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Nanocrystallisation of amorphous alloys: comparison between furnace and current annealing
Intermetallics 8 (2000) 287±291
Nanocrystallisation of amorphous alloys: comparison between furnace and current annealing A. Guptaa, N. Bhagata, G. Principib,*, A. Maddalenab, N. Malhotraa, B.A. Dasannacharyaa, P.S. Goelc, H. Amenitschd, S. Bernstor e a Inter-University Consortium for DAEF, Khandwa Road, Indore - 452 017, India INFM (UnitaÁ di Trento), and UniversitaÁ di Padova, DIM, Settore Materiali, via Marzolo 9, I-35131 Padova, Italy c Inter-University Consortium for DAEF, R-5 Shed, BARC, Mumbai, India d Institute for Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Steyrergasse 17, A-8010 Graz, Austria e Sincrotrone Trieste, SS 14, Km 163.5, I-34012 Basovizza, Trieste, Italy b
Received 5 May 1999; accepted 18 October 1999
Abstract The mechanism of nanocrystallisation of the amorphous Fe72Cu1Nb4.5Si13.5B9 alloy has been studied with particular attention to the early stages of crystallisation. The specimens have been nanocrystallised by furnace (FA) and current (CA) annealing and analysed by MoÈssbauer spectroscopy. X-ray scattering, using the beamline 5.2L at Elettra Synchrotron Source, Trieste, has been carried out on the same FA and CA samples, as well as on a previously untreated sample current heated in situ. It is found that for the same amount of nanocrystalline phase, the crystallite size in CA samples does not dier signi®cantly from that in FA samples. Analysis of the MoÈssbauer spectra shows that in FA and CA samples the nanocrystalline grains consist of non-stoichiometric partially disordered Fe3Si phase, and that there is an indication of the presence of boron atoms in the nanocrystals of CA samples. This implies a lower segregation of boron atoms or of borides at the boundaries of nanocrystallites of CA samples and can be connected to their lower brittleness. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Nanostructured intermetallics; C. Heat treatment
1. Introduction Recent literature [1,2] reports on the formation of ultra®ne grain structure (nanocrystals) when a controlled partial crystallisation is carried out in some Cuand Nb-containing iron-based metallic glasses. These alloys are characterised by interesting soft magnetic properties, namely high saturation magnetisation and low coercivity [3,4]. Partial crystallisation is generally accomplished, starting from the amorphous Fe±metalloid alloy, by annealing at a temperature of 500/600 C. The nanocrystalline phase formation is related to the presence in the alloy of small amounts of niobium and copper: copper, being virtually immiscible in iron, forms local atomic clusters that enhance the nucleation of nanocrystalline structures; the addition of niobium stabilises the residual * Corresponding author. Tel.: +39-49-827-5513; fax: +39-49-8275510. E-mail address: [email protected] (G. Principi).
amorphous phase and hinders the grain growth. It has been reported that the primary phase which precipitates out is a Fe3Si ordered/disordered alloy, possibly with small concentration of other elements as Cu, Nb, B [5,6]. The objective of the present work is a comparative study of furnace-annealed and current-annealed samples of an amorphous FeCuNbSiB alloy using MoÈssbauer spectroscopy and X-ray scattering. Particular interest has been addressed to the early stages of crystallisation. 2. Experimental Ribbons of the Fe72Cu1Nb4.5Si13.5B9 amorphous alloy, obtained from Vacuum-Schmeltze GmbH, Hanau, have been annealed under protective atmosphere at 590 C for 10, 20 and 87 min (FA samples) and current annealed by passing a dc current of 7.8 A for 12 and 90 s (CA samples).
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00107-7
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A. Gupta et al. / Intermetallics 8 (2000) 287±291
Room temperature MoÈssbauer measurements were made using a 57Co:Rh source in transmission geometry. The obtained spectral pro®les were analysed by means of a current minimisation routine, referring the velocity scale and the isomer shifts to metallic iron. X-ray scattering measurements were done using the beamline 5.2L at Elettra Synchrotron Source, Trieste. Two one-dimensional detectors were used in order to cover the small angle (SAXS) as well as the wide-angle (WAXS) regions simultaneously [7]. Static measurements have been done on FA and CA treated samples and, in order to obtain information on the early stages of crystallisation, also on a previously untreated sample heated in situ by passing for a time of 500 s a current of 7.8 A and recording a diraction pattern every second. A run with empty sample holder has also been measured for background subtraction. The calibration of the q scale has been performed with dry rat tail tendon for the SAXS and with p-bromobenzoic acid for the WAXS regime.
fractional area of crystalline component, calculated by ®tting the spectral pro®le as the sum of a broad and a sharp gaussian component, increases gradually with the annealing time and saturates at about 0.3 after 400 s. This ratio of the area of crystalline peak to the total area is a measure of the degree of crystallisation. The variation of grain size, calculated from the linewidth of the nanocrystalline component considering negligible the instrumental broadening, and d-spacing of nanocrystals vs. annealing time are also shown. The grain size increases with the annealing time and reaches the constant value of 18 nm after about 200 s. A slow increase of d-spacing with annealing time is also noticed and may be due to outdiusion of Si from nanocrystalline grains, according to previous observations [5]. The WAXS patterns in Fig. 4 of samples previously treated show the development of nanocrystalline grains
3. Results and discussion The DSC pro®le of the alloy of Fig. 1 displays a twostep crystallisation behaviour. In the ®rst step, at about 540 C, a primary phase precipitates out, while in the second step, at about 680 C, the remaining amorphous matrix crystallises via eutectic transformation. Nanocrystalline phase is obtained after the completion of ®rst crystallisation step. The time-resolved measurements taken during the application of the 7.8 A current to the initially amorphous sample display, in the WAXS region of Fig. 2, the development of a sharp component, due to the progressive formation of nanocrystals, overlapped to the broad pro®le of the amorphous matrix. According to the peak position, this crystalline phase can be either disordered solid solution of silicon in bcc iron or an ordered Fe3Si structure. In Fig. 3 we observe that the
Fig. 1. DSC pro®le of Fe72Cu1Nb4.5Si13.5B9 amorphous alloy.
Fig. 2. Time resolved wide angle scattering patterns of Fe72Cu1Nb4.5Si13.5B9 amorphous alloy submitted to in situ 7.8 A current annealing for the times indicated.
Fig. 3. Fractional area, grain size and lattice parameter of crystalline component vs. annealing time as deduced from the patterns of Fig. 2.
A. Gupta et al. / Intermetallics 8 (2000) 287±291
289
Fig. 4. Wide angle scattering patterns of FA and CA samples.
with the annealing time both in FA and in CA samples. Also in this case, the peak position corresponds to either disordered solid solution of silicon in bcc iron or an ordered Fe3Si structure. Table 1 gives the values of crystalline fraction as well as of grain size obtained by ®tting the patterns with gaussian components. The eect of furnace annealing seems to be very similar to that of current annealing. We note that CA leads to a higher degree of crystallisation with respect to the values obtained dynamically (about 0.45 instead of about 0.30). This can be due to the persistence of heating in the ®rst case. In Fig. 5, room temperature MoÈssbauer spectra of FA and CA samples give further evidence of the formation of crystalline phase with the treatment time. The spectrum pro®le of untreated sample is a very broad sextet typical of an amorphous substance, in which there is a continuous distribution of sites for iron atoms corresponding to a multiplicity of close sextets. As the treatment progresses the spectra display net components, corresponding to well de®ned iron sites in the lattice, superposed to a very broad contribution due to the remaining amorphous matrix. The spectra of samples FA 590 C/87 min and CA 7.8A/90 s look very similar, suggesting, in agreement with X-ray scattering data, that the degree of crystallisation in the two samples is more or less the same. According to early work [5] and to the indication of X-ray scattering, the nanocrystalline Table 1 Values of crystalline fraction and of grain size calculated from the wide-angle X-ray scattering data of samples current and furnace annealed for dierent times Sample
Crystalline fraction
Grain size (nm)
CA 7.8A/90 s CA 7.8A/12 s FA 590 C/87 min FA 590 C/20 min
0.46 0.19 0.45 0.14
20 16 22 15
Fig. 5. Room temperature MoÈssbauer spectra of Fe72Cu1Nb4.5Si13.5B9 alloy: (a) as-received; (b) FA 590 C/87 min; (c) CA 7.8A/90 s.
phase is distinctly of Fe3Si type (DO3). If such structure is ordered and stoichiometric, there are two nonequivalent iron sites, designated as A4 (4 Fe nn, 4 Si nn and 6 Fe nnn) and D8 (8 Fe nn, and 6 Si nnn), with their internal magnetic ®elds Bhf equal to 20.1 and 31.0 T (a Si nn instead of Fe nn depresses the internal ®eld at iron atoms of 3±4 T) and their relative populations 2:1 [8]. Therefore, one should expect a spectrum of only two sextets in case of stoichiometric and ordered Fe3Si phase. The best ®t of our spectral pro®les, however, is obtained with four sharp sextets, corresponding to four dierent iron sites in the nanocrystalline grains, superposed to the above mentioned broad component. The additional sextets with about 24.3 and 28.5 T are easily associated to iron sites dierent than the above considered A4 and D8. In fact, the structure can be nonstoichiometric (silicon de®cient) either ordered and with some degree of disorder. In the ®rst case the additional components may be ascribed to iron in sites A5 (5 Fe nn, 3 Si nn and 6 Fe nnn) and A6 (6 Fe nn, 2 Si nn and 6 Fe nnn), respectively. In the second case a contribution can arise also from iron in sites D6 (6 Fe nn, 2 Si nn and 6 Si nnn) and D7 (7 Fe nn, 1 Si nn and 6 Si nnn), respectively. The ®elds experienced by iron atoms at A6 and D7 sites and at A5 and D6 sites are about the same due to the in¯uence also of nnn atoms, which are prevalently Fe for A sites and Si for D sites. The best ®ts of experimental pro®les for internal ®eld, Bhf, and relative area, RA, of each spectral component of crystalline fraction are reported in Table 2 for samples FA 590 C/87 min and CA 7.8A/90 s. While the Bhf values of each component in the two samples are very close, their RA are considerably dierent. This leads to the following probabilistic considerations:
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A. Gupta et al. / Intermetallics 8 (2000) 287±291
Table 2 Experimental and calculated values of spectral parameters of FA and CA Fe72Cu1Nb4.5Si13.5B9 samples (see below and text)a Specimen
Bhf (T)
RA (%)
RAc (%)
cD (%)
cA (%)
cSi (%)
Furnace-annealed 590 C/87 min
31.2 28.3 24.3 19.4
33 12 27 28
33 13 25 27
40.6
1.6
21.1
Current-annealed 7.8A/90 s
31.1 28.8 24.2 19.6
27 8 32 32
28 15 25 31
42
3.8
22.9
a Bhf (T), hyper®ne ®eld in tesla; RA, relative spectral area; RAc, relative spectral area calculated assuming the nanocrystalline grains to have a partially disordered non-stoichiometric Fe3Si phase; cA, fraction of A sites occupied by Si; cD, fraction of D sites occupied by Si; cSi, overall Si concentration.
Because of disorder Si atoms occupy a small fraction, cA , of A sites. The probability that r out of the eight Fe sites nearest-neighbours of iron atoms in D sites are occupied by Si atoms will be given by P D8 ÿ r C8r crA 1 ÿ cA 8ÿr ; where C8r is the number of combinations of eight objects taken r at a time. On the other hand, because of non-stoichiometry, the fraction of D sites occupied by Si atoms, cD, will be less than 0.5. The probability for iron atoms in A sites to have more than four Si atoms as nearest-neighbours will be P A4 r C4r cr 1 ÿ c 4ÿr ; cD and the overall concentration of with c 4 0:25 ÿ 2 cA cD : Si atoms cSi 2 The relative areas RAc of Table 2, calculated assuming the nanocrystalline grains having a partially disordered non-stoichiometric Fe3Si phase, well correspond to the experimental values in the case of FA sample, while a certain discrepancy is found in the case of CA sample. Table 2 reports also the cA, cD and cSi values calculated on the basis of the previous considerations. In the case of CA sample no combination of cA and cD can match correctly the experimental observed relative areas of the four crystalline iron sites. We suggest that the MoÈssbauer spectrum of CA sample cannot be understood in terms of partially disordered non-stoichiometric Fe3Si phase, probably because some boron atoms are retained in the nanocrystalline grains.
Boron goes interstitially and reduces the hyper®ne ®eld of neighbouring iron atoms. This results in larger relative areas of components having lower hyper®ne ®elds (in particular, the component with Bhf=24.2 T). Presence of retained boron atoms in the nanocrystals of CA sample can be one of the reasons to account for the lower brittleness with respect FA. Our earlier MoÈssbauer studies [5,6] have shown that the width of the hyper®ne ®eld distribution of the remaining amorphous component increases with the amount of crystallisation at a faster rate in alloys containing Nb and Cu as compared with, for example, Fe78Si9B13. It may be noted that during the ®rst crystallisation step, as the primary phase precipitates out, B is expelled out of the crystalline grains. Because of the limited diusivity of B in the amorphous matrix, a B concentration gradient builds up in the amorphous matrix with a higher concentration in the immediate surroundings of the crystalline grains. This inhomogeneous distribution of boron atoms in the remaining amorphous phase gives rise to a broadening of the hyper®ne ®eld distribution. Presence of Cu and Nb seems then to decrease the diffusivity of B in the amorphous matrix, with the result that B concentration around the nanocrystalline grains is higher. It is this high concentration of B at the boundaries of the nanocrystalline grains, probably in form of borides [9] that may be the cause of high brittleness of these alloys. In the case of CA samples, since a part of B is retained inside the nanocrystals, the concentration of B at the grain boundary will be less, thus causing a lower brittleness. The conjecture that some amount of boron atoms is retained inside the nanocrystals is also supported by preliminary SAXS data on the same samples, which will be presented in a further work. 4. Conclusions We have shown that current annealing (CA) of Fe± Si±B±Cu±Nb alloys is as eective as furnace annealing (FA) in producing nanocrystals dispersed in an amorphous matrix. MoÈssbauer spectroscopy and X-ray scattering measurements indicate some dierences in the obtained nanocrystals. In particular, CA samples display a lower grain size and retention of boron atoms, which can be related to a lower brittleness. Acknowledgements This work has been partly supported by the Italian Ministero dell'UniversitaÁ e della Ricerca Scienti®ca e Tecnologica, in the frame of the Italian National Research Project ``Leghe e composti intermetallici: stabilitaÁ termodinamica, proprietaÁ ®siche e reattivitaÁ''.
A. Gupta et al. / Intermetallics 8 (2000) 287±291
Thanks are due by G.P. to the Oce of External Activities of ICTP, Trieste, for a mobility grant. References [1] Herzer G. Phys Scr 1993;T49:307 (and references therein). [2] Rixecker G, Schaaf P, Gonser U. J Phys Condens Matter 1992; 4:10295. [3] Yoshizawa Y, Oguma S, Yamaguchi K. J Appl Phys 1988; 64:6044.
291
[4] Herzer G. IEEE Trans Magn 1989;25:3327 (and references therein). [5] Gupta A, Bhagat N, Principi G, Hernando A. J Magn Magn Mater 1994;133:291. [6] Gupta A, Bhagat N, Principi G. J Phys Condens Matter 1995; 7:2237±48. [7] Amenitsch H, Bernstor S, Rappolt M, Kriechbaum, Mio H, Laggner P. J Synchrotron Rad 1998;5:506. [8] Stearns MB. Phys Rev, 1968;168:588. [9] Garcia del Muro M, Zquiak R, Batle X, Parellada J. J Magn Magn Mater 1995;140±144:475.
Nanocrystallisation of amorphous alloys: comparison between furnace and current annealing A. Guptaa, N. Bhagata, G. Principib,*, A. Maddalenab, N. Malhotraa, B.A. Dasannacharyaa, P.S. Goelc, H. Amenitschd, S. Bernstor e a Inter-University Consortium for DAEF, Khandwa Road, Indore - 452 017, India INFM (UnitaÁ di Trento), and UniversitaÁ di Padova, DIM, Settore Materiali, via Marzolo 9, I-35131 Padova, Italy c Inter-University Consortium for DAEF, R-5 Shed, BARC, Mumbai, India d Institute for Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Steyrergasse 17, A-8010 Graz, Austria e Sincrotrone Trieste, SS 14, Km 163.5, I-34012 Basovizza, Trieste, Italy b
Received 5 May 1999; accepted 18 October 1999
Abstract The mechanism of nanocrystallisation of the amorphous Fe72Cu1Nb4.5Si13.5B9 alloy has been studied with particular attention to the early stages of crystallisation. The specimens have been nanocrystallised by furnace (FA) and current (CA) annealing and analysed by MoÈssbauer spectroscopy. X-ray scattering, using the beamline 5.2L at Elettra Synchrotron Source, Trieste, has been carried out on the same FA and CA samples, as well as on a previously untreated sample current heated in situ. It is found that for the same amount of nanocrystalline phase, the crystallite size in CA samples does not dier signi®cantly from that in FA samples. Analysis of the MoÈssbauer spectra shows that in FA and CA samples the nanocrystalline grains consist of non-stoichiometric partially disordered Fe3Si phase, and that there is an indication of the presence of boron atoms in the nanocrystals of CA samples. This implies a lower segregation of boron atoms or of borides at the boundaries of nanocrystallites of CA samples and can be connected to their lower brittleness. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Nanostructured intermetallics; C. Heat treatment
1. Introduction Recent literature [1,2] reports on the formation of ultra®ne grain structure (nanocrystals) when a controlled partial crystallisation is carried out in some Cuand Nb-containing iron-based metallic glasses. These alloys are characterised by interesting soft magnetic properties, namely high saturation magnetisation and low coercivity [3,4]. Partial crystallisation is generally accomplished, starting from the amorphous Fe±metalloid alloy, by annealing at a temperature of 500/600 C. The nanocrystalline phase formation is related to the presence in the alloy of small amounts of niobium and copper: copper, being virtually immiscible in iron, forms local atomic clusters that enhance the nucleation of nanocrystalline structures; the addition of niobium stabilises the residual * Corresponding author. Tel.: +39-49-827-5513; fax: +39-49-8275510. E-mail address: [email protected] (G. Principi).
amorphous phase and hinders the grain growth. It has been reported that the primary phase which precipitates out is a Fe3Si ordered/disordered alloy, possibly with small concentration of other elements as Cu, Nb, B [5,6]. The objective of the present work is a comparative study of furnace-annealed and current-annealed samples of an amorphous FeCuNbSiB alloy using MoÈssbauer spectroscopy and X-ray scattering. Particular interest has been addressed to the early stages of crystallisation. 2. Experimental Ribbons of the Fe72Cu1Nb4.5Si13.5B9 amorphous alloy, obtained from Vacuum-Schmeltze GmbH, Hanau, have been annealed under protective atmosphere at 590 C for 10, 20 and 87 min (FA samples) and current annealed by passing a dc current of 7.8 A for 12 and 90 s (CA samples).
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00107-7
288
A. Gupta et al. / Intermetallics 8 (2000) 287±291
Room temperature MoÈssbauer measurements were made using a 57Co:Rh source in transmission geometry. The obtained spectral pro®les were analysed by means of a current minimisation routine, referring the velocity scale and the isomer shifts to metallic iron. X-ray scattering measurements were done using the beamline 5.2L at Elettra Synchrotron Source, Trieste. Two one-dimensional detectors were used in order to cover the small angle (SAXS) as well as the wide-angle (WAXS) regions simultaneously [7]. Static measurements have been done on FA and CA treated samples and, in order to obtain information on the early stages of crystallisation, also on a previously untreated sample heated in situ by passing for a time of 500 s a current of 7.8 A and recording a diraction pattern every second. A run with empty sample holder has also been measured for background subtraction. The calibration of the q scale has been performed with dry rat tail tendon for the SAXS and with p-bromobenzoic acid for the WAXS regime.
fractional area of crystalline component, calculated by ®tting the spectral pro®le as the sum of a broad and a sharp gaussian component, increases gradually with the annealing time and saturates at about 0.3 after 400 s. This ratio of the area of crystalline peak to the total area is a measure of the degree of crystallisation. The variation of grain size, calculated from the linewidth of the nanocrystalline component considering negligible the instrumental broadening, and d-spacing of nanocrystals vs. annealing time are also shown. The grain size increases with the annealing time and reaches the constant value of 18 nm after about 200 s. A slow increase of d-spacing with annealing time is also noticed and may be due to outdiusion of Si from nanocrystalline grains, according to previous observations [5]. The WAXS patterns in Fig. 4 of samples previously treated show the development of nanocrystalline grains
3. Results and discussion The DSC pro®le of the alloy of Fig. 1 displays a twostep crystallisation behaviour. In the ®rst step, at about 540 C, a primary phase precipitates out, while in the second step, at about 680 C, the remaining amorphous matrix crystallises via eutectic transformation. Nanocrystalline phase is obtained after the completion of ®rst crystallisation step. The time-resolved measurements taken during the application of the 7.8 A current to the initially amorphous sample display, in the WAXS region of Fig. 2, the development of a sharp component, due to the progressive formation of nanocrystals, overlapped to the broad pro®le of the amorphous matrix. According to the peak position, this crystalline phase can be either disordered solid solution of silicon in bcc iron or an ordered Fe3Si structure. In Fig. 3 we observe that the
Fig. 1. DSC pro®le of Fe72Cu1Nb4.5Si13.5B9 amorphous alloy.
Fig. 2. Time resolved wide angle scattering patterns of Fe72Cu1Nb4.5Si13.5B9 amorphous alloy submitted to in situ 7.8 A current annealing for the times indicated.
Fig. 3. Fractional area, grain size and lattice parameter of crystalline component vs. annealing time as deduced from the patterns of Fig. 2.
A. Gupta et al. / Intermetallics 8 (2000) 287±291
289
Fig. 4. Wide angle scattering patterns of FA and CA samples.
with the annealing time both in FA and in CA samples. Also in this case, the peak position corresponds to either disordered solid solution of silicon in bcc iron or an ordered Fe3Si structure. Table 1 gives the values of crystalline fraction as well as of grain size obtained by ®tting the patterns with gaussian components. The eect of furnace annealing seems to be very similar to that of current annealing. We note that CA leads to a higher degree of crystallisation with respect to the values obtained dynamically (about 0.45 instead of about 0.30). This can be due to the persistence of heating in the ®rst case. In Fig. 5, room temperature MoÈssbauer spectra of FA and CA samples give further evidence of the formation of crystalline phase with the treatment time. The spectrum pro®le of untreated sample is a very broad sextet typical of an amorphous substance, in which there is a continuous distribution of sites for iron atoms corresponding to a multiplicity of close sextets. As the treatment progresses the spectra display net components, corresponding to well de®ned iron sites in the lattice, superposed to a very broad contribution due to the remaining amorphous matrix. The spectra of samples FA 590 C/87 min and CA 7.8A/90 s look very similar, suggesting, in agreement with X-ray scattering data, that the degree of crystallisation in the two samples is more or less the same. According to early work [5] and to the indication of X-ray scattering, the nanocrystalline Table 1 Values of crystalline fraction and of grain size calculated from the wide-angle X-ray scattering data of samples current and furnace annealed for dierent times Sample
Crystalline fraction
Grain size (nm)
CA 7.8A/90 s CA 7.8A/12 s FA 590 C/87 min FA 590 C/20 min
0.46 0.19 0.45 0.14
20 16 22 15
Fig. 5. Room temperature MoÈssbauer spectra of Fe72Cu1Nb4.5Si13.5B9 alloy: (a) as-received; (b) FA 590 C/87 min; (c) CA 7.8A/90 s.
phase is distinctly of Fe3Si type (DO3). If such structure is ordered and stoichiometric, there are two nonequivalent iron sites, designated as A4 (4 Fe nn, 4 Si nn and 6 Fe nnn) and D8 (8 Fe nn, and 6 Si nnn), with their internal magnetic ®elds Bhf equal to 20.1 and 31.0 T (a Si nn instead of Fe nn depresses the internal ®eld at iron atoms of 3±4 T) and their relative populations 2:1 [8]. Therefore, one should expect a spectrum of only two sextets in case of stoichiometric and ordered Fe3Si phase. The best ®t of our spectral pro®les, however, is obtained with four sharp sextets, corresponding to four dierent iron sites in the nanocrystalline grains, superposed to the above mentioned broad component. The additional sextets with about 24.3 and 28.5 T are easily associated to iron sites dierent than the above considered A4 and D8. In fact, the structure can be nonstoichiometric (silicon de®cient) either ordered and with some degree of disorder. In the ®rst case the additional components may be ascribed to iron in sites A5 (5 Fe nn, 3 Si nn and 6 Fe nnn) and A6 (6 Fe nn, 2 Si nn and 6 Fe nnn), respectively. In the second case a contribution can arise also from iron in sites D6 (6 Fe nn, 2 Si nn and 6 Si nnn) and D7 (7 Fe nn, 1 Si nn and 6 Si nnn), respectively. The ®elds experienced by iron atoms at A6 and D7 sites and at A5 and D6 sites are about the same due to the in¯uence also of nnn atoms, which are prevalently Fe for A sites and Si for D sites. The best ®ts of experimental pro®les for internal ®eld, Bhf, and relative area, RA, of each spectral component of crystalline fraction are reported in Table 2 for samples FA 590 C/87 min and CA 7.8A/90 s. While the Bhf values of each component in the two samples are very close, their RA are considerably dierent. This leads to the following probabilistic considerations:
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A. Gupta et al. / Intermetallics 8 (2000) 287±291
Table 2 Experimental and calculated values of spectral parameters of FA and CA Fe72Cu1Nb4.5Si13.5B9 samples (see below and text)a Specimen
Bhf (T)
RA (%)
RAc (%)
cD (%)
cA (%)
cSi (%)
Furnace-annealed 590 C/87 min
31.2 28.3 24.3 19.4
33 12 27 28
33 13 25 27
40.6
1.6
21.1
Current-annealed 7.8A/90 s
31.1 28.8 24.2 19.6
27 8 32 32
28 15 25 31
42
3.8
22.9
a Bhf (T), hyper®ne ®eld in tesla; RA, relative spectral area; RAc, relative spectral area calculated assuming the nanocrystalline grains to have a partially disordered non-stoichiometric Fe3Si phase; cA, fraction of A sites occupied by Si; cD, fraction of D sites occupied by Si; cSi, overall Si concentration.
Because of disorder Si atoms occupy a small fraction, cA , of A sites. The probability that r out of the eight Fe sites nearest-neighbours of iron atoms in D sites are occupied by Si atoms will be given by P D8 ÿ r C8r crA 1 ÿ cA 8ÿr ; where C8r is the number of combinations of eight objects taken r at a time. On the other hand, because of non-stoichiometry, the fraction of D sites occupied by Si atoms, cD, will be less than 0.5. The probability for iron atoms in A sites to have more than four Si atoms as nearest-neighbours will be P A4 r C4r cr 1 ÿ c 4ÿr ; cD and the overall concentration of with c 4 0:25 ÿ 2 cA cD : Si atoms cSi 2 The relative areas RAc of Table 2, calculated assuming the nanocrystalline grains having a partially disordered non-stoichiometric Fe3Si phase, well correspond to the experimental values in the case of FA sample, while a certain discrepancy is found in the case of CA sample. Table 2 reports also the cA, cD and cSi values calculated on the basis of the previous considerations. In the case of CA sample no combination of cA and cD can match correctly the experimental observed relative areas of the four crystalline iron sites. We suggest that the MoÈssbauer spectrum of CA sample cannot be understood in terms of partially disordered non-stoichiometric Fe3Si phase, probably because some boron atoms are retained in the nanocrystalline grains.
Boron goes interstitially and reduces the hyper®ne ®eld of neighbouring iron atoms. This results in larger relative areas of components having lower hyper®ne ®elds (in particular, the component with Bhf=24.2 T). Presence of retained boron atoms in the nanocrystals of CA sample can be one of the reasons to account for the lower brittleness with respect FA. Our earlier MoÈssbauer studies [5,6] have shown that the width of the hyper®ne ®eld distribution of the remaining amorphous component increases with the amount of crystallisation at a faster rate in alloys containing Nb and Cu as compared with, for example, Fe78Si9B13. It may be noted that during the ®rst crystallisation step, as the primary phase precipitates out, B is expelled out of the crystalline grains. Because of the limited diusivity of B in the amorphous matrix, a B concentration gradient builds up in the amorphous matrix with a higher concentration in the immediate surroundings of the crystalline grains. This inhomogeneous distribution of boron atoms in the remaining amorphous phase gives rise to a broadening of the hyper®ne ®eld distribution. Presence of Cu and Nb seems then to decrease the diffusivity of B in the amorphous matrix, with the result that B concentration around the nanocrystalline grains is higher. It is this high concentration of B at the boundaries of the nanocrystalline grains, probably in form of borides [9] that may be the cause of high brittleness of these alloys. In the case of CA samples, since a part of B is retained inside the nanocrystals, the concentration of B at the grain boundary will be less, thus causing a lower brittleness. The conjecture that some amount of boron atoms is retained inside the nanocrystals is also supported by preliminary SAXS data on the same samples, which will be presented in a further work. 4. Conclusions We have shown that current annealing (CA) of Fe± Si±B±Cu±Nb alloys is as eective as furnace annealing (FA) in producing nanocrystals dispersed in an amorphous matrix. MoÈssbauer spectroscopy and X-ray scattering measurements indicate some dierences in the obtained nanocrystals. In particular, CA samples display a lower grain size and retention of boron atoms, which can be related to a lower brittleness. Acknowledgements This work has been partly supported by the Italian Ministero dell'UniversitaÁ e della Ricerca Scienti®ca e Tecnologica, in the frame of the Italian National Research Project ``Leghe e composti intermetallici: stabilitaÁ termodinamica, proprietaÁ ®siche e reattivitaÁ''.
A. Gupta et al. / Intermetallics 8 (2000) 287±291
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