Crystallization kinetics of high iron concentration amorphous alloys under high magnetic
Reisho Onodera a ,⇑,Shojiro Kimura a ,Kazuo Watanabe a ,Yoshihiko Yokoyama b ,Akihiro Makino b ,Keiichi Koyama c
High Field Laboratory for Superconducting Materials,Institute for Materials Research,Tohoku University,Sendai 980-8577,Japan
Cooperative Research and Development Center for Advanced Materials,Institute for Materials Research,Tohoku University,Sendai 980-8577,Japan c
Graduate School of Science and Engineering,Kagoshima University,Kagoshima 890-0065,Japan
a r t i c l e i n f o Article history:
Received 3February 2014
Received in revised form 13March 2014Accepted 13March 2014
Available online 22March 2014Keywords:
Differential thermal analysis High magnetic ﬁeld
a b s t r a c t
Crystallization kinetics of a Fe 83.3Si 4.2B 12.5amorphous alloy and a Fe 83.3Si 4B 8P 4Cu 0.7hetero-amorphous alloy under magnetic ﬁelds was investigated by the differential thermal analysis (DTA).In DTA for Fe 83.3Si 4.2B 12.5,the ﬁrst crystallization peak,which indicates the precipitation of bcc-Fe shifts about 10K toward the lower temperature side,whereas the second peak which indicates the precipitation of iron-compounds,shifts about 4K toward the higher temperature side by applying a magnetic ﬁeld of B =20T.On the other hand,in the case of Fe 83.3Si 4B 8P 4Cu 0.7,the ﬁrst peak at B =20T does not change in comparison with that at a zero magnetic ﬁeld,whereas the second peak shifts toward a higher tem-perature side as well as Fe 83.3Si 4.2B 12.5.
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Recently,the crystallization of amorphous alloys in a high mag-netic ﬁeld has been extensively investigated [1–6].The aim of these research is a development of a new process for nano-crystalline alloys using a magnetic ﬁeld,which is prepared by crys-tallization of amorphous alloys .In general,the distribution of nano-crystals in an amorphous matrix provides an enhancement of their functionalities.In the case of magnetic amorphous alloys,it dominates their magnetic properties,such as the saturation magnetization and the coercivity.Especially,the coercivity of nano-crystalline alloys strongly depends on a grain size of their precipitated crystals .Hence,in order to acquire the excellent soft magnetic nano-crystalline alloys,the accurate control of their crystallization reaction such as nucleation and grain growth processes,is required.
There are several reports that dealt with the crystallization kinetics of amorphous alloys in a high magnetic ﬁeld [1,3,4].It was reported that the acceleration of crystallization takes place un-der magnetic ﬁelds,and this magnetic ﬁeld effect was discussed by taking into account the contribution of the magnetic energy to the nucleation [1,3,4].According to their model,under the magnetic
ﬁelds,the activation energy and the critical radius for the nucle-ation are decreased by contribution of Zeeman energy in the case of the crystallization of ferromagnetic crystal from paramagnetic amorphous phase.These effects mean the acceleration of the nucleation by applying magnetic ﬁelds.Thus,it can be expected that an increase of the nucleation rate under the high magnetic ﬁeld.The increase of the nucleation rate leads to the increase of the number of ferromagnetic crystal grains,which gives the improvement of the saturation magnetization.On the other hand,in our recent work,we found a suppression of the growth process on the crystallization in Fe 79Si 12B 9amorphous alloy,although this crystallization reaction is the precipitation of ferromagnetic bcc-Fe from the paramagnetic amorphous phase [9,10].This magnetic ﬁeld effect cannot be explained by the contribution of the Zeeman energy to the nucleation.We concluded that the origin of this mag-netic ﬁeld effect is the suppression of atomic diffusion by applying the magnetic ﬁeld .The suppression of crystal growth is useful for obtaining small size crystal grains,which results in a low coer-civity.Therefore,the application of the magnetic ﬁeld to the crys-tallization process has a possibility to produce nano-crystalline alloys with excellent soft magnetic properties.
In our previous study above-mentioned,we have investigated the crystallization kinetics of the basic iron-based amorphous al-loys [9,10],which do not nano-crystallize.However,in order to ap-ply the magnetic ﬁeld effects to material processing for soft magnetic nano-crystalline alloys,an investigation of the magnetic
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E-mail address:[email protected] (R.Onodera).
ﬁeld effect on the crystallization kinetics of high
compounds,which exhibit the
this study,we have carried out differential
measurements to investigate the
magneticﬁelds of a typical hetero-amorphous
Fe83.3Si4B8P4Cu0.7,which is a precursor
netic nano-crystalline alloy,and Fe83.3Si4.2B12.5
that is a basic composition of Fe83.3Si4B8P4Cu0.7.
Sample ribbons were prepared by single-roll
is3mm in width and27–29l m in thickness for Fe83.3Si4.2B
and22–24l m in thickness for Fe83.3Si4B8P4Cu0.7.DTA
ﬁelds[12,13]were carried out in a temperature range of R.T.
neticﬁelds up to B=20T by using a cryocooled
52mm room temperature bore,that was developed in
Superconducting Materials,Institute for Materials
.Samples were heated by using an electrical furnace,
experimental bore of a superconducting magnet.The
tance wire heater combined with water-cooled jacket.
The sample and the reference material temperatures
sheathed Pt–13%Rh thermocouples.The reference
We conﬁrmed that the inﬂuence of a magneticﬁeld on the
the thermocouple was negligibly small.The temperature
ments is within1K.The sample and the reference
aluminum foil,and were then wrapped around the
of applied magneticﬁelds is parallel to the ribbon surface.
10K/min.The sample space was evacuated toÀ3Pa by a
system during the measurements.Crystal phases of each
by X-ray diffraction of annealed samples.
3.Results and discussion
Fig.1shows a DTA curve of Fe83.3Si4.2B12.5at0T.Two exother-
mic peaks due to the crystallization reactions are observed.The ﬁrst peak is caused by a crystallization of bcc-Fe,and the second peak is due to the crystallization of iron-boron compounds such as Fe2B and Fe3B.Theﬁrst and second crystallization temperatures T x1and T x2,which are determined by the onset of the DTA peaks, are T x1=706K and T x2=795K,and the peak temperatures are T p1=730K and T p2=809K.Fig.2(a and b)show the magneticﬁeld dependence of theﬁrst and second crystallization peaks of suppression of the crystallization of Fe79Si12B9[9,10].On the other hand,the shift toward the lower temperature side of theﬁrst peak in Fe83.3Si4.2B12.5suggests an acceleration of the crystallization un-der high magneticﬁelds,which is consistent with the expectation from the contribution of the gain of the Zeeman energy.It is con-sidered that this difference of the magneticﬁeld dependence of theﬁrst crystallization peak between Fe83.3Si4.2B12.5and Fe79Si12B9 is caused by a difference of the effective magnetic moment be-tween the crystal phases of Fe83.3Si4.2B12.5and Fe79Si12B9.In both cases,bcc-Fe contained Si[bcc-Fe(Si)]precipitates on theﬁrst crys-tallization.However,Si concentration in bcc-Fe(Si)precipitated in Fe79Si12B9is higher than that of Fe83.3Si4.2B12.5,because a larger amount of Si is contained in its alloy composition.Therefore,the effective magnetic moment of bcc-Fe(Si)precipitated in Fe79Si12B9 is likely smaller than that of Fe83.3Si4.2B12.5.In fact,the Curie tem-perature T C,cry.=1026K of the crystal phase in Fe83.3Si4.2B12.5, which is determined by our magnetization measurements(not shown here)is higher than T C,cry.=946K of Fe79Si12B9.The acceler-ation of the crystallization and the suppression of the crystal growth by applying the magneticﬁeld,reported so far,are consid-ered to have different origins.The acceleration occurs in the nucle-ation process due to a gain of the Zeeman energy,while the suppression is caused by the reduction of the atomic diffusion in the crystal growth process.Thus,we speculate that the accel-eration of the nucleation and the suppression of the crystal growth by applying magneticﬁelds compete on the crystallization process of bcc-Fe.When the crystal phase has a large magnetic moment, the contribution of the Zeeman energy becomes dominant.As a result,the acceleration behavior appears in Fe83.3Si4.2B12.5.By contrast,when the magnetic moment of the crystal phase is small, the effect from the Zeeman energy smears out,and then the suppression behavior appears in Fe79Si12B9.Thereby,even though
curve of a Fe83.3Si4.2B12.5amorphous alloy with heating rate10 magneticﬁeld.T x1and T x2are the crystallization temperatures determined onset of the exothermic peak.T p1and T p2are peak temperatures.Magneticﬁeld dependence of(a)ﬁrst and(b)second crystallization
the crystallization of the iron-based
precipitation of the ferromagnetic crystal
amorphous phase in common,two
fects on the crystallization,such as the
ation or the suppression of the crystal
on the effective magnetic moment of the
Fig.2(b)shows that on the second
shifts toward the higher temperature side
ﬁeld as well as Fe79Si12B9.In both Fe83.3Si
iron-boron compounds such as Fe2B and Fe
residual amorphous phase on the second
means the suppression of the crystal
eration of nucleation.It is conﬁrmed by
surements that the gain of Zeeman
crystallization is less than a half of that on
at B=10T.Hence,it is considered that this
accelerate the nucleation.As a result,
tallization,the acceleration behavior
second crystallization.On the other hand,
tion,the composition of the residual
deviate from that of the primary
compositional difference between the
the primary matrix.Especially,a boron
grain boundary of bcc-Fe(Si),should be
concentration of boron in the residual
bility limit of boron in bcc-Fe that is
compounds should be accompanied by
atoms in the residual amorphous phase
Fe(Si)grains to nuclei of iron-boron
consider that the diffusion of boron atoms
phous phase is an interstitial diffusion,
of the atomic radius between Fe and B.
on the interstitial diffusion was reported
.They reported the decrease of the
in a pure iron(c-Fe)under the uniform
pression of the crystal growth from an
is dominated by atomic diffusion,was also
work.Therefore,the reduction of
atoms in the residual amorphous matrix
neticﬁelds,is likely to occur.This
of the suppression of the iron-compounds
second crystallization.As a result,the shift of
the higher temperature side appears,
sion of the second crystallizations.
the characteristic temperatures are shown in
tion temperature T x and the peak
aged values of the results obtained by DTA,
three times at each measurement condition.
magneticﬁeld on the crystallization in Fe83.3
expansion of the coexistence region of
amorphous phase that is deﬁned by D T x=T
expanded to103K,compared with89K at0
Fig.4shows DTA curve of Fe83.3Si4B8P4Cu
mic peaks due to the crystallizations are observed.Theﬁrst peak is
caused by the crystallization of bcc-Fe,and the second peak is due to the crystallization of iron-compounds such as Fe2B and Fe3B.The ﬁrst and second crystallization temperatures are T x1=663K and T x2=807K,and peak temperatures are T p1=680K and T p2=816K,respectively.Fig.5(a)shows theﬁrst crystallization peak at0and20T.Theﬁrst peak shows the very little magnetic ﬁeld dependence.In this composition,bcc-Fe(Si)crystallizes at theﬁrst crystallization temperature as well as Fe83.3Si4.2B12.5.However,the differences of the crystallization and peak tempera-tures between0and20T are negligibly small.This result is differ-ent from Fe83.3Si4.2B12.5.Hetero-amorphous alloys are a precursor material of nano-crystalline alloys[11,17],and its nano-crystalli-zation is believed to be caused by nano size bcc-Fe clusters(less than3nm),which present in an as-quenched texture.On the crystallization process,these nano size bcc-Fe clusters should act as an inhomogeneous nucleation site.It is considered that Magneticﬁeld dependence of(a)T x1and T p1,(b)T x2and T p2,and(c)
DTA curve of a Fe83.3Si4B8P4Cu0.7hetero-amorphous alloy with heating in a zero magneticﬁeld.T x1and T x2are the crystallization temperatures determined by the onset of the exothermic peak.T p1and T p2are peak temperatures.
the magnetic ﬁeld accelerates the homogeneous nucleation through the gain of the Zeeman energy.However,the primary crystallization process of the hetero-amorphous is the inhomoge-neous nucleation,which is completely dominated by the bcc-clusters rather than homogeneous nucleation.Therefore,an acceleration of the crystallization by the ﬁeld is scarcely seen in this material.On the other hand,it is clear from the small grain size (less than 20nm)of the nano-crystallized texture [11,17]that there is very little contribution of the growth process on the crys-tallization.Hence,the suppression behavior by the magnetic ﬁeld on the growth process also does not appear.From these reasons,the ﬁrst crystallization peak does not change in an applied mag-netic ﬁeld.Fig.5(b)shows the second crystallization peak at B =0and 20T.The second crystallization peak shifts toward the higher temperature side similar to the cases of Fe 83.3Si 4.2B 12.5.It is considered that this shift occurs from the same origin with Fe 83.3Si 4.2B 12.5.As a result,the coexistence region of bcc-Fe(Si)and the residual amorphous phase D T x ,slightly expanded under a high magnetic ﬁeld from 144K at 0T to 147K at 20T.
It turned out that the magnetic ﬁeld effect on the crystallization reaction in both compositions causes an expansion of the coexis-tence region of the bcc-Fe(Si)primary crystal and the residual amorphous phases.The stabilization of the coexistence region leads to the advantage for the nano-crystallization to obtain an ideal nano-crystalline alloy with a large amount of nano-crystal and intergranular amorphous thin layer,which realizes excellent soft magnetic properties.Therefore,we suggest that the magnetic ﬁeld can play an important role in materials processing,in order to achieve the further improvement in soft magnetic properties of nano-crystalline alloys.4.Conclusion
The crystallization kinetics of a Fe 83.3Si 4.2B 12.5amorphous alloy and a Fe 83.3Si 4B 8P 4Cu 0.7hetero-amorphous alloy in high magnetic ﬁelds has been investigated by the differential thermal analysis in ﬁelds up to 20T.In Fe 83.3Si 4.2B 12.5,the ﬁrst crystallization peak shifts toward the lower temperature side,and the second crystalli-zation peak shifts toward the higher temperature side under the magnetic ﬁeld.On the other hand,the ﬁrst crystallization peak of Fe 83.3Si 4B 8P 4Cu 0.7does not change,and the second crystallization peak shifts toward the higher temperature side.The shift toward the lower temperature side indicates that the crystallization is accelerated by applying a high magnetic ﬁeld,and the shift toward the higher temperature side indicates the suppression of the crystallization.
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DTA curves of (a)ﬁrst and (b)second crystallization peaks of a hetero-amorphous alloy at B =0and 20T.
and Compounds 604(2014)8–1111