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Nanocrystallisation of FeCuNbB alloys
NanoShuctured
Pergamon PI1 SO96S-9773(99)00189-O
NANOCRYSTALLISATION
Materials,
Vol. 12, pp. 585-588, 1999 Elsevier Science Ltd 8 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 0965-9773/99&see front matter
OF FeCuNbB ALLOYS
T. Girchardt I, B. Friedrichs 2, E. Woldt 2, J. Hcsse I, K. G. Efthimiadis 3, S. C. Chadjivasiliou ’ 1) Institut fiir Metallphysik und Nukleare Festk&perphysik, TU Braunschweig, Mendelssohn&. 3,38106 Braunschweig, Germany 2) Institut Rir Werkstoffkunde, TU Braunschweig, Langer Kamp 8, 38106 Braunschweig, Germany 3) Aristotle University of Thessaloniki, GR-54006 Thessaloniki, Greece Abstract - We irrvestigated the alloy system of Fes,&uINbxB13 (X=4, 5, 7) in order to obtain information on the crystallisation behaviour of the amorphous precursor alloy and the resulting structural composition. Onset temperatures of deferent crystallisation steps were studied with DSC measurements, Dilatometric and thermomagnetic measurements as well as those of the electrical resistivity were applied in order to detect changes not being correlated with a change of the heat capacity. For a structural investigation, Mirssbauer eflect and electron d@action experiments were carried out. Our results show that - up to a certain annealing temperature depending on the Nb content - there is only a nanocrystalline a-Fe phase, embedded in an amorphous matrix. At higher annealing temperatures crystalline FeB phases appear. 01999 Acta Metallurgica hc.
INTRODUCTION In 1988 Yoshizawa et al. reported on a new kind of Fe-based amorphous alloys which form - after an annealing process - a nanocrystalline structure (1). Those alloys have become well known as Finemet or Vitroperm. Their structure is the cause for many interesting physical properties (2,3). The grains in those alloys consist of FeSi, whereas the alloys discussed here do not contain any Si. When the amorphous ribbons of FeCuNbB alloys are annealed they develop in a first crystallisation step a nanocrystalline structure, containing a-Fe grains embedded in an amorphous matrix. At higher annealing temperatures they form additional FeB phases. In order to learn more about the crystallisation behaviour of these alloys, we investigated them with different experimental techniques.
EXPERIMENTAL The amorphous ribbons of the three alloys were heated up with a rate of 3 K/min, starting at room temperature and going up to 1000 “C (or less, due to technical reasons). During that process the measurements were done. In the following we will discuss measurements 585
588
FOURTHINTERNATIONAL CONFERENCE
ON NANOSTRUCTURED
MATERIALS
of the following properties: thermal expansion, heat flow, electrical resistivety and magnetisation. Due to the limited space we will focus on only one alloy, i. e. Fe82CulNb4B 13. The qualitative behaviour of the other two alloys is very similar.
Fig. 1 shows the development of four of the measured properties of Fes2Cu,Nb4B13 versus temperature. In the upper part temperature ranges marked with letters A to E indicate interesting properties or developments. A: invar behaviour In the temperature range A the sample shows an invar like behaviour, concerning length and resistivity. The Curie point of the amorphous sample (Tc=225 “C) is reached. The other alloys have a lower Curie temperature (Nb,: 200 “C, Nb+ 165 “C). B: structural relaxation Temperature range B reveals some structural relaxation. A reduced thermal expansion, indicating an increase of the density due to the reduction of free volume, and a weak heat emission over a wide temperature range can be observed. C: 1” crystallisation
step
The frrst crystallisation takes place in the temperature range C, beginning already in range B. This can be seen by the narrow exothermal peak in the heat flow curve. Even more significant is the decreasing electrical resistivity and the increasing magnetisation, which cannot be caused by the amorphous phase. However there is no significant change observable with the dilatometer. A phase analysis, done via M&sbauer spectrometry and electron di@action clearly shows the formation of crystalline a-Fe grains in a remaining amorphous matrix. The grain size depends on the Nb content. Nb is enriched in the grain boundaries and finally stops the growth of the grains. The transmission electron microscope shows an average grain size of 30 run; the alloy containing 5 at?h Nb has grains of 10 nm diameter and the one with 7 at% has the smallest grains with only 3.5 nm in diameter. Further nucleation processes take place in the range C-D, indicated by small exotherma1 DSC peaks. The step in the magnetisation curve hints to a second ferromagnetic phase besides Fe. D: 2* crystallisation
step
In the temperature range D a second crystallisation takes place, indicated by a narrow exothermal peak in the heat flow curve. This coincides with a notable contraction of the sample as seen in the dilatometer curve. A phase analysis points to the eutectic crystallisation of the remaining amorphous phase into a-Fe and Fe,,B. As we are already above the Curie point
FOURTH INTERNATIONAL
ON NAN~STRUCTURED
587
MATERIALS
A
A
r
CONFERENCE
\ -i
-‘\
\
c
\ ,
4%
\ ’
j
2
heat flow ----4 Y
i
- 100
T-
magnetisation
400
600
600
Temperature[“Cl Figure 1. Relative length, relative resistivity (both normakxi
heat flow and magnetisation of Fe&ulNbbB13
to room temperature), vs. temperature
588
FOURTH INTERNATIONAL CONFERENCE
ON NANOSTRUCTURED
MATERIALS
of Fe,B (500-550 “C, depending on the lattice type), the siight step in the magnetisation curve at about 700 “C might hint to F%B with Tc=742 “C. Attempts to analyse the structure by electron difiaction however produced no clear results in favour of either borides. At the same time already existing grains grow strongly. E: 3rd transformation step In the temperature range E the coefficients of the electrical resistivity change. A strong contraction can be observed, which is caused by the a-y phase transformation of the iron components. This transformation (and the contraction) is reversible when subsequently lowering the temperature, but with a hysteresis of about 100 K.
SUMMARY Phase and structural investigations - done by Mossbauer spectrometry and electron diffraction - show, that in the first crystallisation process only cl-Fe nanocrystals appear. There remains an amorphous phase still containing more than 50 % of the iron atoms in the sample. We have shown, that the crystallisation process of FefZuNbB alloys is very similar to that of the well known FeCubIbSiB. The main difference is - as there is no Si in these alloys - that no FeSi nanocrystals are formed, but a,-Fe grains. The nanostructure is very similar and so is the crystallisation in two steps. In both cases FeB phases develop during the second crystallisation step.
ACKNOWLEDGEMENTS The authors wish to thank Dr. G. Herzer, Vacuumschmelze GmbH, Hanau, Germany, for the amorphous samples. The work was tinancially supported by the Deutsche Forschungsgemeinschaft (DFG) and the “Internationales B&o des BMBF”.
REFERENCES 1. Y. Yoshizawa, S. Oguma and K. Yamauchi, Journal ofAppl. Physics 64 (lo), 6044, 1988 2. G. Herzer, Physica Scripta x, 307,1993 3. T. Graf, M. Kopcewicz and J. Hesse, X Phys: Cordens. Matter &3897,1996
Pergamon PI1 SO96S-9773(99)00189-O
NANOCRYSTALLISATION
Materials,
Vol. 12, pp. 585-588, 1999 Elsevier Science Ltd 8 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 0965-9773/99&see front matter
OF FeCuNbB ALLOYS
T. Girchardt I, B. Friedrichs 2, E. Woldt 2, J. Hcsse I, K. G. Efthimiadis 3, S. C. Chadjivasiliou ’ 1) Institut fiir Metallphysik und Nukleare Festk&perphysik, TU Braunschweig, Mendelssohn&. 3,38106 Braunschweig, Germany 2) Institut Rir Werkstoffkunde, TU Braunschweig, Langer Kamp 8, 38106 Braunschweig, Germany 3) Aristotle University of Thessaloniki, GR-54006 Thessaloniki, Greece Abstract - We irrvestigated the alloy system of Fes,&uINbxB13 (X=4, 5, 7) in order to obtain information on the crystallisation behaviour of the amorphous precursor alloy and the resulting structural composition. Onset temperatures of deferent crystallisation steps were studied with DSC measurements, Dilatometric and thermomagnetic measurements as well as those of the electrical resistivity were applied in order to detect changes not being correlated with a change of the heat capacity. For a structural investigation, Mirssbauer eflect and electron d@action experiments were carried out. Our results show that - up to a certain annealing temperature depending on the Nb content - there is only a nanocrystalline a-Fe phase, embedded in an amorphous matrix. At higher annealing temperatures crystalline FeB phases appear. 01999 Acta Metallurgica hc.
INTRODUCTION In 1988 Yoshizawa et al. reported on a new kind of Fe-based amorphous alloys which form - after an annealing process - a nanocrystalline structure (1). Those alloys have become well known as Finemet or Vitroperm. Their structure is the cause for many interesting physical properties (2,3). The grains in those alloys consist of FeSi, whereas the alloys discussed here do not contain any Si. When the amorphous ribbons of FeCuNbB alloys are annealed they develop in a first crystallisation step a nanocrystalline structure, containing a-Fe grains embedded in an amorphous matrix. At higher annealing temperatures they form additional FeB phases. In order to learn more about the crystallisation behaviour of these alloys, we investigated them with different experimental techniques.
EXPERIMENTAL The amorphous ribbons of the three alloys were heated up with a rate of 3 K/min, starting at room temperature and going up to 1000 “C (or less, due to technical reasons). During that process the measurements were done. In the following we will discuss measurements 585
588
FOURTHINTERNATIONAL CONFERENCE
ON NANOSTRUCTURED
MATERIALS
of the following properties: thermal expansion, heat flow, electrical resistivety and magnetisation. Due to the limited space we will focus on only one alloy, i. e. Fe82CulNb4B 13. The qualitative behaviour of the other two alloys is very similar.
Fig. 1 shows the development of four of the measured properties of Fes2Cu,Nb4B13 versus temperature. In the upper part temperature ranges marked with letters A to E indicate interesting properties or developments. A: invar behaviour In the temperature range A the sample shows an invar like behaviour, concerning length and resistivity. The Curie point of the amorphous sample (Tc=225 “C) is reached. The other alloys have a lower Curie temperature (Nb,: 200 “C, Nb+ 165 “C). B: structural relaxation Temperature range B reveals some structural relaxation. A reduced thermal expansion, indicating an increase of the density due to the reduction of free volume, and a weak heat emission over a wide temperature range can be observed. C: 1” crystallisation
step
The frrst crystallisation takes place in the temperature range C, beginning already in range B. This can be seen by the narrow exothermal peak in the heat flow curve. Even more significant is the decreasing electrical resistivity and the increasing magnetisation, which cannot be caused by the amorphous phase. However there is no significant change observable with the dilatometer. A phase analysis, done via M&sbauer spectrometry and electron di@action clearly shows the formation of crystalline a-Fe grains in a remaining amorphous matrix. The grain size depends on the Nb content. Nb is enriched in the grain boundaries and finally stops the growth of the grains. The transmission electron microscope shows an average grain size of 30 run; the alloy containing 5 at?h Nb has grains of 10 nm diameter and the one with 7 at% has the smallest grains with only 3.5 nm in diameter. Further nucleation processes take place in the range C-D, indicated by small exotherma1 DSC peaks. The step in the magnetisation curve hints to a second ferromagnetic phase besides Fe. D: 2* crystallisation
step
In the temperature range D a second crystallisation takes place, indicated by a narrow exothermal peak in the heat flow curve. This coincides with a notable contraction of the sample as seen in the dilatometer curve. A phase analysis points to the eutectic crystallisation of the remaining amorphous phase into a-Fe and Fe,,B. As we are already above the Curie point
FOURTH INTERNATIONAL
ON NAN~STRUCTURED
587
MATERIALS
A
A
r
CONFERENCE
\ -i
-‘\
\
c
\ ,
4%
\ ’
j
2
heat flow ----4 Y
i
- 100
T-
magnetisation
400
600
600
Temperature[“Cl Figure 1. Relative length, relative resistivity (both normakxi
heat flow and magnetisation of Fe&ulNbbB13
to room temperature), vs. temperature
588
FOURTH INTERNATIONAL CONFERENCE
ON NANOSTRUCTURED
MATERIALS
of Fe,B (500-550 “C, depending on the lattice type), the siight step in the magnetisation curve at about 700 “C might hint to F%B with Tc=742 “C. Attempts to analyse the structure by electron difiaction however produced no clear results in favour of either borides. At the same time already existing grains grow strongly. E: 3rd transformation step In the temperature range E the coefficients of the electrical resistivity change. A strong contraction can be observed, which is caused by the a-y phase transformation of the iron components. This transformation (and the contraction) is reversible when subsequently lowering the temperature, but with a hysteresis of about 100 K.
SUMMARY Phase and structural investigations - done by Mossbauer spectrometry and electron diffraction - show, that in the first crystallisation process only cl-Fe nanocrystals appear. There remains an amorphous phase still containing more than 50 % of the iron atoms in the sample. We have shown, that the crystallisation process of FefZuNbB alloys is very similar to that of the well known FeCubIbSiB. The main difference is - as there is no Si in these alloys - that no FeSi nanocrystals are formed, but a,-Fe grains. The nanostructure is very similar and so is the crystallisation in two steps. In both cases FeB phases develop during the second crystallisation step.
ACKNOWLEDGEMENTS The authors wish to thank Dr. G. Herzer, Vacuumschmelze GmbH, Hanau, Germany, for the amorphous samples. The work was tinancially supported by the Deutsche Forschungsgemeinschaft (DFG) and the “Internationales B&o des BMBF”.
REFERENCES 1. Y. Yoshizawa, S. Oguma and K. Yamauchi, Journal ofAppl. Physics 64 (lo), 6044, 1988 2. G. Herzer, Physica Scripta x, 307,1993 3. T. Graf, M. Kopcewicz and J. Hesse, X Phys: Cordens. Matter &3897,1996