Materials Science Forum Vols. 561-565 (2007) pp 1361-1366
online at http://www.scientific.net
© (2007) Trans Tech Publications, Switzerland
Online available since 2007/Oct/02
Fe-Metalloid Metallic Glasses with High Magnetic Flux Density
and High Glass-Forming Ability
Akihiro Makino1,a, Takeshi Kubota1,b, Masahiro Makabe2,c, ChunTao Chang1,d
and Akihisa Inoue3,e
1
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2
Makabe R&D co., LTD, Sendai 983-0036, Japan
3
Tohoku University, Sendai 980-8577, Japan
a
[email protected], [email protected], [email protected],
d
[email protected], [email protected]
Keywords: Fe-based metallic glass, glass-forming ability, high magnetic flux density
Abstract. Fe-based bulk metallic glasses with good soft magnetic softness, high strength and
relatively low material cost should have greatest potential for wide variety of applications among
many kinds of bulk metallic glasses (BMGs). However, the glass-forming metal elements such as Al,
Ga, Nb, Mo and so forth in the Fe-based BMGs significantly decrease saturation magnetization (Js)
which is a essential property as soft magnetic materials. Since the coexistence of high Js and high
glass-forming ability (GFA) has been earnestly desired from academia to industry, however, has been
left unrealized over many years. Here, we present a Fe76Si9B10P5 bulk glassy alloy exhibiting with
unusual combination of high Js of 1.51 T comparable to the Fe-Si-B amorphous alloy ribbons with
thickness of about 25 µm in now practical use, because of not-containing the glass-forming metal
elements, and high GFA leading to a rod with a diameter of 2.5 mm. This alloy composed of familiar
and low-priced elements also has extremely low coercivity which should enable ultra-high efficient
transformers, therefore, has a great advantage for engineering and industry, and thus significantly
improves energy saving, conservation of earth resources and environment.
Introduction
Recently, Fe-metalloid (B, C, Si, P)-based BMGs containing glass-forming metal elements such
as Al, Ga, Nb, Mo and Y have been developed [1-9]. These alloys exhibit glass transition and
supercooled liquid regions before crystallization and have relatively high GFA leading to the
formation of BMG rods with diameters of mm-order prepared by Cu-mold casting with much lower
cooling rates than that of melt-spinning used for the production of amorphous alloy thin ribbons
without glass transition. The outer shapes of BMGs are extremely larger than the melt-spun
amorphous ribbons with a thickness of less than about 100 µm. Melt-spun (rapidly solidified) Fe-Si-B
amorphous alloy ribbons, developed three to four decade ago, are now one of major soft magnetic
materials, because of their superior magnetic properties such as the relatively high Js of 1.5-1.7 T,
good magnetic softness due to lack of intrinsic magnetocrystalline anisotropy, and also relatively low
material cost and excellent productivity [10, 11]. The amount of production of the alloys has been
significantly increasing by the reason of improving energy saving for conservation of environment,
today’s urgent subject, due to the low magnetic core losses attributable to the good magnetic softness.
However, the preparation of the ordinary magnetic amorphous alloys such as Fe-B [12], Fe-Si-B [10,
13], Fe-P-C [14, 15], Fe-P-B-C [16], Fe-Zr-B [17, 18] etc, requires higher cooling rates than 104 K/s
due to the low GFA, thus restricting the material outer shape [19]. For example, the thickness of the
typical Fe-Si-B amorphous alloy ribbon produced by a melt-spinning method, in commercial
production for many kinds of high efficient transformer, is about 25 µm [20, 21], which is much
thinner than the other ordinary crystalline soft magnetic alloy sheets. Fe-3mass%Si crystalline alloy
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(silicon steel) sheet [22] produced by cold rolling, occupying the largest market share in the soft
magnetic alloy field, has some advantages of no-limitation for the thickness and high Js of about 2 T
and very low material cost. However, the silicon steel for general purpose has a great disadvantage of
inferior magnetic softness fundamentally caused by a large magneto-crystalline anisotropy.
Comparing the Fe-based amorphous alloy ribbon and the silicon steel sheet, the realization of large
thickness of the ribbon without decrease in Js and increase in the material cost should be the first
priority for the amorphous alloy.
As mentioned above, BMGs offer the relaxing the restricted material outer shape [1-4] of rapidly
quenched amorphous alloys. However, the glass-forming metal elements in BMGs result in a
remarkable decrease in Js to less than about 1.3 T, which is much lower than 1.5-1.7 T of the
representative Fe-Si-B amorphous alloys widely utilized by industries. The magnetic elements such as
Co and Ni also have some beneficial effects on the GFA [23, 24] in most cases, however, the addition
of these elements to Fe-based BMGs significantly decreases Js in the same manner as the
glass-forming elements; the substitution of Co for Fe at 50 % in (Fe0.75Si0.10B0.15)96Nb4 significantly
increases the critical rod-size of 1.5 mm to 5 mm in diameter, which is the largest size among soft
magnetic BMGs without containning rare-earth elements, produced by Cu-mold casting. However,
rapidly decreases Js to 1.1 T [23]. Thus, the soft magnetic BMGs have a great advantage of relaxing
the restricted material outer shape due to their high GFA, however, have a great disadvantage of
remarkable decrease in Js. In addition, the elements such as Ga, Nb, Mo, Y, Co and Ni enhancing
GFA for the Fe-based BMGs are rare and expensive metals, and Al, Nb, Mo and Y also decrease the
productivity due to the easy oxidation. It should be, thus, better to avoid the use of these metal
elements for high Js as well as conservation of environment and low material cost. Therefore, the
development of Fe-based BMGs without any metal element other than Fe and with relatively high Fe
content have been desired for the last one decade, and a great deal of effort has been devoted,
however, has been left unsolved matter over many years.
This work reports a novel Fe-rich Fe-based BMG with high Js of 1.51 T comparable to the
ordinary Fe-Si-B amorphous alloy in now practical use as well as high GFA leading to a rod-shaped
specimen of 2.5 mm in diameter by Cu-mold casting in air. That BMG is composed of a large amount
of Fe of 76 % and small amounts of metalloid elements of Si, B and P. The elements, Si and P, are the
constituent elements in pig-iron produced by the reduction of iron ores typically in a blast furnace.
The Fe-Si and Fe-P ferroalloys are in mass production, thus, there is no restriction in available raw
materials.
Experimental
Fe-Si-B-P ingots were prepared by induction melting the mixture of pure metals of Fe (99.98
mass%), premelted Fe-P (99.9 %), and pure metalloid of crystal B (99.5 %) and Si (99.999 %) in an
argon atmosphere. The alloy compositions represent nominal atomic percent. Amorphous and/or
glassy alloys were produced in a ribbon form by the single roller melt-spinning method and in rod
forms by the Cu-mold casting method in air. The as-melt-spun and as-cast structures were examined
by X-ray diffraction (XRD) with Cu-kα line. The glass transition temperature (Tg) and crystallization
temperature (Tx) were estimated by differential scanning calorimetry at a heating rate of 0.67 K/s and
liquidus temperature (Tl) was also estimated by thermogravimetry differential thermal analyzer at a
cooling rate of 0.17 K/s. The density (ρ) was measured by the archemedian method using a
pycnometer with helium gas at 298 K. Cross section of a bulk rod with 2.5 mm in diameter was etched
for 30 s at room temperature in a solution of 1% hydrofluoric acid and 99% distilled water, and was
observed by optical microscopy. The Js was measured under an applied field of 800 kA/m with a
vibrating sample magnetometer. The coercivity (Hc) was measured under a field of 1 kA/m with a DC
B-H loop tracer. Theρ, Js and Hc were measured at room temperature.
Materials Science Forum Vols. 561-565
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Results and Discussion
Figure 1 shows the compositional dependence of as-quenched structure and supercooled liquid
region (∆Tx), known to be one of the indicators for GFA [25], for melt-spun (Fe-Si9)-B-P alloys. The
number in the figure represents the temperature interval (∆Tx) between Tg and Tx. In Fig.1, ”glassy”
and “amorphous” alloys are distinguished by the presence of glass transition. An amorphous phase
was observed in a wide compositional range, and the crystalline phase was formed at lower B content
than 9-10 % for ternary Fe-Si9-B, lower P content than about 13 % for ternary Fe-Si9-P, and lower (B
+ P) contents than the interpolated values for quasi-ternary (Fe-Si9)-B-P alloy system. Obvious glass
transition was observed at limited quaternary compositions in the amorphous- forming range and the
large ∆Tx of over 40 K is observed in the confined range of 84-86 %(Fe+Si9) and 14-16 %(B+P),
indicated with gray color. Fig.2 shows the Tg, Tx, Tl, reduced transition temperature (Tg/Tl) and ∆Tx
for rapidly solidified Fe76Si9B15-XPX alloy ribbons as a function of P content (X). The Tg decreases
with increasing X, and reaches minimum temperature of 788 K at X = 5. Then, that temperature
increases, slightly. Otherwise, the Tx monotonically and gradually decreases with increasing X. The Tl
shows same behavior as the Tg, reaches a minimum temperature at X = 7.5. The Tg/Tl and ∆Tx
calculated based on results of Tg, Tx and Tl, increasing with X, and reach maximum value of 0.62 at X
= 7.5 for Tg/Tl and 52 K at X = 5 for ∆Tx, respectively. From these results, it is considered that the
higher GFA is obtained in the range of 5-7.5 %P.
Fig.1 The compositional dependence of asquenched structure and ∆Tx for melt-spun
(Fe-Si9)-B-P quasi- ternary alloy system.
Fig.2 The changes of Tg, Tx, Tl, Tg/Tl and ∆Tx
for rapidly solidified Fe76Si9B15-XPX alloy
ribbons as a function of P content.
Figure 3 shows (a) the optical micrograph and (b) the XRD pattern taken from the cross section
of the as-cast Fe76Si9B10P5 rod with diameter of 2.5 mm. The optical micrograph shows smooth and
homogeneous polished surface without spots of corrosion by existing crystalline phases. Both XRD
pattern reveal only typical halos, and no peaks corresponding to crystalline phases are visible. It is
thus to be noticed that a single amorphous phase is formed in the diameter range up to 2.5 mm.
Figure 4 shows the Hc of Fe76Si9B10P5 bulk glassy alloy with ∆Tx of 52 K and Fe76Si9B15 alloy,
representative magnetic amorphous material, without Tg as a function of annealing temperature. The
Hc of the ternary amorphous alloy reaches a minimum value of 8.0 A/m at 653 K and rapidly increases
with increasing annealing temperature above 653 K, even though an amorphous structure should
remain in the temperature range, which is considerably lower than the Tx of 841 K. On the other hands,
the Hc of the bulk glassy alloy reaches a minimum value of 0.8 A/m at 748 K, 10 times smaller than
the minimum Hc of 8.0 A/m for the amorphous alloy, and then slightly to 1.4 A/m at 773 K just below
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Figure 3 (a) the optical micrograph and (b)
the XRD pattern taken from the cross section
of the as-cast Fe76Si9B10P5 rod with a diameter
of 2.5 mm. XRD pattern of rapidly solidified
ribbon is also shown for comparison.
Fig.4 The Hc of Fe76Si9B10P5 bulk glassy
alloy and Fe76Si9B15 amorphous alloy as a
function of annealing temperature.
the Tg of 780 K. The ρ in as-quenched state was 7094 kg/m3 for the Fe76Si9B15 amorphous alloy and
7054 kg/m3 for the bulk glassy alloy. The ρ of the bulk glassy alloy significantly increases from 7054
kg/m3 to 7207 kg/m3 by 2.1 % upon the optimum annealing at 748 K. This change is considered to
originate from the glass transition from the as-quenched amorphous phase with some inhomogeneity
to a glassy phase with a higher degree of atomic density of dense random packed structure. On the
contrary, the ρ of the amorphous alloy little changes with annealing because of lack of glass transition.
Considering the minimum values of Hc and the annealed structure for the alloys, as described above,
the extremely low Hc of 0.8 A/m for the bulk glassy alloy can be explained as due to the dense packed
glassy structure. Taking account of the difference in the optimum annealing temperature for Hc, 748 K
for the bulk glassy alloy and 653 K for the amorphous alloy, we can see that the glassy alloy exhibits
the higher thermal stability of magnetic softness, which should indicate the higher stability of the
composition
Tg (K)
Tx (K)
∆Tx (K)
Tg/Tl
γ
J s (T)
Hc (A/m)
Dcr (mm)
Fe76Si9B10P5
(Fe0.75Si0.1B0.15)96Nb4
Fe74Al4Ga2P12B4Si4
Fe73Al5Ga2P11C5B4
Fe72Al5Ga2P10C6B4Si1
Fe76Mo2Ga2P10C4B4Si2
Fe30Co30Ni15Si8B17
Fe74Nb6Y3B17
780
835
733
732
732
736
780
831
832
885
782
785
785
788
834
789
52
50
49
53
53
52
54
48
0.62
0.61
0.59
-
0.41
0.40
0.40
-
1.51
1.40
1.14
1.29
1.14
1.32
0.92
0.81
0.8
2.9
6.4
6.3
2.8
2.9
3.4
15
2.5
1.5
1
2
2
1.2
2
Table1 The glass transition temperature (Tg), the crystallization temperature (Tx), the critical
diameter (Dcr), the parameters for GFA (∆Tx, Tg/Tl and γ), and magnetic properties (Js, Hc) for the
bulk glassy alloy and the typical Fe-based ferromagnetic BMGs previously reported.
Materials Science Forum Vols. 561-565
1365
glassy phase than the amorphous phase [26, 27].
The remarkable increase in GFA by the addition of P, a familiar element, to well known Fe-Si-B
amorphous alloy should be very interesting. Table1 summarizes Tg, Tx, the critical diameter (Dcr) and
the parameters for GFA, and magnetic properties (Js, Hc) for the bulk glassy alloy and the typical
Fe-based ferromagnetic BMGs previously reported [5, 6, 9, 28-31]. Here, ∆Tx, Tg/Tl [32] and γ [33] (=
Tx/(Tg + Tl)) are adopted as the parameters, and the larger all the parameters, the higher is the GFA.
The Fe76Si9B10P5 bulk glassy alloy has the relatively high ∆Tx of 52 K, comparable to the other BMGs,
and higher Tg/Tl of 0.62 and γ of 0.41 than those of the other Fe-based BMGs, which are probably one
of the reasons for the high GFA of the bulk glassy alloy leading to the 2.5mm-diameter spacemen.
In addition, thick melt-spun amorphous continuous ribbon was successfully prepared by
substituting P for B at 1% in the typical Fe78Si9B13 amorphous alloy (2605SA-II) now in mass
production. The critical thickness of 100 µm with width of 200 mm for a single amorphous phase was
obtained for Fe78Si9B12P1 alloy, near the alloy composition with glass transition (see Fig.1), whose
thickness is 2 times larger than 50 µm of the alloy without P. The Js of 1.54 T for the P-added
amorphous alloy is almost the same as 1.56 T of the ternary amorphous alloy. The addition of P also
should give a solution to the today’s urgent subject of the Fe-based amorphous alloy in mass
production.
Summary
The bulk glassy alloy exhibits the highest Js of 1.51 T among the previously reported Fe-based
BMGs due to the high Fe content of 76 % and without any metal element except Fe as well as high
GFA leading to the 2.5mm-diameter spacemen. Therefore, we can say that this is the first
achievement for simultaneous realization of high Js and high GFA in Fe-based BMGs. In addition, the
bulk glassy alloy also exhibits the very excellent magnetic softness, the lowest Hc, which should
enable ultra-high efficient transformers. Therefore, the alloy offers great advantages of much lower
material cost due to the use of “ubiquitous” elements, Fe, Si and P leading to conservation of earth
resources, unusual combination of high Js and high GFA, very excellent magnetic softness and the
good productivity for engineering and industry, and also significantly improves energy saving and
conservation of environment.
Acknowledgements
This work ahs been supported by a Grant-in-Aid for Scientific Research on Priority Areas,
"Materials Science of Bulk Metallic Glasses", from the Ministry of Education, Science, Sports and
Culture of Japan.
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