STRUCTURAL RELAXATION AND ATOMIC MOBILITY BY MAGNETIC ANISOTROPY MEASUREMENTS IN SOME METALLIC GLASSES

Transcription

1 STRUCTURAL RELAXATION AND ATOMIC MOBILITY BY MAGNETIC ANISOTROPY MEASUREMENTS IN SOME METALLIC GLASSES W. Chabron, A. Chaberod To cite this version: W. Chabron, A. Chaberod. STRUCTURAL RELAXATION AND ATOMIC MOBILITY BY MAGNETIC ANISOTROPY MEASUREMENTS IN SOME METALLIC GLASSES. Journal de Physique Colloques, 1981, 42 (C5), pp.c5-511-c < /jphyscol: >. <jpa > HAL Id: jpa Subitted on 1 Jan 1981 HAL is a ulti-disciplinary open access archive for the deposit and disseination of scientific research docuents, whether they are published or not. The docuents ay coe fro teaching and research institutions in France or abroad, or fro public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de docuents scientifiques de niveau recherche, publiés ou non, éanant des établisseents d enseigneent et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE CoZZoque CS, suppz&ent au no1o, Toe 42, octobre 1981 page C STRUCTURAL RELAXATION AND ATOMIC MOBILITY BY MAGNETIC ANISOTROPY MEASUREMENTS IN SOME METALLIC GLASSES W. Chabron and A. Chaberod Centre d 'Etudes NucZdaires de GrenobZe, Dgpteent de Recherche FondaentaZe Section de Physique du SoZide, 85 X GrenobZe Cedex, France Abstract. - 1Je copare the agnetic anisotropy induced by theroagnetic treatents in soe aorphous etallic alloys, considered in three different states : as-received, annealed, annealed and subsequently quenched. The atoic obility is shown to decrease during an anneal, and to be partially restored by a quench. It is concluded that soe processes involved in what is called "structural relaxation" are reversible, at least in part. 1. Introduction.- Internal friction (IF) easureents have been used for a long tie to study the atoic obility of crystalline alloys [l]. By such a ethod, anelastic relaxation produced by the short range directional order, indltced by a stress, is investigated. The Snoek peaks, observed in insertion solid solutions, allow to deter ine interstitial atos obility, while Zener peaks, insubstitution solid solutions, characterize constituent atos obility. In ferroagnetic crystalline alloys, a directional order (DO) can also be ob- tained by theral treatent under agnetic field, called theroagnetic treatent (T'fT). (I'IA). This DO is detected by easuring the associated induced agnetic anisotropy As in the case of stress-induced DO, the phenoenon is observed both in in- sertion and substitution solid solutions 121. In aorphous etallic alloys, an exponential increase of IF is observed at high teperature [31. It is due both to an anelastic,and aviscoplastic deforation [&l. The latter prevails at high teperature 151, but it is often difficult to se- parate the two contributions. Let us now perfor a TFIT on a ferroagnetic aorphous alloy : one observes an IHA which is - contrary to the deforation - entirely reversible as a function of teperature, and direction of the applied agnetic field 161. This PfA is probably due to a short range DO [7]. Analogy with crystalline aterials leads to think that IMA, and anelastic relaxation, arise fro atoic displaceents. However, when a quantitative coparison has been attepted, restrictions have been expressed [6], because of differences between activation energies. In this paper, we discuss the influence of structural state (as--received, annealed, annealed and subsequently quenched) on the kinetics of establishent of Article published online by EDP Sciences and available at

3 C JOURNAL DE PHYSIQUE IYA, and therefore on the atoic obility. 2. Experients.- The aorphous alloys used are thin ribbons (50u thick) fro ALLIED CHEaIICAL COSPOMTION (Fe4o Ni40 PI B6), VACUP4SCBIELZE (Cogg Ni10 Fe5 Sill B, 6), and GENERAL ELECTRIC B14,5 Si4). Saples are disks of 7 diaeter, cut in these ribbons. The IYA is easured with an autoatic torque agnetoeter, under a agnetic field of 0.34 T, with an accuracy of T.b3. situ" in the torque agnetoeter. The II!A TMT are perfored "in energy density is denoted KU. Fig. 1-3 show the results obtained during isochronal TtfT, 19 C/in. Cle use the "crossed-field" ethod [S]. In this ethod, like in an IF experient where the stress is reversed positive to negative at each half-oscillation of the pendulu, the agnetic field agplied during a TIT is directed perpendicular to the one applied during the previous TYT. Therefore the IYA changes alternatively fro one direction to the other, at each TW. Fig. 1-3 show the variations, ]AK~\, of IMA energy as a

4 We can note that each curve in fig. 1-3 shows a axiu. On the ascending part, the tie (10 in) of each TMT is short, copared to the tie constant of the IE?A es- tablishent. Then, one obtains only a fraction (a = AK /K, ) of the asyptotic value, u KU, and this fraction increases with teperature, because the process is therally activated. On the decreasing side, the tie constant of the IMA i s short enough so that 0 is close to 1 ; then IAK,~ varies as KU, and one observes the decrease of KU, as a function of T, as it does to be zero at the Curie teperature. a) "as-received" state. Curves "a", in fig. 1-3, are obtained on saples in as- received state. They are not repeatidly reproducible on a sae saple : if the series of isochronal TMT is stopped at a given teperature, then taken again at a lower teperature, the second curve is located below the first one, and overtakes this one practically at the stop teperature. So, an evolution, generally considered as irreversible, takes place all along the theral sequence of curves a. It corresponds to a progressive decrease of (AK I, and of the atoic obility. This evolution is called "structural relaxation". b) "annealed" state. Curves "b", in fig. 1-3, correspond to saples "stabili- zed", say annealed at a teperature a little lower than the crystallization onset, and cooled slowly. They can be repeatidly reproduced, with the sae saple, because the IMA is henceforth reversible : the structural relaxation has been achieved du- ring the stabilization anneal, and does no ore interfere to odify the phenoena. Such an experient is often called "isoconfigurational". Fig. 4. Isotheral TMT on Fe40Ni40P14B6 al- Fig. 5.. Arrhenius plot of the tie-- loy. Noralized value of the SMA energy vs constants for the kinetics of fig. 4. tie

5 C5-514 JOURNAL DE PHYSIQUE On fig. 4 are plotted the isotheral kinetics of IYA establishent for the alloy Fe40 Ni40 P14 B6, annealed 30 in at 360 C. These curves can be deduced one fro each other by a shift along the log t axis. They are well described by an expo- nential~ su, with a gaussian distribution of the tie constants [ g], whose the width P is about 4 [10]. The ean tie constants of the curves on fig. 4 are drawn on fig. 5, in the shape of an Arrhenius plot. They can be observed to obey very well an Arrhenius law, with an activation energy of 1.74?! 0.04 ev, and a preexponential ter of about 10-15s ; this value is close to that characterizing atoic jups in etallic crystals. A siilar study on an alloy C O ~ ~ N ~ gives ~ ~ analogous F ~ ~ results S ~ : ~ ~ B ~ ~ 1.85 ev and 5 x 10-l6 S, c) "annealed and subsequently quenched" state. Curves "c", in fig. 1-3, cor- respond to saples annealed as in "b", but quenched, instead of slowly cooled. For each alloy one observes an enhanceent of the atoic obility, induced by quench, and this enhanceent disappears gradually during the following TFIT. For a given saple, the effect is larger the higher the quench teperature, and is a reversible function of this teperature. Let us note that it is specially outstanding for the alloy Ni10 Fe5 Sill B16; the reason is that it has been possible to quench this alloy fro a higher teperature than the others, thanks to a higher crystalli- zation teperature (500 C). Fig. 6. Soe isotheral TMT on Fe40Ni40P14B6 Fig. 7. Arrhenius plot of the tie alloy, at a fixed teperature, TA = 160'~. required to obtain a fractional va- The initial state is obtained by 2 hours an- lue, CS= 0.03, of the IPfA energy. nea1,at T2, followed by a quench. On fig. 6 we have reported the variations of 0 during isotheral TFlT at 160 c, following a quench fro different teperatures (Fe4o Ni40 P14 B6). Fig. 7 gives a

6 plot of the tie necessary to obtain a fixed value of U (0.03), as a function of the reciprocal quench teperature, TQ. Points are observed to be practically aligned, except for low values of TQ. That could be due to a tie of anneal at Tq too short, so that the alloy does not reach its configurational equilibriu [Ill. Then, we ought to consider only the high teperature part of the diagra, which gives an apparent activation energy of 0.26 ev. a) Origin of IHA. In crystalline alloys, the obility of sall size atos, in interstitial position, differs arkedly fro that of substitufionalatos. For ins- tance, in Fe-Ni-C alloys, the DO due to carbon atos appears between 100 and 200 C [12], while the one due to Fe and Ni atos occurs between 400 and 500"~ [13]. In aorphous alloys, as shown on fig. 1-3, only one stage in observed, very broad in teperature, and without substructure. We neither observe any fundaental difference between alloys constituted of two or three etals (fig. 1-2), or only one (fig. 3). So, it sees that, in aorphous alloys, it should be considered a siultaneous rearrangeent of all atos, etals and etalloyds, rather than a sepa- rated contribution of 2 types of DO, as proposed by Luborsky 171. b) Structural relaxation and quenching-effect. The difference between curves a and b on fig. 1-3 shows that the structural relaxation accounts for a reduction of atoic obility, as in diffusitivity [l41 or creep [l51 experients. On the other hand, the quenching experients show that the atoic obility is partially restored by a quench fro T4. Such a quench results in freezing the structural state esta- blished at T On the other hand, the as-received alloy, obtained by the quench fro Q' the elt, is in a structural state which can be considered as frozen fro a fictive teperature, Tf > TQ ; then, the atoic obility is greater. As a conclusion, these experients show that the evolution occuring during structural relaxation is, at least partially, reversible. The process can involve localized holes [161, or "distributed free volue" [l 7 1, or short-range ordering [ References [l] A.S. Nowick and B.s. ic Press, New-York (1972). Berry, Anelastic Relaxation in Crystalline Solids, Acade- [21 G.W. Rathenau and G. De Vries, in Magnetis and Metallurgy, edited by A.E. Ber- L31 B.S. kowitz and E. Kneller, Acadeic Press, New York (1969). Berry, in Metallic Glasses, The Aerican Society for "letals, lletals Park, Ohio (1978). [4] H.S. Chen and V. Goldstein, J. Appli. Phys. 3, 1642 (1972) [5] J. Perez, J.Y. Cavaille, S. Etienne and F. Fouquet, J. Phys. 5, C8-850 (1980). [61 B.S. Berry and W.C. Pritchet, AIP Conf. Proceedings 3, 292 (1976). [71 F.E. Luborsky and J.L. Walter, IEEE Trans. 'lagn. LUG-13, 1635 (1977). [81 W. Chabron and A. Chaberod, Solid State Counications 3, 61 (1980).

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