Keywords: Magnetization; Hard magnetic materials; Crystallization Famorphous systems; Rare-earth alloys Amorphous alloys are conventionally synthesized by rapidly quenching a melt below
Trang 1Influence of B content substituting for Al on the magnetic
Nguyen Chaua,*, Nguyen Hoang Luonga, Cao Xuan Huua, Nguyen Xuan Phucb,
Nguyen Huy Danb
a Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam
b Institute of Materials Science, NCST, Hoang Quoc Viet Road, Hanoi, Viet Nam
Abstract
The Nd60Fe30Al10xBx(x ¼ 0; 2, 4, 6, 8 and 10) alloys were prepared by copper mold casting using arc-melting Investigation shows that with increasing B content magnetization and remanence decrease while coercivity and Curie temperature increase The hard magnetic properties are achieved not only in the amorphous state but also in partly crystallizing state r 2002 Elsevier Science B.V All rights reserved
Keywords: Magnetization; Hard magnetic materials; Crystallization Famorphous systems; Rare-earth alloys
Amorphous alloys are conventionally synthesized by
rapidly quenching a melt below the glass transition
temperature Tg; therefore suppressing crystal nucleation
and/or growth The ribbons prepared by rapid
solidifi-cation are usually restricted with a thickness of about
20–50 mm Recently, it was found that multicomponent
metallic alloys can be cast into bulk amorphous
cylinders from the liquid state at cooling rate as low as
1–1001C/s with diameter of several milimeters The
success of synthesizing these bulk amorphous alloys has
been attributed, as proposed by Inoue et al [1], to the
following three empirical rules for the achievement of
large glass-forming ability: (1) multicomponent alloy
systems consisting of more than three components, (2)
significantly different atomic size ratios above about
12% among the main constituent elements, and (3) large
negative heats of mixing among their main elements
These new alloys are of significant technological
importance, promising new engineering applications of
bulk metallic glasses [2]
Inoue et al [3] have extensively studied the
glass-forming ability of Nd90xFexAl10 They found that the
glass-forming ability for these alloys increases with
increasing iron content x and reaching a maximum for
x ¼ 20; and decreases with further increase of the iron amount In the papers [1–3] the authors succeeded in finding Nd–Fe–Al and Pr–Fe–Al bulk amorphous alloys with hard magnetic properties at room temperature The aim of this paper is to present study of the influence of B content substituting for Al on the glass-forming ability of Nd60Fe30Al10xBxas well as the hard magnetic properties of these alloys
Ternary Nd60Fe30Al10xBx(x ¼ 0; 2, 4, 6, 8 and 10) alloys were prepared by arc-melting in a mixture of Nd,
Fe, Al and B with a purity of 99.9% in an argon atmosphere From the pre-alloyed ingots, the parallele-piped samples (dimensions 3 3 70 mm3) were pre-pared by suction-cast of the molten alloy into copper mold with water cooling The structure of as-cast samples was examined by X-ray diffractometer D5005, Bruker, using Cu-Ka radiation with the wavelength
l ¼ 1:54056 (A The composition of alloy was determined
by energy dispersion spectrometer (EDS) included in scanning electron microscopy 5410LV, Jeol The ther-mal stability associated with crystallization and melting temperature was determined by differential scanning calorimetry (DSC) using apparatus SDT 2960, TA Instruments, with a heating rate of 201C/min Magne-tization and hysteresis loops under an applied magnetic
*Corresponding author Tel./fax: +84-4-858-9496.
E-mail address: chau@cms.edu.vn (N Chau).
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 2 7 5 - 6
Trang 2field up to 13.5 kOe were measured by a vibrating
sample magnetometer DMS 880, Digital Measurement
Systems
Fig 1 shows the X-ray dispersion spectrum for
Nd60Fe30Al10sample measured by EDS This spectrum
indicates the elements existing in our sample As can be
seen in this figure, besides the three constituent elements
Nd, Fe, Al, in the sample there is a small amount of
oxygen which could be related to the purity of starting
materials as well as to some oxidation during sample
preparation The analysis based on the X-ray dispersion
spectrum showed that the alloy has the nominal
composition
X-ray diffraction patterns taken from cast
parallele-piped rods at cast surface indicate that cast surfaces of
all rods prepared by suction-casting into copper molds
with water cooling are largely crystalline, probably due
to crystallization nucleations originating from copper
mold wall To study the variation of crystallinity with
respect to position within the samples, we have taken
X-ray diffraction patterns at depth d ¼ 0:2 mm beneath
the surface of the samples The results are shown in Fig 2 X-ray diffraction scans reveal the broad peak around 2y ¼ 30:51; indicating the change of amorphous– crystalline character with increasing depth in the rods This is in agreement with results presented in Ref [4] for
Nd60Fe30Al10 alloy Also, there could be a critical cooling rate which allows the samples to become bulk amorphous In order to investigate the influence of cooling rate of the melts on their atomic structure, we have remelt the ingots in quartz tubes (which initially have been evacuated with high vacuum and then filled with Ar gas) Then the remelts have been quenched in water, oil and air, corresponding to average estimated cooling rate of 1001C/min, 301C/min and 101C/min, respectively The X-ray diffraction analysis showed that the surface of the samples is partly crystalline but the sample quenched in the oil exhibits a more amorphiza-tion compared to other samples Thus it is very difficult
to establish the optimal and uniform cooling rate for the melts to be in amorphous state in whole bulk samples regardless of the position within the samples
The DSC curves for all as-cast samples show that with increasing B content the sharpness of the crystallization peak decreases In other words, large fraction of volume
of the samples is already crystalline These results on the influence of B content on the amorphous character of the alloys are in good agreement with those obtained by X-ray diffraction analysis (see Fig 2) It is interesting to note that in the sample with x ¼ 0 there exists a glass state before crystallization, and this state is not clearly seen in the DSC curves and disappearing with increasing
B content Thus we observe the decrease of glass-forming ability in Nd60Fe30Al10xBxalloys with increas-ing x:
The hysteresis loops of the Nd60Fe30Al10xBxalloys have been measured at room temperature Representa-tive hysteresis loops for the samples with x ¼ 6; 8 and 10 are shown in Fig 3 With increasing B content, both magnetization at 13.5 kOe and remanence decrease,
Fig 1 X-ray dispersion spectrum for as-cast Nd 60 Fe 30 Al 10
alloy.
Fig 2 X-ray diffraction patterns taken from cast rod at depth
d ¼ 0:2 mm (from cast surface) of Nd Fe Al B alloys.
Fig 3 Hysteresis loops of as-cast samples Nd 60 Fe 30 Al 10x B x
(x ¼ 6; 8, 10) at room temperature.
Trang 3whereas coercivity Hc increases It seems that the
substitution of B for Al leads to the enhancement of
hard magnetic, probably Nd2Fe14B, phase, causing the
increase of Hc:
Magnetization of the Nd60Fe30Al10xBxsamples has
been measured as a function of temperature at 1 kOe,
showing that the Curie temperature increases with
increasing B content Namely, Curie temperatures for
samples with x ¼ 0; 2, 4, 6, 8 and 10 are 477, 480, 480,
484, 486 and 494 K, respectively
Fig 4 shows hysteresis curves for Nd60Fe30Al10
prepared with different cooling rates for the melt As
can be clearly seen, water quenched (with highest cooling rate) sample exhibits the best hard magnetic properties with coercivity Hc¼ 3120 Oe compared to the value of Hc equal to 1980 and 1590 Oe for the samples cooled in oil and in air, respectively We note that the fraction of crystallization in this sample is higher According to Inoue et al [3], the hard magnetic properties at room temperature are obtained only for the bulk amorphous alloy However, as indicated above,
in our case partly bulk amorphous samples always exhibit hard magnetic properties at room temperature The authors are grateful to the Vietnam National Program for Natural Sciences for financial support Research performed within the Vietnamese-US coopera-tion project supported by the US Nacoopera-tional Science Foundation
References
[1] A Inoue, T Zhang, A Takeuchi, IEEE Trans Magn 33 (1997) 3814.
[2] A Inoue, Mater Sci Eng A 226–228 (1997) 357 [3] A Inoue, T Zhang, A Takeuchi, Sci Rep RITU A 44 (1997) 261.
[4] N.H Dan, N.X Phuc, V.H Ky, N.M Hong, N Chau, N.H Luong, C.X Huu, R.W McCallum, M.J Kramer, A.S O’Connor, K.W Dennis, L.H Lewis, L.D Tung, presented at the Materials Research Society Spring Meeting,
SA, USA, April 16–20, 2001, submitted for publication Fig 4 Hysteresis loops of Nd 60 Fe 30 Al 10 prepared with
differ-ent cooling rates from the melt.