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A SAXS observation of the crystallization of an Al85Y10Ni5 amorphous alloy in the supercooled liquid state
Journal of Non-Crystalline Solids 150 (1992) 172-178 North-Holland
:OURNA L or
~ / ~ I ~
I~IIIl~
A SAXS observation of the crystallization of an A185Y10Ni5 amorphous alloy in the supercooled liquid state T o m o a k i K a m i y a m a , S a t o r u M a t s u o i a n d K e n j i Suzuki Institute for Materials Research, Tohoku University, Sendai 980, Japan
The medium-range structure of a heat-treated AI85Y10Ni5 amorphous alloy was investigated by small-angle X-ray scattering, calorimetric analysis, wide-angle X-ray diffraction and transmission electron microscope measurements. It is observed that crystalline particles having a definite boundary are precipitated by annealing at supercooled liquid temperatures and that most of the precipitates consist of the AI fcc crystalline phase. The integrated intensities of the small-angle scattering and the Guinier radii of the precipitates increase on annealing for time duration < 5.4 ks. However, as the annealing time becomes > ~ 5.4 ks, the integrated intensities and the Guinier radii are invariant, i.e., growth of the precipitates ceases. After annealing for 518.4 ks, the diameters of the precipitates exhibit a narrow distribution centered around 200 .A.
1. Introduction A melt-spun AI85Y10Ni 5 amorphous alloy has a high strength, good ductility and a wide supercooled liquid temperature range [1]. However, the Young's modulus and the tensile strength of A185Y10Ni 5 amorphous alloys decrease steeply after heat treatment around the glass transition temperature [2]. Small-angle X-ray scattering (SAXS) is a powerful method for investigating inhomogeneities in the range from an atomic scale to several hundreds of ,~ngstr6m units [3]. The medium-range structural changes for P d - A u - S i amorphous alloys near the glass transition temperature were investigated by Chou et al. [4] using the SAXS technique. This work reports the medium-range structural changes for an AI85Y10Ni 5 amorphous alloy caused by heat treatment in and around the supercooled liquid region, observed using SAXS, 1 Present address: Mitsubishi Materials Corporation, 1-297, Kitabukuro-cho, Omiya 33, Japan. Correspondence to: Dr T. Kamiyama, Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980, Japan. Tel: +81-22 215 2077. Telefax: +81-22 215 2076.
differential scanning calorimetry (DSC), X-ray diffraction ( X R D ) and transmission electron microscopy (TEM).
2. Experimental The initial AissY10Ni 5 alloy was prepared by melting a mixture of metallic A1 (99.9 wt%), Y (99.9 wt%) and Ni (99.9 wt%) in an argon gas atmosphere using an arc furnace. Amorphous alloy ribbons of about 30 ~ m thickness and 3 m m width were prepared from the initial alloy by melt-quenching using a rotating copper wheel in an argon gas atmosphere. The chemical composition of the as-prepared amorphous alloy ribbon was determined to be 85.3 + 0.9 at.% AI, 9.73 + 0.10 at.% Y and 5.83 ___0.06 at.% Ni by the inductively coupled p l a s m a - a t o m i c emission spectrometry ( I C P - A E S ) . The amorphous alloy ribbon was checked by X-ray diffraction to ensure that there was no crystallinity present. A silicon oil bath was used for annealing the amorphous alloy ribbon specimens. To provide rapid heating, specimens were plunged in the bath of silicon oil (Shin-Etsu Chem. Co., Ltd., KF
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
T. Kamiyama et al. / Crystallization of AIssYloNi 5 amorphous alloy
965-100 cs) for a specified time duration and then quenched into ice-water. Finally the specimens were ultrasonically washed in acetone and used for the measurements. Calorimetric analyses of the as-prepared and annealed specimens were made using a Seikoh DSC-200 calorimeter. Both the specimens of about 10 mg and ot-Al203 powder as a reference were heated in an argon gas atmosphere to avoid oxidation. Heating rates were 5-40 K / m i n . SAXS measurements were performed at room temperature using an apparatus with a 0.5 mm × 0.5 mm point focus Cu Ke~ incident beam [5]. The count rate, n, was corrected only for the background scattering without a specimen. The scattered intensity per channel was determined in absolute units (cm2/unit volume) from the equation
I ( h ) - tlo AOe_Ut,
(1)
where t is the sample thickness, AI2 is the solid angle of the detector at the specimen and e -~" is the transparency of the specimen [6]. The primary beam intensity, I 0, was determined by extrapolating the beam intensities attenuated by multiple foils [7]. The amplitude of the scattering vector is given by h = 4w sin 0/A, where A is the radiation wavelength and 20 is the scattering angle. To check the structure of the as-prepared and annealed alloys, wide-angle X-ray diffraction measurements were carried out using a conventional 0 - 2 0 diffractometer. The precipitates in the annealed alloys were also observed with a transmission electron microscope.
3. Results
3.1. Thermal analysis DSC measurements were performed for the as-quenched and isothermally annealed AI85YI0Ni 5 amorphous alloys. The glass-transition temperature, Tg, and the crystallization temperature, Tx, of the as-quenched alloy are Tg = 498 K and Tx = 525 K. DSC curves of Al8sYl0Ni 5 amorphous alloys annealed at different temperatures show
T~
AIssY, oNi5 40K / min
Ta =513K
400
n
173
500
600
Temperature / K
Fig. 1. Calorimetric thermograms of A185Yt0Ni5 amorphous alloy as-quenched and heat-treated at 513 K for different lengths of time. The heating rate is 40 K/min.
that 1 h annealing at 500 K, near Tg, results only in an enthalpy relaxation, whereas 1 h annealing at 528 K results in the disappearance of the first stage of the exothermic peak in addition to enthalpy relaxation. Differential scanning calorimetry curves are shown in fig. 1 for A185Y10Ni5 amorphous alloys as-quenched and isothermally annealed at 513 K for various time periods. The scan speed is 40 K / m i n in all measurements. Peaks due to enthalpy relaxation, the glass-transition and exothermic crystallization are observed in the DSC curve for the as-quenched specimen. The first stage of the crystallization peak becomes weak with increasing annealing time and is not observed for annealing times longer than 5.4 region.
3.2. Small-angle X-ray scattering In fig. 2, the scattered intensities for the specimens as-quenched and annealed for 1 h at different temperatures, ranging from 473 to 528 K, are
T. Kamiyama et aL / Crystallization of AlssYloNi 5 amorphous alloy
174 1 0s
X \
I07
'
'
~h
'Allsfl°l~i'5
4 -
' '
T~ = 525 K
106
1 04 a ^ A.
\
"
"~
x :473 K
%a\ -aa~
0•
....... 4 "NX,h-
A
O : as-quenched
\ "~
X
annealed for 3.6 ksec Tg = 498 K
•
a
"~.'~10 s
:517K :528K
.....
Z
_
i AIssYl°Ni5 annealed at 513 K
tt a
~aaAaX -%a'X.
o •
x XXXXXxx~xx-a~'
xxx~a~ ^x'%
x a
: as-quenched : 0.12 ksec : 1.8 ksec : 5.4 ksec
0 oo:iio
•
~
z
10
b,~
•
o
•
0
x c~
x
•
8"~_~ 103 0
x
1 01
e
2
,
3
,
4
, 5
0.01
,0 J , l~l,j . e'~• 6 789 -
0000000
/~a/~
0.1
10:0.01
2
3
4
5 6 7 890.1
2
h ( k l) h ( k "l) Fig. 2. Scattered intensities for AI8sY10Ni 5 amorphous alloys as-quenched and annealed for I h at different temperatures.
shown in relative units, corrected for sample thickness and primary beam intensity. Scattered intensities for the alloys as-quenched and annealed at 513 K for different lengths of time are shown in fig. 3. It is clearly observed that annealing at 513 K causes variations in both the amplitude and shape of the scattered intensities.
Fig. 3. Scattered intensities for Al8sY10Nis amorphous alloys as-quenched and annealed at 513 K for different lengths of time.
the glass-transition temperature clearly changes the SAXS intensities both in shape and amplitude. The integrated SAXS intensities are plotted in fig. 4 versus the annealing temperatures. The integrated intensity increases abruptly on passing through the glass-transition temperature and the crystallization temperature. It was shown by In-
4. Discussion
4.1. Annealing at various temperatures
As shown in fig. 2, no differences in the scattered intensities can be detected between the as-prepared alloy and those annealed below the glass-transition temperature, both in shape and amplitude. In the DSC measurements for the alloys annealed below the glass-transition temperature, only thermal relaxation can be detected. Annealing below the glass-transition temperature may cause short-range structural relaxation but it does not cause a change in the medium-range structure, while annealing above
10 "~
i
i
u~
10":
A185Y1oNi5 one hour annealing
l[
o
10-: Tg 10460
480
5()0
Tx 5 2' 0
5 4' 0
Temperature ( K ) Fig. 4. Integrated scattered intensities for AlssYloNi 5 amorphous alloys annealed for 1 h at different temperatures.
T. Kamiyama et al. / Crystallization of Als5YioNi 5 amorphous alloy
175
oue et al. [2] that the fracture strength of AI85Yt0Ni 5 amorphous alloy decreases gradually from 920 to 700 MPa in the amorphous solid region below 453 K, whereas it rapidly decreases to 240 MPa in the supercooled liquid region at 508 K. The integrated SAXS intensities increase at the temperatures where the decrease of fracture strength occurs.
4.2. Isothermal annealing in the supercooled liquid region As shown in fig. 3, the scattered intensity for as-prepared sample is weak compared with that for the annealed samples, while a peak near h = 0.04 , ~ - t and a Porod decay [8] (i.e., I(h) h n) are observed even in the scattered intensity for the specimen annealed for 120 s. The Porod law for the scattered intensity can be written as [9,10] 2"rr
1(h) = - ~ ( p , - p o ) 2 S
(2)
+ Ib,
where p~ and Po are the electronic densities of the precipitated particles and the surrounding
0.5
i
i
,
~[
Ala~YI0Ni5
"[
i as-quenched
S
_/ I
0.4
C
,-s 0.3
,.j' "~ 0.2 e-
0.1
o o
o
o
o o
oo o° P
0.0
o
L
100
200
300
4 0
500
h4 ( k 4) xl0~ Fig. 5. Porod plots (i.e., hal(h) vs. h a) for AIssY10Ni 5 amorphous alloys as-quenched and annealed at 513 K for 120 s.
Fig. 6. Transmission electron micrograph of an AlssYl0Ni 5 amorphous alloy as-quenched (a), annealed at 513 K for 14.4 ks (b) and for 518.4 ks (c).
T. Kamiyama et a L / Crystallization o f A l ss Ym Ni 5 amorphous alloy
176
media respectively, and S and I b are constants. As shown in fig. 5, the scattered intensity for the specimen annealed for 120 s is well approximated by eq. (2), whereas that for the as-quenched specimen does not follow the Porod law. This difference shows that the scattering is caused by precipitated particles with a well-defined boundary in an amorphous medium, although Bragg peaks characteristic of crystalline phases cannot be detected by wide-angle X-ray diffraction for the samples annealed for either 120 or 300 s. On heat treatment at 513 K for longer than 120 s, the scattered intensities increase and the peaks or shoulders shift towards smaller scattering vectors. The scattered intensities still obey the Porod law. The SAXS result for the alloy annealed for 5.4 s shows the presence of precipitated particles of a crystalline phase as confirmed by the DSC result (fig. 1) in which the first stage of the crystallization peak has disappeared. To confirm the SAXS results, TEM observations were performed for AI85YmNi5 amorphous alloys as-quenched and annealed at 513 K for various time periods. The micrographs for the as-quenched specimen and for those annealed for 14.4 ks and for 518.4 ks are shown in fig. 6. No precipitates are observed in the as-quenched
100
~
~
(a)
1°+I
Guinier Plot of AlssYt0Ni5
105
1%
J
015
t5
1.0
I
1.
2.0
2.5
wave number (/~-2) xlO-3 100
(b)
. . . . . . . .
I
,"'"
,.
. . . . . . . .
Ostwald'"
%
i
•
. . . . . . . .
•
i
•
•
. . . .
•
.,J"
I= o
AlssY10Ni5 annealed at 513 K ,.¢
I~.I
~
annealed at 513 K for 1.8 ksec
....
....
t
I
i
i
i
,,,
%
......
~'6o
~
,
,
Annealing Time (ksec) Fig. 8. (a) Guinier plot of an AlssYxoNi 5 amorphous alloy annealed at 513 K for 1.8 ks and (b) the variation in the radius of gyration as a function of annealing time.
80
60
AlssYt0Ni5 annealed at 513 K
40
specimen. However, in the micrographs for the specimens annealed for 14.4 ks and 518.4 ks there are precipitated particles having nearly spherical shapes with definite boundaries and dimensions in the range of several tens to 200 A, in good agreement with the result from Guinier plots of the SAXS intensities. In the X-ray diffraction patterns of AI85Y10Ni5 amorphous alloys as-quenched and annealed at 513 K, only an amorphous halo is observed for the as-quenched specimen and those annealed for < 300 s, while some diffraction peaks are o
N
20
O0
110
I
20
310
410
I
50
60
Annealing Time (ksec) Fig. 7. Integrated scattered intensities for AlssY10Ni 5 amorphous alloys annealed at 513 K for different lengths of time.
T. Kamiyama et al. / Crystallization of AlssY/oNi5 amorphous alloy
detected for the sample annealed for 3.6 ks. These peaks increase in height and become sharp as the annealing time increases. Some of the peaks coincide in position with the AI (fcc) Bragg peaks observed in the same alloy at equilibrium. Except for the AI (fcc) peaks, only a few peaks coincide with those of equilibrium phases and the other peaks could not be identified. In the specimen annealed in the supercooled liquid region, most of the precipitates are of the AI (fcc) crystalline phase while those remaining consist of the AI3Ni crystalline phase and Some other non-equilibrium crystalline phase.
4.3. Particle growth The growth of the precipitates is examined using the integrated intensity, defined by eq. (3), as a function of their volume fraction, Vf [8]:
~-~2f° h2l(h) dh = leVf(1 - Vr)(p 1 - p 0 ) 2, (3) where the scattered intensity, l(h), is in absolute units (cmZ/unit volume) and I e = 7.94 × 10 -26
177
cm 2. The integrated intensities for the specimens annealed at 513 K can be calculated from the fact that the scattered intensity follows the Porod law in the high h region and the results are plotted versus annealing time in fig. 7. The integrated intensities increase rapidly with short annealing times and reach a value of 0.46 × 1021 cm -4 after annealing for 5.4 ks. The integrated intensity increases little for annealing times longer than 5.4 ks. Assuming that the precipitates consist of AI (fcc) and that the composition of the medium is not changed during the precipitation process, the volume fraction, Vf, of the precipitates is estimated to be about 10% using eq. (3). It is concluded from the integrated intensity that (i) the precipitates grow rapidly at the beginning, and (ii) when the precipitates occupy a somewhat small volume fraction of about 10%, the growth of the precipitates stops or the precipitates grow by Ostwald ripening. The growth of the precipitates is investigated by estimating the radius of gyration of the precipitated particles. The radius of gyration is estimated by a weighted least squares method and its uncertainty is due only to statistical errors in the
1 05
(a) A185Ylo Ni5 annealed at 513 K for 518.4 ksec
12x10 "3
I
I
I
I
I
"~I 0 '~
Ib) Is
10
8
~1 03 Z
.!
~
6
4
J 1 02 0.1
0.01
h (/~-1)
2
0
I
100
150
200
250
300
r/A
Fig. 9. (a) Scattered intensity for an AI85YloNi 5 amorphous alloy annealed at 513 K for 518.4 ks and (b) the particle diameter-distribution function, p(r), calculated using the scattered intensity and eq. (4).
T. Kamiyama et al. / Crystallization of AlssYwNi 5 amorphous alloy
178
count rates. In Guinier plots of AIssY10Ni 5 amorphous alloy annealed at 513 K, the linear regions are extended on annealing for 1.8 ks as shown in fig. 8(a). Such a linearity is indicative of a fairly narrow size distribution for the precipitated particles. The radii of gyration for specimens annealed for various time periods are illustrated in fig. 8(b). The radii of gyration increase, but their growth rate becomes low with increasing annealing time. For Ostwald ripening, the average radius of the precipitates increases with annealing time, t, proportional to t 1/3, if a diffusion limited process is assumed [11,12]. However, as shown in fig. 8(b), the radius of gyration increases at a rate less than t 1/3 and, when it reaches about 80 .&, the precipitates no longer grow by further annealing.
4.4. Precipitates formed by annealing at 513 K for six days Finally, we examine the behavior of the precipitates annealed for a much longer time than that within which the growth of the precipitates ceases. The scattered intensity for specimens annealed for 518.4 ks is shown in fig. 9(a). An oscillation about the h - a line is observed in the scattered intensity that is characteristic of scattering from a system of monodispersed particles. For the assemblies of spherical particles observed by TEM in this work, the particle-diameter distribution function, p(r), is calculated from eq. (4) [13]: 1
p(r)
--~-~r2 fo dh{h41(h) - C}o~(hr),
where C = lim hal(h) h~
and ( 8 )
a( hr ) = cos hr 1 - ~
4sinhr
hr
The result is illustrated in fig. 9(b).
(4)
5. Conclusions
(1) On annealing for > 120 s in the supercooled liquid region (513 K), a phase separation occurs in AI85Y10Ni5 amorphous alloy with the precipitation of particles with a well-defined boundary in the amorphous medium. (2) The precipitates in AlssYl0Ni s amorphous alloy annealed at 513 K consist mostly of the AI (fcc) crystalline phase and the other precipitates are the AI3Ni crystalline phase and some other non-equilibrium crystalline phase. (3) The growth rate of the precipitates decreases on annealing at 513 K for > 3.6 ks, and the precipitates do not grow when annealing time exceeds 10 ks. On annealing for 518.4 ks, the diameters of the precipitates exhibit a narrow distribution centered at ~ 200 ~,.
References [ll A. Inoue, K. Ohtera and T. Masumoto, Sci. Rep. RITU A-35 (1990) 115. [2] A. Inoue, K. Ohtera, A.P. Tsai, H. Kimura and T. Masumoto, Jpn. J. Appl. Phys. 27 (1988) L1579. [3] A. Guinier and G. Fournet, Small-angle Scattering of X-rays (Wiley, New York; Chapman and Hall, London, 1955). [4] C.P.P. Chou and D. Turnbull, J. Non-Cryst. Solids 17 (1975) 169. [5] T. Kamiyama, T. Itoh and K. Suzuki, J. Non-Cryst. Solids 100 (1988) 466. [6] R.W. Hendricks, J. Appl. Crystallogr. 5 (1972) 315. [7] D.L. Weinberg, Rev. Sci. lnstrum. 34 (1963) 691. [8] G. Porod, Kolloid Z. 124 (1951) 83. [9] V. Luzzati, J. Witz and A. Nicolaeff, J. Molec. Biol. 3 (1961) 367. [10] A. Naudon and J. Caisso, J. Aopl. Crystallogr. 7 (1974) 25. [11] I.M. Lifshitz and V.V. Slyozov, J. Phys. Chem. Solids 19 (1961) 35. [12] C. Wagner, Z. Elektrochem. 65 (1961) 581. [13] J.H. Letcher and P.W. Schmidt, J. Aopl. Phys. 37 (1966) 649.
:OURNA L or
~ / ~ I ~
I~IIIl~
A SAXS observation of the crystallization of an A185Y10Ni5 amorphous alloy in the supercooled liquid state T o m o a k i K a m i y a m a , S a t o r u M a t s u o i a n d K e n j i Suzuki Institute for Materials Research, Tohoku University, Sendai 980, Japan
The medium-range structure of a heat-treated AI85Y10Ni5 amorphous alloy was investigated by small-angle X-ray scattering, calorimetric analysis, wide-angle X-ray diffraction and transmission electron microscope measurements. It is observed that crystalline particles having a definite boundary are precipitated by annealing at supercooled liquid temperatures and that most of the precipitates consist of the AI fcc crystalline phase. The integrated intensities of the small-angle scattering and the Guinier radii of the precipitates increase on annealing for time duration < 5.4 ks. However, as the annealing time becomes > ~ 5.4 ks, the integrated intensities and the Guinier radii are invariant, i.e., growth of the precipitates ceases. After annealing for 518.4 ks, the diameters of the precipitates exhibit a narrow distribution centered around 200 .A.
1. Introduction A melt-spun AI85Y10Ni 5 amorphous alloy has a high strength, good ductility and a wide supercooled liquid temperature range [1]. However, the Young's modulus and the tensile strength of A185Y10Ni 5 amorphous alloys decrease steeply after heat treatment around the glass transition temperature [2]. Small-angle X-ray scattering (SAXS) is a powerful method for investigating inhomogeneities in the range from an atomic scale to several hundreds of ,~ngstr6m units [3]. The medium-range structural changes for P d - A u - S i amorphous alloys near the glass transition temperature were investigated by Chou et al. [4] using the SAXS technique. This work reports the medium-range structural changes for an AI85Y10Ni 5 amorphous alloy caused by heat treatment in and around the supercooled liquid region, observed using SAXS, 1 Present address: Mitsubishi Materials Corporation, 1-297, Kitabukuro-cho, Omiya 33, Japan. Correspondence to: Dr T. Kamiyama, Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980, Japan. Tel: +81-22 215 2077. Telefax: +81-22 215 2076.
differential scanning calorimetry (DSC), X-ray diffraction ( X R D ) and transmission electron microscopy (TEM).
2. Experimental The initial AissY10Ni 5 alloy was prepared by melting a mixture of metallic A1 (99.9 wt%), Y (99.9 wt%) and Ni (99.9 wt%) in an argon gas atmosphere using an arc furnace. Amorphous alloy ribbons of about 30 ~ m thickness and 3 m m width were prepared from the initial alloy by melt-quenching using a rotating copper wheel in an argon gas atmosphere. The chemical composition of the as-prepared amorphous alloy ribbon was determined to be 85.3 + 0.9 at.% AI, 9.73 + 0.10 at.% Y and 5.83 ___0.06 at.% Ni by the inductively coupled p l a s m a - a t o m i c emission spectrometry ( I C P - A E S ) . The amorphous alloy ribbon was checked by X-ray diffraction to ensure that there was no crystallinity present. A silicon oil bath was used for annealing the amorphous alloy ribbon specimens. To provide rapid heating, specimens were plunged in the bath of silicon oil (Shin-Etsu Chem. Co., Ltd., KF
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
T. Kamiyama et al. / Crystallization of AIssYloNi 5 amorphous alloy
965-100 cs) for a specified time duration and then quenched into ice-water. Finally the specimens were ultrasonically washed in acetone and used for the measurements. Calorimetric analyses of the as-prepared and annealed specimens were made using a Seikoh DSC-200 calorimeter. Both the specimens of about 10 mg and ot-Al203 powder as a reference were heated in an argon gas atmosphere to avoid oxidation. Heating rates were 5-40 K / m i n . SAXS measurements were performed at room temperature using an apparatus with a 0.5 mm × 0.5 mm point focus Cu Ke~ incident beam [5]. The count rate, n, was corrected only for the background scattering without a specimen. The scattered intensity per channel was determined in absolute units (cm2/unit volume) from the equation
I ( h ) - tlo AOe_Ut,
(1)
where t is the sample thickness, AI2 is the solid angle of the detector at the specimen and e -~" is the transparency of the specimen [6]. The primary beam intensity, I 0, was determined by extrapolating the beam intensities attenuated by multiple foils [7]. The amplitude of the scattering vector is given by h = 4w sin 0/A, where A is the radiation wavelength and 20 is the scattering angle. To check the structure of the as-prepared and annealed alloys, wide-angle X-ray diffraction measurements were carried out using a conventional 0 - 2 0 diffractometer. The precipitates in the annealed alloys were also observed with a transmission electron microscope.
3. Results
3.1. Thermal analysis DSC measurements were performed for the as-quenched and isothermally annealed AI85YI0Ni 5 amorphous alloys. The glass-transition temperature, Tg, and the crystallization temperature, Tx, of the as-quenched alloy are Tg = 498 K and Tx = 525 K. DSC curves of Al8sYl0Ni 5 amorphous alloys annealed at different temperatures show
T~
AIssY, oNi5 40K / min
Ta =513K
400
n
173
500
600
Temperature / K
Fig. 1. Calorimetric thermograms of A185Yt0Ni5 amorphous alloy as-quenched and heat-treated at 513 K for different lengths of time. The heating rate is 40 K/min.
that 1 h annealing at 500 K, near Tg, results only in an enthalpy relaxation, whereas 1 h annealing at 528 K results in the disappearance of the first stage of the exothermic peak in addition to enthalpy relaxation. Differential scanning calorimetry curves are shown in fig. 1 for A185Y10Ni5 amorphous alloys as-quenched and isothermally annealed at 513 K for various time periods. The scan speed is 40 K / m i n in all measurements. Peaks due to enthalpy relaxation, the glass-transition and exothermic crystallization are observed in the DSC curve for the as-quenched specimen. The first stage of the crystallization peak becomes weak with increasing annealing time and is not observed for annealing times longer than 5.4 region.
3.2. Small-angle X-ray scattering In fig. 2, the scattered intensities for the specimens as-quenched and annealed for 1 h at different temperatures, ranging from 473 to 528 K, are
T. Kamiyama et aL / Crystallization of AlssYloNi 5 amorphous alloy
174 1 0s
X \
I07
'
'
~h
'Allsfl°l~i'5
4 -
' '
T~ = 525 K
106
1 04 a ^ A.
\
"
"~
x :473 K
%a\ -aa~
0•
....... 4 "NX,h-
A
O : as-quenched
\ "~
X
annealed for 3.6 ksec Tg = 498 K
•
a
"~.'~10 s
:517K :528K
.....
Z
_
i AIssYl°Ni5 annealed at 513 K
tt a
~aaAaX -%a'X.
o •
x XXXXXxx~xx-a~'
xxx~a~ ^x'%
x a
: as-quenched : 0.12 ksec : 1.8 ksec : 5.4 ksec
0 oo:iio
•
~
z
10
b,~
•
o
•
0
x c~
x
•
8"~_~ 103 0
x
1 01
e
2
,
3
,
4
, 5
0.01
,0 J , l~l,j . e'~• 6 789 -
0000000
/~a/~
0.1
10:0.01
2
3
4
5 6 7 890.1
2
h ( k l) h ( k "l) Fig. 2. Scattered intensities for AI8sY10Ni 5 amorphous alloys as-quenched and annealed for I h at different temperatures.
shown in relative units, corrected for sample thickness and primary beam intensity. Scattered intensities for the alloys as-quenched and annealed at 513 K for different lengths of time are shown in fig. 3. It is clearly observed that annealing at 513 K causes variations in both the amplitude and shape of the scattered intensities.
Fig. 3. Scattered intensities for Al8sY10Nis amorphous alloys as-quenched and annealed at 513 K for different lengths of time.
the glass-transition temperature clearly changes the SAXS intensities both in shape and amplitude. The integrated SAXS intensities are plotted in fig. 4 versus the annealing temperatures. The integrated intensity increases abruptly on passing through the glass-transition temperature and the crystallization temperature. It was shown by In-
4. Discussion
4.1. Annealing at various temperatures
As shown in fig. 2, no differences in the scattered intensities can be detected between the as-prepared alloy and those annealed below the glass-transition temperature, both in shape and amplitude. In the DSC measurements for the alloys annealed below the glass-transition temperature, only thermal relaxation can be detected. Annealing below the glass-transition temperature may cause short-range structural relaxation but it does not cause a change in the medium-range structure, while annealing above
10 "~
i
i
u~
10":
A185Y1oNi5 one hour annealing
l[
o
10-: Tg 10460
480
5()0
Tx 5 2' 0
5 4' 0
Temperature ( K ) Fig. 4. Integrated scattered intensities for AlssYloNi 5 amorphous alloys annealed for 1 h at different temperatures.
T. Kamiyama et al. / Crystallization of Als5YioNi 5 amorphous alloy
175
oue et al. [2] that the fracture strength of AI85Yt0Ni 5 amorphous alloy decreases gradually from 920 to 700 MPa in the amorphous solid region below 453 K, whereas it rapidly decreases to 240 MPa in the supercooled liquid region at 508 K. The integrated SAXS intensities increase at the temperatures where the decrease of fracture strength occurs.
4.2. Isothermal annealing in the supercooled liquid region As shown in fig. 3, the scattered intensity for as-prepared sample is weak compared with that for the annealed samples, while a peak near h = 0.04 , ~ - t and a Porod decay [8] (i.e., I(h) h n) are observed even in the scattered intensity for the specimen annealed for 120 s. The Porod law for the scattered intensity can be written as [9,10] 2"rr
1(h) = - ~ ( p , - p o ) 2 S
(2)
+ Ib,
where p~ and Po are the electronic densities of the precipitated particles and the surrounding
0.5
i
i
,
~[
Ala~YI0Ni5
"[
i as-quenched
S
_/ I
0.4
C
,-s 0.3
,.j' "~ 0.2 e-
0.1
o o
o
o
o o
oo o° P
0.0
o
L
100
200
300
4 0
500
h4 ( k 4) xl0~ Fig. 5. Porod plots (i.e., hal(h) vs. h a) for AIssY10Ni 5 amorphous alloys as-quenched and annealed at 513 K for 120 s.
Fig. 6. Transmission electron micrograph of an AlssYl0Ni 5 amorphous alloy as-quenched (a), annealed at 513 K for 14.4 ks (b) and for 518.4 ks (c).
T. Kamiyama et a L / Crystallization o f A l ss Ym Ni 5 amorphous alloy
176
media respectively, and S and I b are constants. As shown in fig. 5, the scattered intensity for the specimen annealed for 120 s is well approximated by eq. (2), whereas that for the as-quenched specimen does not follow the Porod law. This difference shows that the scattering is caused by precipitated particles with a well-defined boundary in an amorphous medium, although Bragg peaks characteristic of crystalline phases cannot be detected by wide-angle X-ray diffraction for the samples annealed for either 120 or 300 s. On heat treatment at 513 K for longer than 120 s, the scattered intensities increase and the peaks or shoulders shift towards smaller scattering vectors. The scattered intensities still obey the Porod law. The SAXS result for the alloy annealed for 5.4 s shows the presence of precipitated particles of a crystalline phase as confirmed by the DSC result (fig. 1) in which the first stage of the crystallization peak has disappeared. To confirm the SAXS results, TEM observations were performed for AI85YmNi5 amorphous alloys as-quenched and annealed at 513 K for various time periods. The micrographs for the as-quenched specimen and for those annealed for 14.4 ks and for 518.4 ks are shown in fig. 6. No precipitates are observed in the as-quenched
100
~
~
(a)
1°+I
Guinier Plot of AlssYt0Ni5
105
1%
J
015
t5
1.0
I
1.
2.0
2.5
wave number (/~-2) xlO-3 100
(b)
. . . . . . . .
I
,"'"
,.
. . . . . . . .
Ostwald'"
%
i
•
. . . . . . . .
•
i
•
•
. . . .
•
.,J"
I= o
AlssY10Ni5 annealed at 513 K ,.¢
I~.I
~
annealed at 513 K for 1.8 ksec
....
....
t
I
i
i
i
,,,
%
......
~'6o
~
,
,
Annealing Time (ksec) Fig. 8. (a) Guinier plot of an AlssYxoNi 5 amorphous alloy annealed at 513 K for 1.8 ks and (b) the variation in the radius of gyration as a function of annealing time.
80
60
AlssYt0Ni5 annealed at 513 K
40
specimen. However, in the micrographs for the specimens annealed for 14.4 ks and 518.4 ks there are precipitated particles having nearly spherical shapes with definite boundaries and dimensions in the range of several tens to 200 A, in good agreement with the result from Guinier plots of the SAXS intensities. In the X-ray diffraction patterns of AI85Y10Ni5 amorphous alloys as-quenched and annealed at 513 K, only an amorphous halo is observed for the as-quenched specimen and those annealed for < 300 s, while some diffraction peaks are o
N
20
O0
110
I
20
310
410
I
50
60
Annealing Time (ksec) Fig. 7. Integrated scattered intensities for AlssY10Ni 5 amorphous alloys annealed at 513 K for different lengths of time.
T. Kamiyama et al. / Crystallization of AlssY/oNi5 amorphous alloy
detected for the sample annealed for 3.6 ks. These peaks increase in height and become sharp as the annealing time increases. Some of the peaks coincide in position with the AI (fcc) Bragg peaks observed in the same alloy at equilibrium. Except for the AI (fcc) peaks, only a few peaks coincide with those of equilibrium phases and the other peaks could not be identified. In the specimen annealed in the supercooled liquid region, most of the precipitates are of the AI (fcc) crystalline phase while those remaining consist of the AI3Ni crystalline phase and Some other non-equilibrium crystalline phase.
4.3. Particle growth The growth of the precipitates is examined using the integrated intensity, defined by eq. (3), as a function of their volume fraction, Vf [8]:
~-~2f° h2l(h) dh = leVf(1 - Vr)(p 1 - p 0 ) 2, (3) where the scattered intensity, l(h), is in absolute units (cmZ/unit volume) and I e = 7.94 × 10 -26
177
cm 2. The integrated intensities for the specimens annealed at 513 K can be calculated from the fact that the scattered intensity follows the Porod law in the high h region and the results are plotted versus annealing time in fig. 7. The integrated intensities increase rapidly with short annealing times and reach a value of 0.46 × 1021 cm -4 after annealing for 5.4 ks. The integrated intensity increases little for annealing times longer than 5.4 ks. Assuming that the precipitates consist of AI (fcc) and that the composition of the medium is not changed during the precipitation process, the volume fraction, Vf, of the precipitates is estimated to be about 10% using eq. (3). It is concluded from the integrated intensity that (i) the precipitates grow rapidly at the beginning, and (ii) when the precipitates occupy a somewhat small volume fraction of about 10%, the growth of the precipitates stops or the precipitates grow by Ostwald ripening. The growth of the precipitates is investigated by estimating the radius of gyration of the precipitated particles. The radius of gyration is estimated by a weighted least squares method and its uncertainty is due only to statistical errors in the
1 05
(a) A185Ylo Ni5 annealed at 513 K for 518.4 ksec
12x10 "3
I
I
I
I
I
"~I 0 '~
Ib) Is
10
8
~1 03 Z
.!
~
6
4
J 1 02 0.1
0.01
h (/~-1)
2
0
I
100
150
200
250
300
r/A
Fig. 9. (a) Scattered intensity for an AI85YloNi 5 amorphous alloy annealed at 513 K for 518.4 ks and (b) the particle diameter-distribution function, p(r), calculated using the scattered intensity and eq. (4).
T. Kamiyama et al. / Crystallization of AlssYwNi 5 amorphous alloy
178
count rates. In Guinier plots of AIssY10Ni 5 amorphous alloy annealed at 513 K, the linear regions are extended on annealing for 1.8 ks as shown in fig. 8(a). Such a linearity is indicative of a fairly narrow size distribution for the precipitated particles. The radii of gyration for specimens annealed for various time periods are illustrated in fig. 8(b). The radii of gyration increase, but their growth rate becomes low with increasing annealing time. For Ostwald ripening, the average radius of the precipitates increases with annealing time, t, proportional to t 1/3, if a diffusion limited process is assumed [11,12]. However, as shown in fig. 8(b), the radius of gyration increases at a rate less than t 1/3 and, when it reaches about 80 .&, the precipitates no longer grow by further annealing.
4.4. Precipitates formed by annealing at 513 K for six days Finally, we examine the behavior of the precipitates annealed for a much longer time than that within which the growth of the precipitates ceases. The scattered intensity for specimens annealed for 518.4 ks is shown in fig. 9(a). An oscillation about the h - a line is observed in the scattered intensity that is characteristic of scattering from a system of monodispersed particles. For the assemblies of spherical particles observed by TEM in this work, the particle-diameter distribution function, p(r), is calculated from eq. (4) [13]: 1
p(r)
--~-~r2 fo dh{h41(h) - C}o~(hr),
where C = lim hal(h) h~
and ( 8 )
a( hr ) = cos hr 1 - ~
4sinhr
hr
The result is illustrated in fig. 9(b).
(4)
5. Conclusions
(1) On annealing for > 120 s in the supercooled liquid region (513 K), a phase separation occurs in AI85Y10Ni5 amorphous alloy with the precipitation of particles with a well-defined boundary in the amorphous medium. (2) The precipitates in AlssYl0Ni s amorphous alloy annealed at 513 K consist mostly of the AI (fcc) crystalline phase and the other precipitates are the AI3Ni crystalline phase and some other non-equilibrium crystalline phase. (3) The growth rate of the precipitates decreases on annealing at 513 K for > 3.6 ks, and the precipitates do not grow when annealing time exceeds 10 ks. On annealing for 518.4 ks, the diameters of the precipitates exhibit a narrow distribution centered at ~ 200 ~,.
References [ll A. Inoue, K. Ohtera and T. Masumoto, Sci. Rep. RITU A-35 (1990) 115. [2] A. Inoue, K. Ohtera, A.P. Tsai, H. Kimura and T. Masumoto, Jpn. J. Appl. Phys. 27 (1988) L1579. [3] A. Guinier and G. Fournet, Small-angle Scattering of X-rays (Wiley, New York; Chapman and Hall, London, 1955). [4] C.P.P. Chou and D. Turnbull, J. Non-Cryst. Solids 17 (1975) 169. [5] T. Kamiyama, T. Itoh and K. Suzuki, J. Non-Cryst. Solids 100 (1988) 466. [6] R.W. Hendricks, J. Appl. Crystallogr. 5 (1972) 315. [7] D.L. Weinberg, Rev. Sci. lnstrum. 34 (1963) 691. [8] G. Porod, Kolloid Z. 124 (1951) 83. [9] V. Luzzati, J. Witz and A. Nicolaeff, J. Molec. Biol. 3 (1961) 367. [10] A. Naudon and J. Caisso, J. Aopl. Crystallogr. 7 (1974) 25. [11] I.M. Lifshitz and V.V. Slyozov, J. Phys. Chem. Solids 19 (1961) 35. [12] C. Wagner, Z. Elektrochem. 65 (1961) 581. [13] J.H. Letcher and P.W. Schmidt, J. Aopl. Phys. 37 (1966) 649.