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Crystallization behaviour of Fe40Ni40SixP20 − x (x=6, 10, 14) amorphous alloys
Journal of Non-Crystalline Solids 276 (2000) 113±121
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Crystallization behaviour of Fe40Ni40Six P20 ÿ x (x 6, 10, 14) amorphous alloys T. Pradell a, J.J. Su~ nol b, N. Clavaguera c,*, M.T. Clavaguera-Mora d a ESAB, Universitat Polit ecnica de Catalunya, Urgell 187, 08036 Barcelona, Spain Grup de Recerca en Materials, Universitat de Girona, Santal o s/n. 17071 Girona, Spain c Grup de Fõsica de l'Estat S olid, Departament ECM, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain d Grup de Fõsica de Materials I, Departament de Fõsica, Universitat Aut onoma de Barcelona, 08193 Bellaterra, Spain b
Received 17 February 2000; received in revised form 6 April 2000
Abstract The progress in crystallization has been examined for three amorphous alloys produced by melt spinning (Fe40 Ni40 Six P20 ÿ x with x 6, 10 and 14) by use of X-ray diraction (XRD), transmission M ossbauer spectroscopy (TMS) and dierential scanning calorimetry (DSC). Two main crystallization peaks are observed calorimetrically on heating. Analysis of the crystallization products reveals the formation of Ni-rich silicides and Fe(Si) bcc phases in the early stages and subsequent formation of Fe-rich phosphides and ferromagnetic Fe(Ni) fcc phases at higher temperatures. Both the Si-rich and the P-rich alloys have lower glass stability than the equiatomic one. At high temperatures, there is competition between the Fe(Si) bcc phase and its transformation in the ferromagnetic Fe(Ni) phase. Such a transformation is clearly apparent for the P-rich alloy. On the contrary, in the Si-rich alloy a third exothermic peak is obtained at high temperatures due to the formation of an fcc paramagnetic Ni(Fe) phase. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Metallic glass formation by rapid quenching from the liquid state was ®rst reported in 1970 [1]. This proved to be a watershed in opening up new ®elds of research. These include amorphous alloy phases, which in some cases have unique combinations of properties [2,3]. Moreover, glassy alloys can be produced over a fairly wide range of compositions. As an example, it is known that Febased alloys prepared by rapid solidi®cation techniques in ribbon form exhibit superior soft
*
Corresponding author. Fax: +34-93 402 1182. E-mail address: [email protected] (N. Clavaguera).
magnetic properties. These alloys were widely investigated during the last two decades [4,5]. Nevertheless, they did not attain an important level of applicability because ribbon form limits its technological use. Amorphous (Fe,Ni)-metalloid alloys are currently produced in ribbon form by melt spinning and planar ¯ow casting [6±10]. Among the most studied Fe±Ni-based quaternary systems, there are Fe±Ni±Si±B [11] and Fe±Ni±P±B [12]. The quaternary alloys analyzed here, which are chosen to be 80% metal±20% metalloid, include both Si and P. These metalloid elements are not habitual in the bibliography, apart from recent studies [13±15] which show their potential for applications. The nominal compositions of the metallic glass ribbons
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 2 8 5 - 4
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T. Pradell et al. / Journal of Non-Crystalline Solids 276 (2000) 113±121
studied are Fe40 Ni40 Six P20 ÿ x with x 6, 10 and 14, labelled as X6, X10 and X14, respectively. The aim of this work is to analyse the process of crystallization, on amorphous alloys produced by melt spinning, as a function of the amount of Si/P content. Structural and calorimetric analyses are performed by X-ray diraction (XRD), transmission M ossbauer spectroscopy (TMS) and dierential scanning calorimetry (DSC). Partial crystallization is monitored by isothermal annealing of the initially amorphous alloys at selected temperatures. 2. Materials and methods In this work, the Fe±Ni-based quaternary alloys were obtained by rapid solidi®cation, i.e., melt spinning. The precursors used were pressed powders of elemental Fe, Ni, Si and the Fe3 P compound (to prevent P sublimation). Pure elements (<99.9 at.%) and small particle sizes were chosen (smaller than 25 mm). The samples were produced in ribbon form; they were about 0.1-cm wide and 20-lm thick. The working conditions of the device were chosen to obtain amorphous alloys. To reduce in¯uences of the production conditions on the ribbons' properties the same parameters were employed to obtain these ribbons. Ribbons were produced by quenching the molten alloy on the surface of a rapidly spinning (about 30 m/s) Cu wheel. The working atmosphere was inert Ar. Calorimetric measurements were carried out in a Perkin±Elmer DSC-7 dierential scanning calorimeter using a 20 K/min heating rate. The usual temperature and enthalpy calibration procedures were applied. From DSC scans, selected temperatures above each crystallization peak were chosen for isothermal measurements to obtain information after every crystallization step. The isothermal measurements were performed ®rst by a heating rate of 160 K/min until the temperature for the isothermal treatment was reached. The cooling rate, 200 K/min, was chosen to preserve, as far as possible, the thermodynamic state of the samples after isothermal annealing. The baseline of the calorimeter was checked and subtracted from all data.
The structural analysis of the rapidly quenched samples and their crystallization stages were performed by XRD and TMS. XRD patterns were obtained from a D-500 Siemens diractometer, using monochromatic Cu (ka ) radiation; low scan rates were used in order to obtain good statistics. ossbauer spectra The room temperature 57 Fe M were recorded by a standard constant acceleration spectrometer using 25 mCi of 57 Co in a Rh matrix source, and were calibrated with an a-Fe foil. The spectra were analyzed using a conventional NORMOS program (version 1990), by obtaining the hyper®ne ®eld distributions using the Hesse± R ubarstch methods [16], and by including a linear correlation between the isomer shift (IS) and magnetic ®eld to take into account, at least partially, the peak asymmetries typical for Fe±P-based amorphous alloys [17]. 3. Results The DSC curves obtained under continuous heating conditions are shown in Fig. 1 for the three alloys. A main exothermic peak labelled 1 in the ®gure followed by one or two exothermic peaks, labelled 2 and 3, appear. The onset and peak temperatures and the enthalpy of crystallization for the several crystallization peaks are given in Table 1. Isothermal anneals after selected temperatures corresponding to the ends of each transformation were made to identify the crystalline phases formed. The time/temperature paths chosen for the three alloys are 160 K/min heating rate and 40 min isothermal anneal at 723 and 793 K for alloy X6; 743 and 813 K for alloy X10, corresponding to the end of the ®rst and second crystallization peaks, respectively. For alloy X14, the annealing temperatures of 733, 813 and 853 K were chosen, corresponding to the end of the ®rst, second and third crystallization peaks, respectively. Fig. 2 shows the XRD patterns obtained for alloy X14. Table 2 includes the phases determined by XRD and their relative abundance. The phases identi®ed are bcc Fe(Si) (between 4 at.% Si and 14 at.% Si), fcc Ni±Fe alloy (between 36 at.% Fe and 50 at.% Fe), Ni-rich silicide (close to a phase-like Ni31 Si12 )
T. Pradell et al. / Journal of Non-Crystalline Solids 276 (2000) 113±121
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Fig. 1. Continuous DSC analysis of the three ribbons obtained for a heating rate of 20 K/min. The crystallization peaks are marked.
Table 1 Characteristic temperatures, onset (To ) and peak (Tp ), and enthalpy of the crystallization processes Ribbon
Process
To (K)
Tp (K)
DH (J/g)
X6
1 2
693 758
706 768
86 5
X10
1 2
708 781
720 801
67 4
X14
1 2 3
683 779 813
699 796 833
73 3 9
and (Fe,Ni)3 P (between 50 at.% Fe and 72 at.% Fe). The M ossbauer analysis was made by ®tting two hyper®ne magnetic ®eld distributions, one related to a ferromagnetic Fe-rich phase and the other related to either an amorphous phase or crystalline Fe±Ni phosphide. These phases are clearly distinguished by the value of the IS; the Ferich alloy has a typical value of )0.08 mm/s and the phosphide and the amorphous phases have
Fig. 2. XRD patterns corresponding to the three crystallization peaks of ribbon X14.
values of 0.10±0.20 mm/s. Two paramagnetic phases were also a singlet related to an Fe alloy (IS 0 mm/s) and a doublet related most probably to Ni-rich silicides (IS 0.20 mm/s and QS 0.70 mm/s). A typical M ossbauer spectra with the ®tted curves and the two hyper®ne ®eld distribution ®ts are shown in Fig. 3 for alloy X14. The phases and at.% Fe relative to total iron content of the alloy are also given in Table 2. One of the main problems was to establish a relationship between the XRD and the M ossbauer data. XRD gives the structure (bcc, fcc phases) and composition, but M ossbauer data show for both structures a cubic neighborhood, with the main distinction of whether they are either paramagnetic or ferromagnetic. BCC Fe-based alloys are ferromagnetic. FCC Fe±Ni alloys may be either paramagnetic or ferromagnetic, and the
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Table 2 X-ray diraction phases (main phase in bold letters) detected and parameters calculateda Ribbon
X6
X10
X14
Thermal treatment (40 min.)
X-ray diraction
a.q. 723 K
Amorphous Bcc Fe(Si) Ni31 Si12
793 K
#fcc Ni(Fe) (Fe,Ni)3 P Bcc Fe(Si) Ni31 Si12
a.q. 743 K
Amorphous Bcc Fe(Si) Ni31 Si12
813 K
Bcc Fe(Si) Ni31 Si12 #fcc Ni(Fe) (Fe,Ni)3 P
Phase
M ossbauer spectroscopy Estimated composition 8 at.% Si
50 at.% Fe Fe2 NiP 4 at.% Si
11 at.% Si
11 at.% Si 42 at%Fe Fe2 NiP
a.q. 733 K
Amorphous Bcc Fe(Si) Ni31 Si12
813 K
Bcc Fe(Si) (Fe,Ni)3 P Ni31 Si12
10 at.% Si Fe2:45 Ni0:55 P
853 K
Bcc Fe(Si) #fcc Ni(Fe) Ni31 Si12 (Fe,Ni)3 P
11 at.% Si 42 at.% Fe
14 at.% Si
Fe2:9 Ni0:1 P
Phase
%
Amorphous Ferromagnetic Silicide Am. Fe3 P order Paramagnetic Ferromagnetic (Fe,Ni)3 P Silicide Paramagnetic
100 70.1 17.2 11.4 1.3 57.4 23.0 17.6 2.1
Amorphous Ferromagnetic Silicide Am. Fe3 P order Paramagnetic Ferromagnetic Silicide (Fe,Ni)3 P Paramagnetic
100 70.5 16.6 12.9 ± 62.7 17.6 16.1 3.6
Amorphous Ferromagnetic Am. Fe3 P order Silicide Paramagnetic Ferromagnetic Silicide (Fe,Ni)3 P Paramagnetic Paramagnetic Ferromagnetic Silicide (Fe,Ni)3 P
100 74.8 12.7 12.6 ± 76.3 15.9 5.7 2.1 44.3 32.0 18.6 5.1
a
# corresponds to an fcc Ni(Fe) phase rich in Fe, either paramagnetic or ferromagnetic. M ossbauer phases determined from ®tted spectra and %Fe content in the phase relative to total Fe.
magnetic hyper®ne ®elds are quite similar for Nirich fcc Ni(Fe) alloys ()32 to 32.5 T) and for Fe(Si) bcc alloys (33±31 T, depending on the Si content). The magnetic behaviour of these alloys is therefore of high interest. A short summary of the magnetic-induced phase transformations in the Fe±Ni phase diagram is presented (see Fig. 4). The respective solubilities of Ni in the stable bcc Fe phase and of Fe in the fcc Ni phase are very small at room temperature, and both alloys are ferromagnetic. In the Fe-rich region (<40 at.% Ni) at high temperatures (>662 K),
the fcc phase becomes paramagnetic (c), and a phase separation between a ferromagnetic bcc Ferich phase and a paramagnetic fcc Fe±Ni phase occurs. In the region between 40 at.% Ni and 46 at.% Ni, a magnetic-induced tricritical point [18] appears, as shown in Fig. 4. At temperatures below the tricritical point (760 K), the fcc ferromagnetic Ni-rich phase undergoes a second-order phase transition from the ferromagnetic state to a paramagnetic state (c2 ±c1 ). In the Ni-richer regions, the Curie temperature of the alloy is maximum at about 60 at.% Ni of 885 K.
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Fig. 4. Schematic diagram of the Fe±Ni phase diagram at low temperatures after Chang [18].
magnetic phase may be related to either the Fe(Si) bcc phase or to the Ni(Fe) fcc phase. The presence of a paramagnetic phase is also common for fcc phases that are richer in Fe. XRD indicates whether the phases present are bcc or fcc. The peak positions of the fcc phase indicate whether it corresponds to a Ni-rich or to a Ni-poor phase; the compositions given in Table 2 are estimated from the derived lattice parameters. With this summary, a detailed description of the results for the three crystallization events follows. 3.1. First crystallization peak Fig. 3. M ossbauer spectra corresponding to the crystallization peaks of ribbon X14; the crosses correspond to the experimental data and the lines to the subspectra and ®nal spectrum ®tted. The resulting magnetic hyper®ne distributions are also shown in the ®gure for the three crystallization processes.
As mentioned, experimentally, from the M ossbauer analysis the phases are determined to be ferromagnetic and/or paramagnetic. A ferro-
XRD results show that as a general trend for the three alloys, a bcc Fe(Si) alloy and a Ni-rich silicide are formed, together with the remains of an amorphous phase. The Si concentration in the bcc Fe(Si) phase increases with increasing Si content in the amorphous alloy (from X6 to X14). The higher the Si contents of the amorphous alloy, the broader the diraction peaks found. This is related to a low crystal size, the presence of microstrains
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Fig. 6. AHF corresponding to the ferromagnetic alloy. The full spots and solid lines correspond to the ®rst crystallization and open spots and dashed lines to the second crystallization, the full square corresponds to the third crystallization of ribbon X14.
Fig. 5. Percentages relative to the total iron content in the samples of the dierent phases, determined by M ossbauer spectroscopy for the three ribbons. The full symbols and solid lines correspond to the ®rst crystallization and open symbols and dashed lines to the second crystallization.
and/or a lack of order. Broad peaks appear, in general, for all the phases formed during the ®rst crystallization event in sample X14, as shown in Fig. 2. In the following, crystalline phases showing broad diraction peaks will be named with loworder phases. M ossbauer results, shown in Fig. 5, reveal that most of the iron appears in a ferromagnetic phase, which is related mainly to the bcc Fe(Si), determined by XRD. The quantity of Fe in this phase increases and the average hyper®ne magnetic ®eld (AHF) (see Fig. 6) decreases (the phase is richer in Si) with increasing Si in the amorphous alloy (see Fig. 5). Therefore, a higher quantity of a Si-richer bcc Fe(Si) phase is formed. Moreover, a paramagnetic phase (QS 0.6 mm/s; IS 0.26 mm/s) is related to the Ni-rich silicide with 15 at.% Fe concentration. The total iron contained in this phase decreases with increasing Si in the amorphous phase (Fig. 5). The rest of the iron appears in a magnetic phase showing a broad magnetic hyper®ne ®eld distribution with three main peaks at maximum values
of 10, 15 and 20 T, (see Fig. 3), which may be related to an Fe-rich phosphide of the type (Fe,Ni)3 P. Although XRD data do not show the presence of a crystalline phosphide, the M ossbauer results indicate an Fe3 P-type local order in the amorphous phase. The iron in this phase constitutes about 13 at.% of the total iron (Fig. 5). From these results, about 90 at.% of the total iron in the alloys appears in crystalline phases. Therefore, although we do not have a direct information about the quantity of Ni transformed, assuming a similar quantity of Ni in the crystal phases, we may conclude that only about 10% of the amorphous alloy remains untransformed. 3.2. Second crystallization The second crystallization stage is characterized (Table 2) mainly by the precipitation of an Fe±Ni phosphide of the type (Fe,Ni)3 P, and the formation of an fcc Ni(Fe) alloy. The phosphide becomes richer in Fe, and the fcc phase becomes richer in Ni with increasing Si concentration in the amorphous alloy. The Ni(Fe) fcc phase becomes the primary phase for the alloy X6. However, for alloy X10 it appears as the second-most abundant phase, and for the alloy X14 it appears as a trace phase, the bcc Fe(Si) remaining the main phase.
T. Pradell et al. / Journal of Non-Crystalline Solids 276 (2000) 113±121
The M ossbauer results are consistent with the XRD results. The ferromagnetic phase is related to both the bcc Fe(Si) and the ferromagnetic fcc Ni(Fe) phases determined by XRD. Moreover, the formation of a paramagnetic fcc phase containing very low quantities of Fe appears in the three alloys. The coexistence of the two fcc phases, paramagnetic and ferromagnetic, and of the bcc phase may indicate, in conjunction with Fig. 5, that the annealing was performed above the miscibility gap but below the Curie temperature. The AHF, Fig. 6, related to the ferromagnetic phase has the value (about 32.5 T) characteristic of an fcc Ni(Fe) alloy. Moreover, the values of the IS related to the paramagnetic fcc phase, shown in Fig. 7, are also characteristic of an fcc Ni(Fe) alloy. The total amount of Fe in both paramagnetic + ferromagnetic phases from the ®rst to the second crystallization (Fig. 5) shows an increase for alloy X6, and is about the same for alloy X10 and decreases in the case of alloy X14. At the end of this second precipitation the silicides contain similar quantities of Fe (about 17 at.% of the total iron) in all the alloys, as shown in Fig. 5. The local ordering of the phosphides in alloys X6 and X10 is similar and close to Fe3 P, but for alloy X14 it seems close to a phase of the type Fe2 P. XRD results indicate an enrichment in Fe in the phosphide. M ossbauer data show that the total amount of Fe in this phase decreases with in-
Fig. 7. Evolution of the IS corresponding to the paramagnetic alloy for the three crystallization events, and for the three alloy compositions analyzed.
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creasing Si in the alloy (see Fig. 5). However, XRD results indicate an enrichment in their Fe concentration. Therefore, a smaller amount of phosphide is formed with increasing Si concentration of the amorphous alloys (X6±X14). 3.3. Third crystallization This crystallization peak appears only for alloy X14; the main trend is the formation of an fcc paramagnetic Ni(Fe) phase from the original fcc and bcc ferromagnetic phases. M ossbauer analysis shows this transformation to be due to the transformation of the ferromagnetic phases to paramagnetic ones. The IS of the paramagnetic fcc alloy, as shown in Fig. 7, reaches the value ()0.1 mm/s), typical of an Fe(Ni) alloy with a composition (obtained from the XRD lattice spacing) of about 42 at.% Fe. As the XRD data show (see Fig. 2) there is also an increase in the relative amount of Fe in the silicides and a small decrease of Fe in the phosphides. No signi®cant changes appear in the magnetic hyper®ne ®eld distribution related to the phosphides.
4. Discussion The amorphous alloys obtained after melt spinning are homogeneous. Only small dierences are obtained in the SRO M ossbauer parameters, which are related to the dierences in the composition [13]. The values obtained are IS 0.2 mm/s, QS 0 mm/s, AHF 19 T, STD 5 T, which correspond to a unique FeNiP-based amorphous phase. At room temperature and up to high temperatures, there is almost no miscibility between Fe and Ni. Therefore, on heating the amorphous alloys, separation of Fe and Ni results in the precipitation of Ni-rich silicides and Fe(Si) bcc, respectively. Consequently, the ®rst crystallization is activated by the nucleation of a primary bcc Fe(Si), and of a Ni-rich silicide. The remaining amorphous phase after the ®rst crystallization has an Fe3 P-type local order and composition.
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At higher temperatures, during the second crystallization, the formation of the Fe-rich phosphide depends on the initial Si concentration of the alloy. The phosphide has a local ordering of the type (FeNi)3 P, and is richer in Fe with increasing Si in the alloy. The formation of a ferromagnetic fcc Ni(Fe) phase occurs mainly during the second crystallization and becomes the main phase for the P-richer alloy, X6. That is, the formation of this phase seems to be accelerated by the formation of the Fe-rich phosphides, and, consequently, with the increasing P concentration of the alloy. The ferromagnetic fcc + bcc ® paramagnetic fcc transformation is activated only for the Si-richer alloy (X14) and it appears as the third crystallization process. Probably, such a transformation might also be activated for the other two alloys, but at a higher temperature than used in this study. The general sequence of product phases appearing in the thermally activated crystallization of the three alloys is as follows: (1) the formation of Ni-rich silicides and Fe(Si) bcc phases; and (2) the formation of Fe-rich phosphides and ferromagnetic Fe(Ni) fcc phases. Calorimetrically, the most stable alloy is X10 since the ®rst crystallization peak is sharp and delayed in temperature (with respect to X6 and X14). For this alloy, the local ordering does not change signi®cantly from the ®rst to the second crystallization. Moreover, the phases formed during the ®rst crystallization maintain their Fe concentration during the second crystallization. Although, during the ®rst crystallization, Nirich silicides and Fe(Si) bcc phases are also formed in alloy X6, the high P content of this alloy (or its low Si content) seems to retard the end of the ®rst crystallization (see Fig. 1). Such a spread in temperature may be attributed to the need for diusion-controlled growth in the last steps of the ®rst crystallization process. Similarly, such an extension is also seen in alloy X14. That is, it appears as if alloy X10 had the `ideal' composition for an eutectic-like transformation whereas alloys X6 and X14 need diusion over long distances to achieve the transformation. Schematically, the ®rst crystallization for alloy X10 may be written as an eutectic one
X 10amorphous ) Ni-rich silicidescrystal Fe Sibcc Fe-rich phosphidesamorphous : The onset temperature of the ®rst crystallization is advanced in alloys X6 and X14. For alloy X6 the reason may be related to the enhancement of the Ni-rich silicide formation with regard to the Fe(Si) phase since this alloy is Si-poor (the Si concentration of this last phase is about 8 at.%). For alloy X14, richer in Si, the formation of both bcc Fe(Si) and Ni-rich silicide phases seems promoted, but it results in phases with very low order. Also, the mean composition and local order of the remaining amorphous phase after the ®rst crystallization are close to Fe2 P (instead of Fe3 P for alloys X6 and X10). The lack of P results in the formation of an Fe-rich phosphide as the main contribution to the second crystallization peak. Also, at higher temperatures, the formation of a paramagnetic fcc Fe(Ni) alloy is observed. The onset temperature of the second crystallization is advanced in alloy X6. As already mentioned, for this alloy the high P concentration promotes the formation of a high amount of Ferich phosphides and the decomposition of the Fe(Si) bcc phase at the expense of the formation of a ferromagnetic Fe(Ni) phase. All these results agree with the lower stability of the remaining amorphous phase with increasing P content, thus accelerating the second crystallization.
5. Conclusions The thermally activated crystallization of three amorphous alloys (nominal composition Fe40 Ni40 Six P20 ÿ x with x 6, 10 and 14) produced by melt spinning has been examined. The general transformation sequence is a ®rst stage where Fe(Si) bcc- and Ni-rich silicides appear, and a second stage where the remaining amorphous phase transforms to Fe-rich silicides and the Fe(Si) bcc partially transforms to ferromagnetic Fe(Ni) fcc. In both the P-rich and the Si-rich alloys, X6 and X14, the onset temperature of the ®rst crystallization stage is advanced with respect to that
T. Pradell et al. / Journal of Non-Crystalline Solids 276 (2000) 113±121
of the equiatomic alloy, X10. Also for the former alloys, the crystallization calorimetric peak shows a long tail, indicative of diusion-limiting mechanisms. The second crystallization stage is advanced for the P-rich alloy, X6. Its high P content enables a large amount of Fe-rich phosphides to be formed, but their need of Fe results in the decomposition of the main bcc phase and the formation of an fcc ferromagnetic Fe(Ni) alloy. The Si-rich alloy, X14, shows very low order after the ®rst crystallization stage. However, in this alloy as well as in the equiatomic one, X10, the local ordering and overall composition of the remaining amorphous phase are quite similar. Thus, they show close values for the onset temperature of the second crystallization stage. The main dierence between these alloys is that in the Si-rich alloy a third exothermic peak is obtained at high temperatures, due to the formation of an fcc paramagnetic Ni(Fe) phase. Most probably, all three alloys may undergo such a transformation at high temperature. The alloy that is equiatomic in Si and P, X10, has the higher glass stability of the alloys studied. Also, almost no diusion at large distances are needed for the ®rst and second crystallization processes. That is, both may be considered to be eutectic-like transformations.
Acknowledgements The authors wish to thank Dr Parellada for the M ossbauer measurement facilities.The work has
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been ®nanced by CICYT grant MAT96-0692 and Generalitat de Catalunya grant 1995SGR 00514. References [1] K. Klement, R.H. Willens, P. Duwez, Nature 187 (1970) 869. [2] H.A. Davies, Phys. Chem. Glasses 17 (1976) 159. [3] S. Lele, K.S. Dubey, P. Ramanchandrarao, Curr. Sci. 14 (1985) 994. [4] H.J. G untherodt, H. Beck (Eds.), Glassy Metals I&II, Springer, Berlin, 1981±1983. [5] H. Frederiksson, S. Savage, Mater. Sci. Eng. A 133&134 (1991) 151. [6] D.G. Morris, Acta Metall. 29 (1981) 1213. [7] S.J. Thorpe, B. Ramaswami, K.T. Aust, Acta Metall. 36 (1988) 795. [8] K. Russew, S. Budurov, L. Snestiev, in: Fourth International Conference Rapidly Quenched Metals, 1994, p. 1285. [9] A. Dunst, D.M. Herlach, F. Gillessen, Mater. Sci. Eng. A 133 (1991) 785. [10] F. Welz, H. Kronm uller, D. Martinez, J. Rivas, Phys. Status Solidi A 138 (1993) 265. [11] H. Kimura, T. Masimoto, Amorphous Metallic Alloys, Butterworths, London, 1983, p. 187. [12] R. Br uning, Z. Altounian, J.O. Ostr om-Olsen, J. Appl. Phys. 62 (9) (1987) 3633. [13] J.J. Su~ nol, M.T. Clavaguera-Mora, N. Clavaguera, T. Pradell, Mater. Res. Soc. Symp. Proc. 455 (1997) 489. [14] J.J. Su~ nol, PhD thesis, Universitat Aut onoma de Barcelona, 1996. [15] J.J. Su~ nol, M.T. Mora, N. Clavaguera, Rev. Ciencia 7 (1997). [16] J. Hesse, A. R ubarstch, J. Phys. E 7 (1974) 526. [17] R.E. Vanderberghe, D. Ghyroy, E. De Grave, Nucl. Instrum. and Meth. 26 (1987) 603. [18] Y.A. Chang, in: H. Brodowsky, H.J. Schaller (Eds.), Thermochemistry of Alloys: Recent Developments of Experimental Methods, NATO ASI Series C: Mathematical and Physical Sciences, vol. 286, p. 85.
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Crystallization behaviour of Fe40Ni40Six P20 ÿ x (x 6, 10, 14) amorphous alloys T. Pradell a, J.J. Su~ nol b, N. Clavaguera c,*, M.T. Clavaguera-Mora d a ESAB, Universitat Polit ecnica de Catalunya, Urgell 187, 08036 Barcelona, Spain Grup de Recerca en Materials, Universitat de Girona, Santal o s/n. 17071 Girona, Spain c Grup de Fõsica de l'Estat S olid, Departament ECM, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain d Grup de Fõsica de Materials I, Departament de Fõsica, Universitat Aut onoma de Barcelona, 08193 Bellaterra, Spain b
Received 17 February 2000; received in revised form 6 April 2000
Abstract The progress in crystallization has been examined for three amorphous alloys produced by melt spinning (Fe40 Ni40 Six P20 ÿ x with x 6, 10 and 14) by use of X-ray diraction (XRD), transmission M ossbauer spectroscopy (TMS) and dierential scanning calorimetry (DSC). Two main crystallization peaks are observed calorimetrically on heating. Analysis of the crystallization products reveals the formation of Ni-rich silicides and Fe(Si) bcc phases in the early stages and subsequent formation of Fe-rich phosphides and ferromagnetic Fe(Ni) fcc phases at higher temperatures. Both the Si-rich and the P-rich alloys have lower glass stability than the equiatomic one. At high temperatures, there is competition between the Fe(Si) bcc phase and its transformation in the ferromagnetic Fe(Ni) phase. Such a transformation is clearly apparent for the P-rich alloy. On the contrary, in the Si-rich alloy a third exothermic peak is obtained at high temperatures due to the formation of an fcc paramagnetic Ni(Fe) phase. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Metallic glass formation by rapid quenching from the liquid state was ®rst reported in 1970 [1]. This proved to be a watershed in opening up new ®elds of research. These include amorphous alloy phases, which in some cases have unique combinations of properties [2,3]. Moreover, glassy alloys can be produced over a fairly wide range of compositions. As an example, it is known that Febased alloys prepared by rapid solidi®cation techniques in ribbon form exhibit superior soft
*
Corresponding author. Fax: +34-93 402 1182. E-mail address: [email protected] (N. Clavaguera).
magnetic properties. These alloys were widely investigated during the last two decades [4,5]. Nevertheless, they did not attain an important level of applicability because ribbon form limits its technological use. Amorphous (Fe,Ni)-metalloid alloys are currently produced in ribbon form by melt spinning and planar ¯ow casting [6±10]. Among the most studied Fe±Ni-based quaternary systems, there are Fe±Ni±Si±B [11] and Fe±Ni±P±B [12]. The quaternary alloys analyzed here, which are chosen to be 80% metal±20% metalloid, include both Si and P. These metalloid elements are not habitual in the bibliography, apart from recent studies [13±15] which show their potential for applications. The nominal compositions of the metallic glass ribbons
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 2 8 5 - 4
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studied are Fe40 Ni40 Six P20 ÿ x with x 6, 10 and 14, labelled as X6, X10 and X14, respectively. The aim of this work is to analyse the process of crystallization, on amorphous alloys produced by melt spinning, as a function of the amount of Si/P content. Structural and calorimetric analyses are performed by X-ray diraction (XRD), transmission M ossbauer spectroscopy (TMS) and dierential scanning calorimetry (DSC). Partial crystallization is monitored by isothermal annealing of the initially amorphous alloys at selected temperatures. 2. Materials and methods In this work, the Fe±Ni-based quaternary alloys were obtained by rapid solidi®cation, i.e., melt spinning. The precursors used were pressed powders of elemental Fe, Ni, Si and the Fe3 P compound (to prevent P sublimation). Pure elements (<99.9 at.%) and small particle sizes were chosen (smaller than 25 mm). The samples were produced in ribbon form; they were about 0.1-cm wide and 20-lm thick. The working conditions of the device were chosen to obtain amorphous alloys. To reduce in¯uences of the production conditions on the ribbons' properties the same parameters were employed to obtain these ribbons. Ribbons were produced by quenching the molten alloy on the surface of a rapidly spinning (about 30 m/s) Cu wheel. The working atmosphere was inert Ar. Calorimetric measurements were carried out in a Perkin±Elmer DSC-7 dierential scanning calorimeter using a 20 K/min heating rate. The usual temperature and enthalpy calibration procedures were applied. From DSC scans, selected temperatures above each crystallization peak were chosen for isothermal measurements to obtain information after every crystallization step. The isothermal measurements were performed ®rst by a heating rate of 160 K/min until the temperature for the isothermal treatment was reached. The cooling rate, 200 K/min, was chosen to preserve, as far as possible, the thermodynamic state of the samples after isothermal annealing. The baseline of the calorimeter was checked and subtracted from all data.
The structural analysis of the rapidly quenched samples and their crystallization stages were performed by XRD and TMS. XRD patterns were obtained from a D-500 Siemens diractometer, using monochromatic Cu (ka ) radiation; low scan rates were used in order to obtain good statistics. ossbauer spectra The room temperature 57 Fe M were recorded by a standard constant acceleration spectrometer using 25 mCi of 57 Co in a Rh matrix source, and were calibrated with an a-Fe foil. The spectra were analyzed using a conventional NORMOS program (version 1990), by obtaining the hyper®ne ®eld distributions using the Hesse± R ubarstch methods [16], and by including a linear correlation between the isomer shift (IS) and magnetic ®eld to take into account, at least partially, the peak asymmetries typical for Fe±P-based amorphous alloys [17]. 3. Results The DSC curves obtained under continuous heating conditions are shown in Fig. 1 for the three alloys. A main exothermic peak labelled 1 in the ®gure followed by one or two exothermic peaks, labelled 2 and 3, appear. The onset and peak temperatures and the enthalpy of crystallization for the several crystallization peaks are given in Table 1. Isothermal anneals after selected temperatures corresponding to the ends of each transformation were made to identify the crystalline phases formed. The time/temperature paths chosen for the three alloys are 160 K/min heating rate and 40 min isothermal anneal at 723 and 793 K for alloy X6; 743 and 813 K for alloy X10, corresponding to the end of the ®rst and second crystallization peaks, respectively. For alloy X14, the annealing temperatures of 733, 813 and 853 K were chosen, corresponding to the end of the ®rst, second and third crystallization peaks, respectively. Fig. 2 shows the XRD patterns obtained for alloy X14. Table 2 includes the phases determined by XRD and their relative abundance. The phases identi®ed are bcc Fe(Si) (between 4 at.% Si and 14 at.% Si), fcc Ni±Fe alloy (between 36 at.% Fe and 50 at.% Fe), Ni-rich silicide (close to a phase-like Ni31 Si12 )
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Fig. 1. Continuous DSC analysis of the three ribbons obtained for a heating rate of 20 K/min. The crystallization peaks are marked.
Table 1 Characteristic temperatures, onset (To ) and peak (Tp ), and enthalpy of the crystallization processes Ribbon
Process
To (K)
Tp (K)
DH (J/g)
X6
1 2
693 758
706 768
86 5
X10
1 2
708 781
720 801
67 4
X14
1 2 3
683 779 813
699 796 833
73 3 9
and (Fe,Ni)3 P (between 50 at.% Fe and 72 at.% Fe). The M ossbauer analysis was made by ®tting two hyper®ne magnetic ®eld distributions, one related to a ferromagnetic Fe-rich phase and the other related to either an amorphous phase or crystalline Fe±Ni phosphide. These phases are clearly distinguished by the value of the IS; the Ferich alloy has a typical value of )0.08 mm/s and the phosphide and the amorphous phases have
Fig. 2. XRD patterns corresponding to the three crystallization peaks of ribbon X14.
values of 0.10±0.20 mm/s. Two paramagnetic phases were also a singlet related to an Fe alloy (IS 0 mm/s) and a doublet related most probably to Ni-rich silicides (IS 0.20 mm/s and QS 0.70 mm/s). A typical M ossbauer spectra with the ®tted curves and the two hyper®ne ®eld distribution ®ts are shown in Fig. 3 for alloy X14. The phases and at.% Fe relative to total iron content of the alloy are also given in Table 2. One of the main problems was to establish a relationship between the XRD and the M ossbauer data. XRD gives the structure (bcc, fcc phases) and composition, but M ossbauer data show for both structures a cubic neighborhood, with the main distinction of whether they are either paramagnetic or ferromagnetic. BCC Fe-based alloys are ferromagnetic. FCC Fe±Ni alloys may be either paramagnetic or ferromagnetic, and the
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Table 2 X-ray diraction phases (main phase in bold letters) detected and parameters calculateda Ribbon
X6
X10
X14
Thermal treatment (40 min.)
X-ray diraction
a.q. 723 K
Amorphous Bcc Fe(Si) Ni31 Si12
793 K
#fcc Ni(Fe) (Fe,Ni)3 P Bcc Fe(Si) Ni31 Si12
a.q. 743 K
Amorphous Bcc Fe(Si) Ni31 Si12
813 K
Bcc Fe(Si) Ni31 Si12 #fcc Ni(Fe) (Fe,Ni)3 P
Phase
M ossbauer spectroscopy Estimated composition 8 at.% Si
50 at.% Fe Fe2 NiP 4 at.% Si
11 at.% Si
11 at.% Si 42 at%Fe Fe2 NiP
a.q. 733 K
Amorphous Bcc Fe(Si) Ni31 Si12
813 K
Bcc Fe(Si) (Fe,Ni)3 P Ni31 Si12
10 at.% Si Fe2:45 Ni0:55 P
853 K
Bcc Fe(Si) #fcc Ni(Fe) Ni31 Si12 (Fe,Ni)3 P
11 at.% Si 42 at.% Fe
14 at.% Si
Fe2:9 Ni0:1 P
Phase
%
Amorphous Ferromagnetic Silicide Am. Fe3 P order Paramagnetic Ferromagnetic (Fe,Ni)3 P Silicide Paramagnetic
100 70.1 17.2 11.4 1.3 57.4 23.0 17.6 2.1
Amorphous Ferromagnetic Silicide Am. Fe3 P order Paramagnetic Ferromagnetic Silicide (Fe,Ni)3 P Paramagnetic
100 70.5 16.6 12.9 ± 62.7 17.6 16.1 3.6
Amorphous Ferromagnetic Am. Fe3 P order Silicide Paramagnetic Ferromagnetic Silicide (Fe,Ni)3 P Paramagnetic Paramagnetic Ferromagnetic Silicide (Fe,Ni)3 P
100 74.8 12.7 12.6 ± 76.3 15.9 5.7 2.1 44.3 32.0 18.6 5.1
a
# corresponds to an fcc Ni(Fe) phase rich in Fe, either paramagnetic or ferromagnetic. M ossbauer phases determined from ®tted spectra and %Fe content in the phase relative to total Fe.
magnetic hyper®ne ®elds are quite similar for Nirich fcc Ni(Fe) alloys ()32 to 32.5 T) and for Fe(Si) bcc alloys (33±31 T, depending on the Si content). The magnetic behaviour of these alloys is therefore of high interest. A short summary of the magnetic-induced phase transformations in the Fe±Ni phase diagram is presented (see Fig. 4). The respective solubilities of Ni in the stable bcc Fe phase and of Fe in the fcc Ni phase are very small at room temperature, and both alloys are ferromagnetic. In the Fe-rich region (<40 at.% Ni) at high temperatures (>662 K),
the fcc phase becomes paramagnetic (c), and a phase separation between a ferromagnetic bcc Ferich phase and a paramagnetic fcc Fe±Ni phase occurs. In the region between 40 at.% Ni and 46 at.% Ni, a magnetic-induced tricritical point [18] appears, as shown in Fig. 4. At temperatures below the tricritical point (760 K), the fcc ferromagnetic Ni-rich phase undergoes a second-order phase transition from the ferromagnetic state to a paramagnetic state (c2 ±c1 ). In the Ni-richer regions, the Curie temperature of the alloy is maximum at about 60 at.% Ni of 885 K.
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Fig. 4. Schematic diagram of the Fe±Ni phase diagram at low temperatures after Chang [18].
magnetic phase may be related to either the Fe(Si) bcc phase or to the Ni(Fe) fcc phase. The presence of a paramagnetic phase is also common for fcc phases that are richer in Fe. XRD indicates whether the phases present are bcc or fcc. The peak positions of the fcc phase indicate whether it corresponds to a Ni-rich or to a Ni-poor phase; the compositions given in Table 2 are estimated from the derived lattice parameters. With this summary, a detailed description of the results for the three crystallization events follows. 3.1. First crystallization peak Fig. 3. M ossbauer spectra corresponding to the crystallization peaks of ribbon X14; the crosses correspond to the experimental data and the lines to the subspectra and ®nal spectrum ®tted. The resulting magnetic hyper®ne distributions are also shown in the ®gure for the three crystallization processes.
As mentioned, experimentally, from the M ossbauer analysis the phases are determined to be ferromagnetic and/or paramagnetic. A ferro-
XRD results show that as a general trend for the three alloys, a bcc Fe(Si) alloy and a Ni-rich silicide are formed, together with the remains of an amorphous phase. The Si concentration in the bcc Fe(Si) phase increases with increasing Si content in the amorphous alloy (from X6 to X14). The higher the Si contents of the amorphous alloy, the broader the diraction peaks found. This is related to a low crystal size, the presence of microstrains
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Fig. 6. AHF corresponding to the ferromagnetic alloy. The full spots and solid lines correspond to the ®rst crystallization and open spots and dashed lines to the second crystallization, the full square corresponds to the third crystallization of ribbon X14.
Fig. 5. Percentages relative to the total iron content in the samples of the dierent phases, determined by M ossbauer spectroscopy for the three ribbons. The full symbols and solid lines correspond to the ®rst crystallization and open symbols and dashed lines to the second crystallization.
and/or a lack of order. Broad peaks appear, in general, for all the phases formed during the ®rst crystallization event in sample X14, as shown in Fig. 2. In the following, crystalline phases showing broad diraction peaks will be named with loworder phases. M ossbauer results, shown in Fig. 5, reveal that most of the iron appears in a ferromagnetic phase, which is related mainly to the bcc Fe(Si), determined by XRD. The quantity of Fe in this phase increases and the average hyper®ne magnetic ®eld (AHF) (see Fig. 6) decreases (the phase is richer in Si) with increasing Si in the amorphous alloy (see Fig. 5). Therefore, a higher quantity of a Si-richer bcc Fe(Si) phase is formed. Moreover, a paramagnetic phase (QS 0.6 mm/s; IS 0.26 mm/s) is related to the Ni-rich silicide with 15 at.% Fe concentration. The total iron contained in this phase decreases with increasing Si in the amorphous phase (Fig. 5). The rest of the iron appears in a magnetic phase showing a broad magnetic hyper®ne ®eld distribution with three main peaks at maximum values
of 10, 15 and 20 T, (see Fig. 3), which may be related to an Fe-rich phosphide of the type (Fe,Ni)3 P. Although XRD data do not show the presence of a crystalline phosphide, the M ossbauer results indicate an Fe3 P-type local order in the amorphous phase. The iron in this phase constitutes about 13 at.% of the total iron (Fig. 5). From these results, about 90 at.% of the total iron in the alloys appears in crystalline phases. Therefore, although we do not have a direct information about the quantity of Ni transformed, assuming a similar quantity of Ni in the crystal phases, we may conclude that only about 10% of the amorphous alloy remains untransformed. 3.2. Second crystallization The second crystallization stage is characterized (Table 2) mainly by the precipitation of an Fe±Ni phosphide of the type (Fe,Ni)3 P, and the formation of an fcc Ni(Fe) alloy. The phosphide becomes richer in Fe, and the fcc phase becomes richer in Ni with increasing Si concentration in the amorphous alloy. The Ni(Fe) fcc phase becomes the primary phase for the alloy X6. However, for alloy X10 it appears as the second-most abundant phase, and for the alloy X14 it appears as a trace phase, the bcc Fe(Si) remaining the main phase.
T. Pradell et al. / Journal of Non-Crystalline Solids 276 (2000) 113±121
The M ossbauer results are consistent with the XRD results. The ferromagnetic phase is related to both the bcc Fe(Si) and the ferromagnetic fcc Ni(Fe) phases determined by XRD. Moreover, the formation of a paramagnetic fcc phase containing very low quantities of Fe appears in the three alloys. The coexistence of the two fcc phases, paramagnetic and ferromagnetic, and of the bcc phase may indicate, in conjunction with Fig. 5, that the annealing was performed above the miscibility gap but below the Curie temperature. The AHF, Fig. 6, related to the ferromagnetic phase has the value (about 32.5 T) characteristic of an fcc Ni(Fe) alloy. Moreover, the values of the IS related to the paramagnetic fcc phase, shown in Fig. 7, are also characteristic of an fcc Ni(Fe) alloy. The total amount of Fe in both paramagnetic + ferromagnetic phases from the ®rst to the second crystallization (Fig. 5) shows an increase for alloy X6, and is about the same for alloy X10 and decreases in the case of alloy X14. At the end of this second precipitation the silicides contain similar quantities of Fe (about 17 at.% of the total iron) in all the alloys, as shown in Fig. 5. The local ordering of the phosphides in alloys X6 and X10 is similar and close to Fe3 P, but for alloy X14 it seems close to a phase of the type Fe2 P. XRD results indicate an enrichment in Fe in the phosphide. M ossbauer data show that the total amount of Fe in this phase decreases with in-
Fig. 7. Evolution of the IS corresponding to the paramagnetic alloy for the three crystallization events, and for the three alloy compositions analyzed.
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creasing Si in the alloy (see Fig. 5). However, XRD results indicate an enrichment in their Fe concentration. Therefore, a smaller amount of phosphide is formed with increasing Si concentration of the amorphous alloys (X6±X14). 3.3. Third crystallization This crystallization peak appears only for alloy X14; the main trend is the formation of an fcc paramagnetic Ni(Fe) phase from the original fcc and bcc ferromagnetic phases. M ossbauer analysis shows this transformation to be due to the transformation of the ferromagnetic phases to paramagnetic ones. The IS of the paramagnetic fcc alloy, as shown in Fig. 7, reaches the value ()0.1 mm/s), typical of an Fe(Ni) alloy with a composition (obtained from the XRD lattice spacing) of about 42 at.% Fe. As the XRD data show (see Fig. 2) there is also an increase in the relative amount of Fe in the silicides and a small decrease of Fe in the phosphides. No signi®cant changes appear in the magnetic hyper®ne ®eld distribution related to the phosphides.
4. Discussion The amorphous alloys obtained after melt spinning are homogeneous. Only small dierences are obtained in the SRO M ossbauer parameters, which are related to the dierences in the composition [13]. The values obtained are IS 0.2 mm/s, QS 0 mm/s, AHF 19 T, STD 5 T, which correspond to a unique FeNiP-based amorphous phase. At room temperature and up to high temperatures, there is almost no miscibility between Fe and Ni. Therefore, on heating the amorphous alloys, separation of Fe and Ni results in the precipitation of Ni-rich silicides and Fe(Si) bcc, respectively. Consequently, the ®rst crystallization is activated by the nucleation of a primary bcc Fe(Si), and of a Ni-rich silicide. The remaining amorphous phase after the ®rst crystallization has an Fe3 P-type local order and composition.
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At higher temperatures, during the second crystallization, the formation of the Fe-rich phosphide depends on the initial Si concentration of the alloy. The phosphide has a local ordering of the type (FeNi)3 P, and is richer in Fe with increasing Si in the alloy. The formation of a ferromagnetic fcc Ni(Fe) phase occurs mainly during the second crystallization and becomes the main phase for the P-richer alloy, X6. That is, the formation of this phase seems to be accelerated by the formation of the Fe-rich phosphides, and, consequently, with the increasing P concentration of the alloy. The ferromagnetic fcc + bcc ® paramagnetic fcc transformation is activated only for the Si-richer alloy (X14) and it appears as the third crystallization process. Probably, such a transformation might also be activated for the other two alloys, but at a higher temperature than used in this study. The general sequence of product phases appearing in the thermally activated crystallization of the three alloys is as follows: (1) the formation of Ni-rich silicides and Fe(Si) bcc phases; and (2) the formation of Fe-rich phosphides and ferromagnetic Fe(Ni) fcc phases. Calorimetrically, the most stable alloy is X10 since the ®rst crystallization peak is sharp and delayed in temperature (with respect to X6 and X14). For this alloy, the local ordering does not change signi®cantly from the ®rst to the second crystallization. Moreover, the phases formed during the ®rst crystallization maintain their Fe concentration during the second crystallization. Although, during the ®rst crystallization, Nirich silicides and Fe(Si) bcc phases are also formed in alloy X6, the high P content of this alloy (or its low Si content) seems to retard the end of the ®rst crystallization (see Fig. 1). Such a spread in temperature may be attributed to the need for diusion-controlled growth in the last steps of the ®rst crystallization process. Similarly, such an extension is also seen in alloy X14. That is, it appears as if alloy X10 had the `ideal' composition for an eutectic-like transformation whereas alloys X6 and X14 need diusion over long distances to achieve the transformation. Schematically, the ®rst crystallization for alloy X10 may be written as an eutectic one
X 10amorphous ) Ni-rich silicidescrystal Fe Sibcc Fe-rich phosphidesamorphous : The onset temperature of the ®rst crystallization is advanced in alloys X6 and X14. For alloy X6 the reason may be related to the enhancement of the Ni-rich silicide formation with regard to the Fe(Si) phase since this alloy is Si-poor (the Si concentration of this last phase is about 8 at.%). For alloy X14, richer in Si, the formation of both bcc Fe(Si) and Ni-rich silicide phases seems promoted, but it results in phases with very low order. Also, the mean composition and local order of the remaining amorphous phase after the ®rst crystallization are close to Fe2 P (instead of Fe3 P for alloys X6 and X10). The lack of P results in the formation of an Fe-rich phosphide as the main contribution to the second crystallization peak. Also, at higher temperatures, the formation of a paramagnetic fcc Fe(Ni) alloy is observed. The onset temperature of the second crystallization is advanced in alloy X6. As already mentioned, for this alloy the high P concentration promotes the formation of a high amount of Ferich phosphides and the decomposition of the Fe(Si) bcc phase at the expense of the formation of a ferromagnetic Fe(Ni) phase. All these results agree with the lower stability of the remaining amorphous phase with increasing P content, thus accelerating the second crystallization.
5. Conclusions The thermally activated crystallization of three amorphous alloys (nominal composition Fe40 Ni40 Six P20 ÿ x with x 6, 10 and 14) produced by melt spinning has been examined. The general transformation sequence is a ®rst stage where Fe(Si) bcc- and Ni-rich silicides appear, and a second stage where the remaining amorphous phase transforms to Fe-rich silicides and the Fe(Si) bcc partially transforms to ferromagnetic Fe(Ni) fcc. In both the P-rich and the Si-rich alloys, X6 and X14, the onset temperature of the ®rst crystallization stage is advanced with respect to that
T. Pradell et al. / Journal of Non-Crystalline Solids 276 (2000) 113±121
of the equiatomic alloy, X10. Also for the former alloys, the crystallization calorimetric peak shows a long tail, indicative of diusion-limiting mechanisms. The second crystallization stage is advanced for the P-rich alloy, X6. Its high P content enables a large amount of Fe-rich phosphides to be formed, but their need of Fe results in the decomposition of the main bcc phase and the formation of an fcc ferromagnetic Fe(Ni) alloy. The Si-rich alloy, X14, shows very low order after the ®rst crystallization stage. However, in this alloy as well as in the equiatomic one, X10, the local ordering and overall composition of the remaining amorphous phase are quite similar. Thus, they show close values for the onset temperature of the second crystallization stage. The main dierence between these alloys is that in the Si-rich alloy a third exothermic peak is obtained at high temperatures, due to the formation of an fcc paramagnetic Ni(Fe) phase. Most probably, all three alloys may undergo such a transformation at high temperature. The alloy that is equiatomic in Si and P, X10, has the higher glass stability of the alloys studied. Also, almost no diusion at large distances are needed for the ®rst and second crystallization processes. That is, both may be considered to be eutectic-like transformations.
Acknowledgements The authors wish to thank Dr Parellada for the M ossbauer measurement facilities.The work has
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been ®nanced by CICYT grant MAT96-0692 and Generalitat de Catalunya grant 1995SGR 00514. References [1] K. Klement, R.H. Willens, P. Duwez, Nature 187 (1970) 869. [2] H.A. Davies, Phys. Chem. Glasses 17 (1976) 159. [3] S. Lele, K.S. Dubey, P. Ramanchandrarao, Curr. Sci. 14 (1985) 994. [4] H.J. G untherodt, H. Beck (Eds.), Glassy Metals I&II, Springer, Berlin, 1981±1983. [5] H. Frederiksson, S. Savage, Mater. Sci. Eng. A 133&134 (1991) 151. [6] D.G. Morris, Acta Metall. 29 (1981) 1213. [7] S.J. Thorpe, B. Ramaswami, K.T. Aust, Acta Metall. 36 (1988) 795. [8] K. Russew, S. Budurov, L. Snestiev, in: Fourth International Conference Rapidly Quenched Metals, 1994, p. 1285. [9] A. Dunst, D.M. Herlach, F. Gillessen, Mater. Sci. Eng. A 133 (1991) 785. [10] F. Welz, H. Kronm uller, D. Martinez, J. Rivas, Phys. Status Solidi A 138 (1993) 265. [11] H. Kimura, T. Masimoto, Amorphous Metallic Alloys, Butterworths, London, 1983, p. 187. [12] R. Br uning, Z. Altounian, J.O. Ostr om-Olsen, J. Appl. Phys. 62 (9) (1987) 3633. [13] J.J. Su~ nol, M.T. Clavaguera-Mora, N. Clavaguera, T. Pradell, Mater. Res. Soc. Symp. Proc. 455 (1997) 489. [14] J.J. Su~ nol, PhD thesis, Universitat Aut onoma de Barcelona, 1996. [15] J.J. Su~ nol, M.T. Mora, N. Clavaguera, Rev. Ciencia 7 (1997). [16] J. Hesse, A. R ubarstch, J. Phys. E 7 (1974) 526. [17] R.E. Vanderberghe, D. Ghyroy, E. De Grave, Nucl. Instrum. and Meth. 26 (1987) 603. [18] Y.A. Chang, in: H. Brodowsky, H.J. Schaller (Eds.), Thermochemistry of Alloys: Recent Developments of Experimental Methods, NATO ASI Series C: Mathematical and Physical Sciences, vol. 286, p. 85.