. Following the approach of Egami, polychromatic X-ray diffraction measure-

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Figure 1 Steady state electrical resistivity R= (which is proportional to the sample voltage, since a constant current was used) and relaxation time T of lVIetglas 2826, Fe40Ni,oP~,B 6. The dots (o) represent steady state resistivity and inverted triangles (v), the relaxation time r. Electrical resistivity (*+ voltage output) shows a maxi- mum, and a wiggle at the pressures at which r shows maxima.

ments on the pressure treated metallic glass samples were made, and the data is displayed in Fig. 2. White X-ray beam derived from a tungsten target at 60kV and 30mA, was well coll imated and stuck the metallic glass sample held on a goniometer head. The diffracted beam was also coll imated and was received by a Si(Li) detector posit ioned at about 12 ° from the direct beam.

This angle was precisely determined by a cali-

bration run with an iron foil in place of the amorphous alloy and measuring the energies at which the diffraction peaks occurred. All pulse height measurements were done with a 4096 channel multichannel analyser. The useful range of q spanned in this set up was from 27 nm -1 to 130nm -1. The experiment performed on each sample had an integrated count of 2 x 10 7 to minimize the statistical errors and to make mean- ingful comparison of the peaks from samples treated at different pressures. The X-ray spectra obtained on the pressure treated samples were analysed to bring out the features o f structural relaxation induced by pressure.

In Fig. 3a a schematic of the X-ray diffraction pat tern obtained from the amorphous sample is shown, and various parameters derived from it are indicated. The "shoulder" appearing after the second peak is characteristic of an alloy obtained by rapid quenching and distinguishes it from a liquid. The width of this shoulder was taken to be the range of q values appearing between the start and end of the shoulder. Also, the intensity of the shoulder was evaluated at the midpoint of this width region, and used in subsequent comparison. Fig. 3b shows how this width parameter varies with pressure, and it is evident from the figure that sharp changes occur in the shoulder width at

40 and ~ 60 kbar, respectively, nicely corrob- orating the resistivity and relaxation time results. The other quantities of interest which have been plot ted are the intensity ratio IIS/IIP and IIS/IIIP in Figs. 3c and d, respectively. They also show

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Figure 3 (a) Shows the various parameters of the spectrum, in which the first peak (IP), second peak (IIP), shoulder (IIS), shoulder width and the third peak (IIIP) are indicated. (b) Shown here is the variation of the shoulder width with pressure. A decrease at 40 and 60 kbar can be noted. (c) Variation in the intensity ratio IIS/IIP as a function of pressure is plotted, from which sharp decrease correspondingly at 40 and 60 kbar can be noticed. (d) Change in IIS/IIIP intensity ratio with pressure; sharp changes at 40 and 60 khar herald structural rearrangement in the amorphous alloy 2826.

sharp changes at 40 and 60kbar, further con- firming the conclusion that structural relaxations are initiated at these pressures. The experimental results fall into two categories: steady state behaviour of the amorphous alloy under high pressure and relaxation aspects. In the literature, there is an earlier work on the variation of electrical resistivity o f the alloy 2826 in the range of pressure covered in the present work [11]. The present results are in agreement with that work in so far as the shape of the resistivity pressure curve is concerned, but, however, the second peak observed in the pressure studies has not been reported by Vaidya and co-workers [11, 12], possibly due to their insufficient resolution. It is possible to inter- pret the overall shape of the curve by suitably modifying the Ziman's theory of simple liquid metals [13-17] and allowing pressure variations in the structure factor, Fermi energy, etc.

Turning now to the more interesting relaxation aspects, it may first be recalled that any defect

increases the specific volume of the amorphous alloy and under hydrostatic pressure, the energy of the amorphous solid containing larger number of defects will be higher. This extra strain energy will then provide a driving force for the elimination or annealing of the defect. However, the defect must migrate to be removed. The sign and value of the AVm, the activation volume for migration is an indicator of the possibility of the defect migration under pressure and its efficiency. Processes with negative activation volume of migration will be enhanced by pressure. Any possible variation of the activation energy with pressure will also affect the process, but the data is very meagre on this aspect [18]. The calculation by Johnson [19] on 3'-Fe ( f cc ) gave a negative AV m for vacancy migration, and the experiments by Kulcinski [20] on molybdenum showed that pressure indeed decreases the stage III recovery temperature substantially.

The authors conclude the paper by pointing out

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that pressure induced relaxat ion has been observed

in an amorphous alloy, similar to that f ound in a

crystal l ine metal , and by raising the quest ion

whe ther ideas such as act ivat ion vo lume can be

meaningfu l in amorphous systems.

Acknowledgement The authors thank Mr V. S. Sastry for his valuable

help in E D X T measurements .

References 1. M. LASOCKA and H. MATYJA, in "Treatise on

Materials Science and Technology", Vol. 20, Ultra- rapid Quenching of Liquid Alloys, edited by H. Herman (Academic Press, New York, 1981) p. 261.

2. T. EGAMI, K. MAEDA and V. VITEK, Phil. Mag. A 41 (1981) 883.

3. H.S. CHEN, K. J. AUST and Y. WASEDA, d. Non- cryst. Solids46 (1981) 307.

4. H.S. CHEN and S. Y. CHUANG, AppL Phys. Lett. 31 (1977) 255.

5. T. MIHARA, S. OTAKE, H. FUKUSHIMA and M. DOYAMA, J. Phys. F: Met. Phys. 11 (1981) 727.

6. T. EGAMI, in "Topics in Applied Physics", Vol. 46: Glassy Metals I, edited by H. J. Guntherodt and H. Beck (Springer-Veriag, Berlin, 1981) p. 24.

7. T. EGAMI, J. Mater. ScL 13 (1978) 2587.

8. H.S. CHEN, Scripta Metall. 9 (1975) 411. 9. R.S. WILLIAMS and T. EGAMI, IEEE Trans.

Magn. 12 (1978) 927. 10. K. GOVINDA RAJAN, V. SANKARA SASTRY and

RITA KHANNA, Rev. Sci. lnstrum. 52 (1981) 1734. 11. V. VIJAKUMAR, D. IC JOSHI and S. N. VAIDYA,

J. Non-eryst. Solids 43 (1981) 439. 12. Work by Coehrane et al. is limited to only 5 kbax,

Solid State Commun. 35 (1980) 199. 13. H.J. GUNTHERODT and H. U. KUNZI, in"Metallic

Glasses", edited by J .J . Gilman and H.J. Leamy (American Society for Metals, Metal Park, Ohio, 1978) p. 247.

14. G. FRITSCH, J. WlLLER, A. WlLDERMUTH and E. LUSCHER, in "Physics of Solids Under High Pressure", edited by J. S. Schilling and R. N. Shelton (North-Holland Publishing Co., Amsterdam, 1981) p. 239.

15. P.J. COTE and L. V. MEISEL, Phys. Rev. Lett. 39 (1977) 102.

16. ldem, ibid. 40 (1978) 1586. 17. Idem, in "Topics in Applied Physics" Vol. 46, Glassy

Metals I, edited by H. J. Gtmtherodt and H. Beck (Springer-Verlag, Berlin, 1981)p, 141.

18. J.R. BEELAR Jr, Phys. Rev. 150 (1966)470. 19. R.A. JOHNSON, ibid. 145 (1966) 423. 20. G. L. KULCINSKI, ibid. 179 (1969) 676.

Received 25 July

and accepted 26 July 1983

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