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Time-resolved X-ray diffraction study of nanocrystallization in Al-based metallic glasses
Journal of Non-Crystalline Solids 351 (2005) 2383–2387 www.elsevier.com/locate/jnoncrysol
Time-resolved X-ray diffraction study of nanocrystallization in Al-based metallic glasses Jerzy Antonowicz
*
Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warszawa, Poland Received 18 November 2004; received in revised form 23 June 2005
Abstract Nanocrystallization kinetics of five binary Al–rare earth glassy alloys were investigated in situ using time-resolved X-ray diffraction. From the diffraction spectra parameters the crystalline phase volume fraction and mean crystal size were evaluated. The analysis show that nanocrystal growth kinetics exhibits a sharp transition between extremely rapid and very sluggish growth stage. This observation is supported by the TEM analysis. The nanocrystal size reaches from 9 to 16 nm depending on the alloy system. The crystalline volume fraction approaches 20% from which grain density of about 1.5 · 1023 m3 is deduced. It is concluded that the nanocrystallization kinetics is governed by nucleation rate and that diffusion controlled growth plays minor role in the devitrification process. 2005 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 81.05.Kf; 81.07.b; 64.60.Qb
1. Introduction Rapidly quenched aluminium-rich metallic glasses containing rare earth (RE) and transition metal (TM) exhibit high strength and good ductility [1]. These properties can be improved by heat treatment of amorphous alloy to obtain nanocrystalline material. Understanding of the nanocrystallization process is important for controlling the unique mechanical properties of these materials. Devitrification of Al-based metallic glasses occurs in two or three stages with the primary crystallization of fcc-Al nanograins as the first stage. After the primary crystallization is completed the residual amorphous matrix undergoes eutectic transformation into aluminium and Al–RE compounds. The nanocrystalline material consists of dispersion of 1022–1023 m3 of approximately spherical nanocrystals of about 10 nm embedded in
*
Tel.: +48 22 660 8214; fax: +48 22 660 8419. E-mail address: [email protected]
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.06.037
amorphous matrix. This kind of microstructure requires extremely high nucleation rate of order of 1020 m3 s1 and sluggish grain growth. The nanocrystallization of Al–RE and Al–RE–TM glasses was studied extensively over last several years [2–6]. Although the nanocrystallization process in Al-based amorphous alloys is widely investigated, the mechanism is still not well known. In the present paper the nanocrystallization of series of five Al–RE (RE = Y, Sm, Gd, Tb, Dy) amorphous alloys is investigated by means of in situ diffraction methods during isothermal annealing at temperatures near the crystallization point.
2. Experimental Amorphous ribbons with thickness of 20–30 lm were produced by melt-spinning technique in argon atmosphere with wheel velocities 35–65 m/s. The nominal compositions of alloys used for the analysis were:
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J. Antonowicz / Journal of Non-Crystalline Solids 351 (2005) 2383–2387
Al90Y10, Al92Sm8, Al91Gd9, Al91Tb9, Al90Dy10. The Xray diffraction measurements were performed at ID11 of European Synchrotron Radiation Facility [7] using ˚ corresponding to energy wavelength of k = 0.69668 A of 17.8 keV. Pieces of as-quenched ribbons were sealed in Pyrex capillaries under argon atmosphere. Such prepared samples were placed in DSC type Linkam hot stage between the incident beam and two-dimensional detector. The diffraction patterns in transmission mode were collected by 2-D CCD camera every 15 s with acquisition time of 2 s. All investigated alloys exhibited amorphous patterns in as-quenched state. The measurements were carried out during isothermal annealing at temperatures in the vicinity of crystallization temperature. The annealing temperature was reached with a heating rate of 80 K/min and was controlled with precision better then 0.1. The final diffraction spectra were obtained from 2-D patterns after radial integration described in details elsewhere [7]. The transmission electron microscopy (TEM) micrographs were taken using Philips EM300 microscope. The differential scanning calorimetry (DSC) measurements were carried out with Perkin Elmer DSC-7 device.
subtraction and deconvolution of amorphous halo and crystalline peaks. The result is presented in Fig. 2. Crystalline volume fraction was calculated using the recently proposed method described in details in [8]. According to this approach, x is derived from the total and partial scattered intensities and from X-ray properties of constituent atoms: x¼
Ic ; I K
ð1Þ
where K¼
2 fAl lRE mRE ; 2 2 fAl lRE mRE ð1 cÞ þ fRE lAl mAl c
ð2Þ
Primary crystallization of fcc-Al was found to be the first stage of devitrification in all investigated systems. The sample amorphous ! amorphous + fcc-Al transformation of Al90Y10 at 458 K is shown in Fig. 1 where the scattered intensity versus q = (4p sin h)/k is plotted. No changes in fcc-Al lattice parameters in course of nanocrystallization stage were found. From the diffraction spectra taken in lower q-range the crystalline volume fraction x was determined after background
fAl and fRE are atomic scattering factors, lAl and lRE are mass attenuation coefficients, mAl and mRE are atomic masses of Al and RE atoms respectively and c is the initial RE content in amorphous phase. The total scattered intensity I is defined as I(2h)d2h and Ic is crystalline intensity contribution. In present analysis the angular range considered for x calculations was limited to area covering first amorphous halo and two strongest fccAl reflexes (Fig. 2). Atomic scattering factors were obtained from DABAX database [10] available via SCATFAC software [9] and the mass attenuation coefficients were taken from the NIST Standard Reference Database using XCOM software [11]. The total scattered intensity I in the range considered was found to be constant within accuracy of 5% for all measurements. An average nanocrystal size D was obtained from a single peak broadening analysis using classical Debye– Scherrer formula after instrumental broadening subtraction. Due to significant overlap of the (1 1 1) peak with an amorphous halo the (2 0 0) fcc-Al peak was selected for the broadening analysis. The feature common to D in all alloys is a step-like behavior with time, followed
Fig. 1. Evolution of diffraction spectra during annealing of Al90Y10 alloy at 458 K. The fcc-Al primary phase peaks are indexed.
Fig. 2. Deconvolution of diffraction spectra of Al92Sm8 amorphous alloy annealed at 453 K for 1350 s. Circles are experimental points and lines are calculated results.
3. Results
J. Antonowicz / Journal of Non-Crystalline Solids 351 (2005) 2383–2387
by rapid saturation. The peak broadening method validity was verified by the TEM analysis of the nanocrystalline samples. Fig. 3 presents the TEM images of Al92Sm8 samples at two different stages of the nanocrystallization process. The Ôearly stageÕ sample annealed up to 440 K at a heating rate of 10 K/min exhibits an average nanocrystal size D ¼ 11 nm while the Ôlate stageÕ sample annealed up to 470 K, shows D ¼ 13 nm. Fig. 4 shows the DSC run of as-quenched Al92Sm8, where the arrows show temperatures used by TEM observations. In order to detect the early stages of grain growth the measurements in lowered temperatures were carried out. Although the method used in present study allows detection of 0.5% crystalline phase volume fraction, no intermediate mean grain size was observed. The volume fraction of crystalline phase and mean nanocrystal size evolution at two temperatures are plotted in Fig. 5. The comparison of the diffraction peak broadening method with the TEM images proves the validity of crystal size determination method used in present work. It has been found that the saturation value of mean grain size in temperature range considered does not
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Fig. 4. A DSC trace of an as-quenched Al92Sm8 amorphous alloy taken during heating at 10 K/min. The nanocrystallization onset point (at 438 K) together with annealing temperatures used for TEM analysis (arrows at 440 K and 470 K) are shown.
Fig. 5. Crystalline volume fraction and mean grain size during annealing of amorphous Al92Sm8 at 443 K and 453 K.
Fig. 3. Bright field TEM image of the Al92Sm8 amorphous alloy annealed up to 440 K (a) and 470 K (b) using 10 K/min heating rate.
depend on the annealing temperature and is a characteristic parameter for each alloy. Fig. 6 shows the average
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J. Antonowicz / Journal of Non-Crystalline Solids 351 (2005) 2383–2387
Fig. 6. Mean grain size values after isothermal annealing of Al90Y10 for 1000 s at different temperatures.
nanocrystal size values for Al90Y10 annealed for 1000 s at different temperatures. The nanocrystal size saturation values for all the investigated alloys are given in Table 1. If the crystalline volume fraction x and mean grain size D are known and a spherical shape of nanocrystals is assumed, the grain density can be estimated as 3 N ð3xÞ=ð4pðD=2Þ Þ [12]. Thus the nucleation frequency Iv defined as the time derivative of the nuclei density can be evaluated from: ! d 3x Iv . ð3Þ dt 4pðD=2Þ3 Grain density for Al92Sm8 annealed at 443 K and 453 K are evaluated from x and D as a function of time. The result is plotted in Fig. 7(a). It has been found that the nucleation rate calculated using Eq. (3) decreases approximately exponentially with annealing time. The nucleation rate was fitted with exponential decay and plotted in Fig. 7(b). The error bars represent the standard deviation of values derived from Eq. (3). While the nucleation frequency is rather roughly estimated, it clearly shows the main features of nucleation kinetics.
Table 1 Mean grain size saturation values for different alloys Alloy composition
Mean grain size [nm]
Al90Y10 Al92Sm8 Al91Gd9 Al91Tb9 Al90Dy10
9±1 14 ± 1 13 ± 1 15 ± 1 16 ± 1
The uncertainties are estimated from the grain size determination method.
Fig. 7. Grain density (top) and nucleation rate (bottom) during annealing of amorphous Al92Sm8 at 443 K and 453 K. The error bars show the standard deviation of nucleation rate values obtained from Eq. (3).
4. Discussion Time-resolved X-ray diffraction experiments using high brilliance synchrotron radiation allow in situ analysis of the nanocrystallization process. The method gives insight both in the overall transformation progress and microstructure parameters evolution. In this way nucleation and growth kinetics can be determined even during relatively rapid transformations. The results obtained in present work show that similar mechanism acts during crystallization of all investigated Al–RE alloys. The most significant feature of the transformation is the step-like character of the mean grain size time dependence indicating extremely rapid growth in the initial stage of the nucleus evolution. This behavior is not consistent with square-root time proportionality observed during diffusion-controlled growth [13]. After initial rapid growth stage the mean size nearly saturates. Similar nanocrystals growth kinetics in Al-based glasses was reported earlier [3,6]. The comparison of crystalline volume fraction and mean grain size (Fig. 5) shows that nanocrystal growth is nearly terminated already at early stages of transformation
J. Antonowicz / Journal of Non-Crystalline Solids 351 (2005) 2383–2387
(x 0.5%). At such low fraction of crystalline volume the overlap of diffusion fields between adjacent grains (soft impingement) is not expected to be significant. This means that some other mechanism of growth blocking acts during nanocrystallization process. The above observations lead to conclusion that nanocrystallization of Al–RE glasses is controlled by nucleation rate and diffusion-controlled growth plays minor role in overall transformation kinetics. As demonstrated in present work, the mean grain size is nearly constant expect for an initial stage of transformation. The existence of a characteristic microstructure dimension is reminiscent of phase separation occurred by a spinodal decomposition mechanism. Spinodal decomposition of the amorphous phase, occurring on nanometer scale prior to nucleation of crystalline phase was proposed as a model of nanocrystallization in multicomponent Zr-based bulk metallic glass [14]. The phase separation in glassy Al–Gd–La–Ni were also reported [5]. Kinetic model of homogeneous nucleation in partitioning systems proposed by Kelton [15] predicts existence of regular compositional fluctuations similar to those resulting from spinodal decomposition. According to this model the fluctuations facilitate crystal nucleation and restrict their growth. Homogeneous compositional fluctuations in the glass and high mobility difference between Al and RE atoms was recently proposed as an origin of Al nanocrystals formation [4]. The rapid arrest of nanocrystal growth can also be interpreted in a framework of StephensonÕs theory of plastic strain and stress during interdiffusion [16] as discussed previously by Greer [17]. According to this model, in the small distances and short times regime required for nucleation the diffusion is fast. At longer times diffusion becomes controlled by slower component which reduces the crystal growth rate. The diffusion and stress coupling effects are expected to be significant in systems with strong diffusion coefficient asymmetry. As the diffusion coefficient is scaled with the atomic volume [18], strong stress effects on diffusion are expected in Al–RE systems with the volume ratio VRE/VAl of about 2. Further experimental and theoretical work on nanocrystallization mechanism of Al-based metallic glasses are in progress.
2387
Acknowledgments This work was initiated during stay of author at Institut National Poltechnique de Grenoble in the framework of the EU Nano-Al RTN (contract no. HPRN-CT-2000-000388). The author would like to express his thanks to Professor Yavari for his invaluable help, to Dr Vaughan of ESRF for the experimental ideas and to Dr Jaskiewicz for encouragement to this work. The Polish Committee of Scientific Research (grant no. 7T08 A02121) and Faculty of Physics of Warsaw University of Technology are gratefully acknowledged for supporting this work. References [1] A. Inoue, Progr. Mater. Sci . 43 (1998) 365. [2] J.C. Foley, D.R. Allen, J.H. Perepezko, Scr. Mater. 35 (1996) 655. [3] M. Calin, U. Ko¨ster, in: Proceedings of ISMANAN-97, Barcelona, 1997, published in: Mater. Sci. For. 269–272 (1998) 749. [4] P. Rizzi, M. Baricco, S. Borace, L. Battezzati, Mater. Sci. Eng. A 304–306 (2001) 574. [5] T.K. Croat, A.K. Gangopadhyay, K.F. Kelton, Philos. Mag. A 82 (2002) 2483. [6] X.Y. Jiang, Z.C. Zhong, A.L. Greer, Mater. Sci. Eng. A 226–228 (1997) 789. [7] Available from:. [12] M.T. Clavaguera-Mora, N. Clavaguera, D. Crespo, T. Pradell, Progr. Mater. Sci. 47 (2002) 559. [13] J.W. Christian, The Theory of Transformations in Metals and Alloys, Pergamon Press, Oxford, 1981. [14] J.F. Lo¨ffler, W.L. Johnson, Scr. Mater. 44 (2001) 1251. [15] K.F. Kelton, Philos. Mag. Lett. 77 (1998) 337. [16] G.B. Stephenson, Acta Metall. 36 (1988) 2663. [17] A.L. Greer, Defects Diffus. Forum 129&130 (1996) 163. [18] A.L. Greer, N. Karpe, J. Bøttiger, J. Alloys Compds. 194 (1993) 199.
Time-resolved X-ray diffraction study of nanocrystallization in Al-based metallic glasses Jerzy Antonowicz
*
Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warszawa, Poland Received 18 November 2004; received in revised form 23 June 2005
Abstract Nanocrystallization kinetics of five binary Al–rare earth glassy alloys were investigated in situ using time-resolved X-ray diffraction. From the diffraction spectra parameters the crystalline phase volume fraction and mean crystal size were evaluated. The analysis show that nanocrystal growth kinetics exhibits a sharp transition between extremely rapid and very sluggish growth stage. This observation is supported by the TEM analysis. The nanocrystal size reaches from 9 to 16 nm depending on the alloy system. The crystalline volume fraction approaches 20% from which grain density of about 1.5 · 1023 m3 is deduced. It is concluded that the nanocrystallization kinetics is governed by nucleation rate and that diffusion controlled growth plays minor role in the devitrification process. 2005 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 81.05.Kf; 81.07.b; 64.60.Qb
1. Introduction Rapidly quenched aluminium-rich metallic glasses containing rare earth (RE) and transition metal (TM) exhibit high strength and good ductility [1]. These properties can be improved by heat treatment of amorphous alloy to obtain nanocrystalline material. Understanding of the nanocrystallization process is important for controlling the unique mechanical properties of these materials. Devitrification of Al-based metallic glasses occurs in two or three stages with the primary crystallization of fcc-Al nanograins as the first stage. After the primary crystallization is completed the residual amorphous matrix undergoes eutectic transformation into aluminium and Al–RE compounds. The nanocrystalline material consists of dispersion of 1022–1023 m3 of approximately spherical nanocrystals of about 10 nm embedded in
*
Tel.: +48 22 660 8214; fax: +48 22 660 8419. E-mail address: [email protected]
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.06.037
amorphous matrix. This kind of microstructure requires extremely high nucleation rate of order of 1020 m3 s1 and sluggish grain growth. The nanocrystallization of Al–RE and Al–RE–TM glasses was studied extensively over last several years [2–6]. Although the nanocrystallization process in Al-based amorphous alloys is widely investigated, the mechanism is still not well known. In the present paper the nanocrystallization of series of five Al–RE (RE = Y, Sm, Gd, Tb, Dy) amorphous alloys is investigated by means of in situ diffraction methods during isothermal annealing at temperatures near the crystallization point.
2. Experimental Amorphous ribbons with thickness of 20–30 lm were produced by melt-spinning technique in argon atmosphere with wheel velocities 35–65 m/s. The nominal compositions of alloys used for the analysis were:
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J. Antonowicz / Journal of Non-Crystalline Solids 351 (2005) 2383–2387
Al90Y10, Al92Sm8, Al91Gd9, Al91Tb9, Al90Dy10. The Xray diffraction measurements were performed at ID11 of European Synchrotron Radiation Facility [7] using ˚ corresponding to energy wavelength of k = 0.69668 A of 17.8 keV. Pieces of as-quenched ribbons were sealed in Pyrex capillaries under argon atmosphere. Such prepared samples were placed in DSC type Linkam hot stage between the incident beam and two-dimensional detector. The diffraction patterns in transmission mode were collected by 2-D CCD camera every 15 s with acquisition time of 2 s. All investigated alloys exhibited amorphous patterns in as-quenched state. The measurements were carried out during isothermal annealing at temperatures in the vicinity of crystallization temperature. The annealing temperature was reached with a heating rate of 80 K/min and was controlled with precision better then 0.1. The final diffraction spectra were obtained from 2-D patterns after radial integration described in details elsewhere [7]. The transmission electron microscopy (TEM) micrographs were taken using Philips EM300 microscope. The differential scanning calorimetry (DSC) measurements were carried out with Perkin Elmer DSC-7 device.
subtraction and deconvolution of amorphous halo and crystalline peaks. The result is presented in Fig. 2. Crystalline volume fraction was calculated using the recently proposed method described in details in [8]. According to this approach, x is derived from the total and partial scattered intensities and from X-ray properties of constituent atoms: x¼
Ic ; I K
ð1Þ
where K¼
2 fAl lRE mRE ; 2 2 fAl lRE mRE ð1 cÞ þ fRE lAl mAl c
ð2Þ
Primary crystallization of fcc-Al was found to be the first stage of devitrification in all investigated systems. The sample amorphous ! amorphous + fcc-Al transformation of Al90Y10 at 458 K is shown in Fig. 1 where the scattered intensity versus q = (4p sin h)/k is plotted. No changes in fcc-Al lattice parameters in course of nanocrystallization stage were found. From the diffraction spectra taken in lower q-range the crystalline volume fraction x was determined after background
fAl and fRE are atomic scattering factors, lAl and lRE are mass attenuation coefficients, mAl and mRE are atomic masses of Al and RE atoms respectively and c is the initial RE content in amorphous phase. The total scattered intensity I is defined as I(2h)d2h and Ic is crystalline intensity contribution. In present analysis the angular range considered for x calculations was limited to area covering first amorphous halo and two strongest fccAl reflexes (Fig. 2). Atomic scattering factors were obtained from DABAX database [10] available via SCATFAC software [9] and the mass attenuation coefficients were taken from the NIST Standard Reference Database using XCOM software [11]. The total scattered intensity I in the range considered was found to be constant within accuracy of 5% for all measurements. An average nanocrystal size D was obtained from a single peak broadening analysis using classical Debye– Scherrer formula after instrumental broadening subtraction. Due to significant overlap of the (1 1 1) peak with an amorphous halo the (2 0 0) fcc-Al peak was selected for the broadening analysis. The feature common to D in all alloys is a step-like behavior with time, followed
Fig. 1. Evolution of diffraction spectra during annealing of Al90Y10 alloy at 458 K. The fcc-Al primary phase peaks are indexed.
Fig. 2. Deconvolution of diffraction spectra of Al92Sm8 amorphous alloy annealed at 453 K for 1350 s. Circles are experimental points and lines are calculated results.
3. Results
J. Antonowicz / Journal of Non-Crystalline Solids 351 (2005) 2383–2387
by rapid saturation. The peak broadening method validity was verified by the TEM analysis of the nanocrystalline samples. Fig. 3 presents the TEM images of Al92Sm8 samples at two different stages of the nanocrystallization process. The Ôearly stageÕ sample annealed up to 440 K at a heating rate of 10 K/min exhibits an average nanocrystal size D ¼ 11 nm while the Ôlate stageÕ sample annealed up to 470 K, shows D ¼ 13 nm. Fig. 4 shows the DSC run of as-quenched Al92Sm8, where the arrows show temperatures used by TEM observations. In order to detect the early stages of grain growth the measurements in lowered temperatures were carried out. Although the method used in present study allows detection of 0.5% crystalline phase volume fraction, no intermediate mean grain size was observed. The volume fraction of crystalline phase and mean nanocrystal size evolution at two temperatures are plotted in Fig. 5. The comparison of the diffraction peak broadening method with the TEM images proves the validity of crystal size determination method used in present work. It has been found that the saturation value of mean grain size in temperature range considered does not
2385
Fig. 4. A DSC trace of an as-quenched Al92Sm8 amorphous alloy taken during heating at 10 K/min. The nanocrystallization onset point (at 438 K) together with annealing temperatures used for TEM analysis (arrows at 440 K and 470 K) are shown.
Fig. 5. Crystalline volume fraction and mean grain size during annealing of amorphous Al92Sm8 at 443 K and 453 K.
Fig. 3. Bright field TEM image of the Al92Sm8 amorphous alloy annealed up to 440 K (a) and 470 K (b) using 10 K/min heating rate.
depend on the annealing temperature and is a characteristic parameter for each alloy. Fig. 6 shows the average
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J. Antonowicz / Journal of Non-Crystalline Solids 351 (2005) 2383–2387
Fig. 6. Mean grain size values after isothermal annealing of Al90Y10 for 1000 s at different temperatures.
nanocrystal size values for Al90Y10 annealed for 1000 s at different temperatures. The nanocrystal size saturation values for all the investigated alloys are given in Table 1. If the crystalline volume fraction x and mean grain size D are known and a spherical shape of nanocrystals is assumed, the grain density can be estimated as 3 N ð3xÞ=ð4pðD=2Þ Þ [12]. Thus the nucleation frequency Iv defined as the time derivative of the nuclei density can be evaluated from: ! d 3x Iv . ð3Þ dt 4pðD=2Þ3 Grain density for Al92Sm8 annealed at 443 K and 453 K are evaluated from x and D as a function of time. The result is plotted in Fig. 7(a). It has been found that the nucleation rate calculated using Eq. (3) decreases approximately exponentially with annealing time. The nucleation rate was fitted with exponential decay and plotted in Fig. 7(b). The error bars represent the standard deviation of values derived from Eq. (3). While the nucleation frequency is rather roughly estimated, it clearly shows the main features of nucleation kinetics.
Table 1 Mean grain size saturation values for different alloys Alloy composition
Mean grain size [nm]
Al90Y10 Al92Sm8 Al91Gd9 Al91Tb9 Al90Dy10
9±1 14 ± 1 13 ± 1 15 ± 1 16 ± 1
The uncertainties are estimated from the grain size determination method.
Fig. 7. Grain density (top) and nucleation rate (bottom) during annealing of amorphous Al92Sm8 at 443 K and 453 K. The error bars show the standard deviation of nucleation rate values obtained from Eq. (3).
4. Discussion Time-resolved X-ray diffraction experiments using high brilliance synchrotron radiation allow in situ analysis of the nanocrystallization process. The method gives insight both in the overall transformation progress and microstructure parameters evolution. In this way nucleation and growth kinetics can be determined even during relatively rapid transformations. The results obtained in present work show that similar mechanism acts during crystallization of all investigated Al–RE alloys. The most significant feature of the transformation is the step-like character of the mean grain size time dependence indicating extremely rapid growth in the initial stage of the nucleus evolution. This behavior is not consistent with square-root time proportionality observed during diffusion-controlled growth [13]. After initial rapid growth stage the mean size nearly saturates. Similar nanocrystals growth kinetics in Al-based glasses was reported earlier [3,6]. The comparison of crystalline volume fraction and mean grain size (Fig. 5) shows that nanocrystal growth is nearly terminated already at early stages of transformation
J. Antonowicz / Journal of Non-Crystalline Solids 351 (2005) 2383–2387
(x 0.5%). At such low fraction of crystalline volume the overlap of diffusion fields between adjacent grains (soft impingement) is not expected to be significant. This means that some other mechanism of growth blocking acts during nanocrystallization process. The above observations lead to conclusion that nanocrystallization of Al–RE glasses is controlled by nucleation rate and diffusion-controlled growth plays minor role in overall transformation kinetics. As demonstrated in present work, the mean grain size is nearly constant expect for an initial stage of transformation. The existence of a characteristic microstructure dimension is reminiscent of phase separation occurred by a spinodal decomposition mechanism. Spinodal decomposition of the amorphous phase, occurring on nanometer scale prior to nucleation of crystalline phase was proposed as a model of nanocrystallization in multicomponent Zr-based bulk metallic glass [14]. The phase separation in glassy Al–Gd–La–Ni were also reported [5]. Kinetic model of homogeneous nucleation in partitioning systems proposed by Kelton [15] predicts existence of regular compositional fluctuations similar to those resulting from spinodal decomposition. According to this model the fluctuations facilitate crystal nucleation and restrict their growth. Homogeneous compositional fluctuations in the glass and high mobility difference between Al and RE atoms was recently proposed as an origin of Al nanocrystals formation [4]. The rapid arrest of nanocrystal growth can also be interpreted in a framework of StephensonÕs theory of plastic strain and stress during interdiffusion [16] as discussed previously by Greer [17]. According to this model, in the small distances and short times regime required for nucleation the diffusion is fast. At longer times diffusion becomes controlled by slower component which reduces the crystal growth rate. The diffusion and stress coupling effects are expected to be significant in systems with strong diffusion coefficient asymmetry. As the diffusion coefficient is scaled with the atomic volume [18], strong stress effects on diffusion are expected in Al–RE systems with the volume ratio VRE/VAl of about 2. Further experimental and theoretical work on nanocrystallization mechanism of Al-based metallic glasses are in progress.
2387
Acknowledgments This work was initiated during stay of author at Institut National Poltechnique de Grenoble in the framework of the EU Nano-Al RTN (contract no. HPRN-CT-2000-000388). The author would like to express his thanks to Professor Yavari for his invaluable help, to Dr Vaughan of ESRF for the experimental ideas and to Dr Jaskiewicz for encouragement to this work. The Polish Committee of Scientific Research (grant no. 7T08 A02121) and Faculty of Physics of Warsaw University of Technology are gratefully acknowledged for supporting this work. References [1] A. Inoue, Progr. Mater. Sci . 43 (1998) 365. [2] J.C. Foley, D.R. Allen, J.H. Perepezko, Scr. Mater. 35 (1996) 655. [3] M. Calin, U. Ko¨ster, in: Proceedings of ISMANAN-97, Barcelona, 1997, published in: Mater. Sci. For. 269–272 (1998) 749. [4] P. Rizzi, M. Baricco, S. Borace, L. Battezzati, Mater. Sci. Eng. A 304–306 (2001) 574. [5] T.K. Croat, A.K. Gangopadhyay, K.F. Kelton, Philos. Mag. A 82 (2002) 2483. [6] X.Y. Jiang, Z.C. Zhong, A.L. Greer, Mater. Sci. Eng. A 226–228 (1997) 789. [7] Available from: