CLUSTER MANIFESTATIONS IN Fe AND Fe-Ni

CLUSTER MANIFESTATIONS IN Fe AND Fe-Ni BASED GLASSY ALLOYS DURING THEIR CURIE TEMPERATURE RELAXATION K. Bán1), A. Lovas1), J. Kováč2), L. Novák3) 1)

Department of Vehicles Manufacturing and Repairing, Budapest University of Technology and Economics, Budapest, Hungary 2) Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia 3) Department of Physics Technical University of Košice, Košice, Slovakia abstract: In this paper we give a possible interpretation of some phenomenon which appeared at examination of thermal processes (structural relaxation and low temperature treatment) and magnetic properties of Fe-Ni based glassy alloys. These alloys nowadays are more and more popular in magnetic applications. On the other hand their properties in more details were not been understood yet. In our consideration we could successfully alloy the behaviour of crystalline Fe-Ni system and cluster theory which is the best interpretation of the glassy state in these days and connect to our experimental observation.

1. INTRODUCTION Clusters are considered in the condense matter state as an assembly of few atoms being in bonding relation during the time-period of observation [1]. The physical properties of small clusters with free surface are size dependent and strongly differ from free atoms, molecules or bulk solids as well even consisting of same elements. Hence, their size-related properties are always between the individual and collective atomic properties [2]. From thermodynamic point of view the Clusters with free surface are metastable (either topological or compositional sense) [3]. In this paper such cluster–related phenomena will be treated in which the clusters are formed in the glass forming melt without free surface embedded in the glassy matrix. Consequently no sharp compositional or topological change exists between the clusters and their surroundings [4]. Their magnetic manifestations and the metallurgical background will be treated briefly mainly in the Fe-B and Fe-Ni-based glassy alloys obtained by rapid solidification. 1.1. The Bethe-Slater curve [5] and the fluctuating exchange integral in the rapidly quenched Fe and Fe-Ni based glasses The essential feature of metallic glasses is the frozen-in free volume (related to the equilibrium crystalline state) proposed by Turnbull and Cohen which defined as v = vWS vDRP, where vWS is the volume of the Wigner–Seitz (WS) cell around the atom, and vDRP is the volume of the WS cell in the dense random packed (ideal amorphous) glassy structure [6]. However, the distribution of free volume is not uniform in atomic level, but exhibits as density fluctuations either in the short range or over a few atomic distances (which can be observed by the high resolutions transmission electron microscopy (TEM) as a „medium range order” in these materials [7]). As the nature and strength of magnetic coupling is concerned, the magnitude and sign of the exchange integral J depends also sensitively on the

inter-atomic spacing. Consequently, both the inter-atomic spacing and the local chemical environment (CSRO) do fluctuate in the disordered glassy matrix. The original meaning of the Bethe-Slater (BS) curve (plotted for the 3d transition elements) can be understood on the basis of Fig 1. where the sign and magnitude of exchange integral (J) is plotted versus a/d (where a is the interatomic spacing and d is the radius of unfilled d shell). This conception cannot directly applied for the appreciation of the strength of ferromagnetic glassy state because of the mentioned spatial and chemical fluctuation. Consequently, the net magnetisation and the strength of magnetic coupling (represented by the amorphous Curie temperature (TCam)) can be better described by the polycluster conception of metallic glass proposed in Ref. [4]. Similar conclusion was drawn by Egami when the conception of compressed and stressed atomic environments was introduced [8].

Fig.1. The Bethe-Slater curve representing the variation of the exchange integral J with interatomic spacing a and radius d of unfilled d shell. The fluctuating local atomic volume and the associated manifestation in the strength of ferromagnetic coupling is clearly reflected in the extraordinary concentration dependence of magnetic moment/ Fe atom (µFe) as well as in the concentration dependence of amorphous Curie temperature (TCam) in binary Fe-B glasses (Fig. 2. a) and b)).

a)

b)

Fig.2. a) Compositional dependence of magnetisation (µFe) and b) the amorphous Curie temperature (TC) in binary Fe-B glasses [9].

Neither the magnetisation nor the amorphous Curie temperature exhibit increase in the hypoeutectic region (with increasing Fe-content) as it is expected. Contrary, there is a definite decrease in both properties as the Fe content increases. This fact hints to the existence of mixed antiferromagnetic and ferromagnetic coupling between the neighbouring Fe atoms in the hypo-eutectic region of Fe-B. The reason is gradually decreasing ferromagnetic coupling as the fraction of Fe atoms increases. In the spirit of Bethe-Slater curve this mixed magnetic interaction looks to be a reminiscence to the Fe- allotropes in short range order scale which means the co-existence of compressed fcc like and stressed bcc like clusters (inherited from the liquid quench process) in the hypo-eutectic glasses [10]. 1.2. Cluster manifestation as phase reminiscences in the TCam relaxation of Fe-Ni based glasses. The quenching rate dependence of TCam in Fe-Ni based glasses can also be understood on this basis. It is well known that TCam is influenced by the cooling rate applied during the liquid quench [11]. This cooling rate dependence is believed as a consequence of the pulse-like high temperature relaxation when the temperature of the supercooled liquid approaches the glass transition range. In this sense the slowly cooled samples is considered as more relaxed ones. However, the atomic mechanism of coupling between the degree of relaxation and the TCam shift remains unsolved within this conception. The TCam of Fe40Ni40Si6B14 ribbons (thin and thick were prepared by high and slow cooling rates respectively) is depicted in Fig. 3. After the TCam determination the samples were (in situ) annealed in the equipment at 200 °C for 30 min. then the TCam measurements were repeated. In agreement with Ref. [11] TCam is higher for the ribbon prepared by lower quenching rate.

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Fig.3. The evaluation of TCam in Fe40Ni40Si6B14 during the heat treatment cycle. 1.3. TCam shift caused by low temperature treatments “Phase reminiscence effects” were also detected in Fe20Ni60Si6B14 samples. The Ni/Fe ratio was increased to lower the TCam in order to minimize the additional heat treatment effect resulting itself from the measuring process. In Fig. 4. a the evolution of TCam after 1h isothermal heat treatments at various temperatures are plotted. One series of samples were „immersed” to liquid N2 (-196 °C for 20 hours, signed as LN) between the heat treatment and measurement. In Fig. 4. b is depicted the difference between the TCam obtained during the

cooling and heating runs respectively. The TCam decreases when the samples are cooled to LN temperature. On the other hand, the TCam of the supercooled samples increases more rapidly when the sample is heated slightly beyond the TCam and measured again during the cooling run. This effect hints to the existence of possible hysteric phenomena occurring during the measurements. However, the difference between TCam(up) and TCam(down) gradually disappears when the temperature of heat treatment approaches the 300 °C. In addition, the sign of ∆TCam turns to the opposite, when this heat treatment temperature is exceeded. It means that the susceptibility to the „low temperature treatment” depends on the thermal history represented by the temperature of the previous heat treatments.

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Fig.4. a) The evolution of TCam in Fe20Ni60Si6B14 after 1h isothermal heat treatments at various temperatures. A series of samples was previously “immersed” to liquid N2 (-196 °C, signed as LN) temperature for 20 hours between the heat treatment and measurement. b) The difference between the TCam measured during the cooling and heating runs. 2. DISCUSSION 2.1. Two-phase nature of Fe-Ni alloys and its manifestation in the magnetic properties of FeNi based glasses. As it is stressed the strength of ferromagnetic coupling depends on the atomic volume (interatomic distances). Though original meaning of the BS curve cannot be used directly in the ferromagnetic glasses due to the fluctuations of density are present. The concentration dependence as well as the sluggish nature of bcc ↔ fcc transformations in the Fe-Ni crystalline alloys seems to supply sufficient phenomenological background for the interpretation of the experimental results presented in Fig. 3. and 4. As the Fe-Ni phase diagram indicates the stability of γ phase is strongly extended into the lower temperature range by the Ni addition (Fig. 5.). The stability region approaches the 500°C at around 30-40 at. % Ni content. Simultaneously, the hysteretic nature of α(bcc) ↔ γ(fcc) transformation increases [13]. At around 30-40 at. % Ni content the γ → α transition (cooling direction) the appropriate temperature roughly 470 °C which is close to the glass transition temperature (Tg) in the investigated Fe-Ni based glasses. As both the fcc and the bcc crystalline phase are ferromagnetic in this composition range, the atomic positions with high and low momentum exist and randomly mixed due to the low transformation rate even in the crystalline state [14]. We suppose that the appropriate low momentum atomic positions are localised predominantly in the dense, fcc like environments in the as quenched glass.

Fig.5. The equilibrium diagram of Fe-Ni system and schematic TTT diagram for crystal growth in the undercooled FeNiSiB glass forming melt [12]. This is the starting point of our arguments regarding the interpretation of TCam shift as phase reminiscence. The α, γ like (compressed and stressed) environments inherited from the supercooling liquid similarly to the binary Fe-B glasses. This effect is enlarged due to the Ni addition because the γ stability region is extended. Hence, both the irreversible and reversible part of the TCam shift during the relaxation processes are the consequence of quenched-in reminiscence to this α ↔ γ transformation in the short range (or medium range) order scale. At small undercooling (near to the melting temperature when the temperature drops during the liquid quench) the formation of fcc like clusters is predominant. Contrary, with increasing supercooling the bcc cluster formation is favoured. At around the Tg the coexistence of fcc and bcc like clusters is typical. The ratio of two cluster type depends both on the Ni content and applied cooling rate as well, according to the homogeneous equilibrium described by the scheme (1): → increasing T [Fe,Ni(B)]bcc ↔ [Fe,Ni(B)]fcc ← decreasing T

(1)

At high cooling rate the ratio of fcc type clusters with lower ferromagnetic coupling strength is dominant, resulting lower TCam (see Fig. 2.). Cooling the sample to LN temperature the frozen-in ratio of fcc/bcc clusters will be completed (similarly to the martensitic transformations), so the TCam further decreases (see Fig. 4. a)). A more detailed analysis of these cryo-enhanced transformations is going on. 3. CONCLUSION The manifestation of Curie-point shift was studied in Fe-Ni-based glassy alloys. The results are interpreted on the basis of quenched-in phase reminiscences inherited from the liquid state. It is supposed that the rations of dense and stressed clusters are responsible for the direction of TC shift. This ratio is frozen-in at the glass transition temperature. The TC is further decreased due to cryo-treatments at –196 °C which confirm the similarity between the mechanism of glass transformation and the martensitic, non-diffusive transformation. Acknowledgements This work has been supported by the Hungarian Fund (OTKA) through grant No. T 046239 and in part by the Slovak Scientific Research grants APVT-51-052702, VEGA 2/4065, CexNANOSMART and the Slovak Grant Agency for Science (VEGA) through Grant No. 1/1013/04.

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Krisztián Bán Department of Vehicles Manufacturing and Repairing Budapest University of Technology and Economics H-1111 Budapest, Bertalan Lajos u. 2., Hungary e-mail: [email protected]