- Email: [email protected]
Phase selection in undercooled Fe–Nd alloy melts
Materials Science and Engineering A 375–377 (2004) 561–564
Phase selection in undercooled Fe–Nd alloy melts J. Strohmenger a,∗ , T. Volkmann a , J. Gao a,b , D.M. Herlach a b
a Institute of Space Simulation, Cologne D-51170, Germany Department of Applied Physics, Northwestern Polytechnical University, Xian 710072, China
Abstract Undercooling experiments on Fe89.5 Nd10.5 , Fe87.5 Nd12.5 and Fe75 Nd25 alloy melts have been performed by electromagnetic levitation and drop tube experiments. Phase formation in Fe–Nd alloys is of interest due to the ternary compound Fe14 Nd2 B1, which exhibits extraordinary hard magnetic properties. Under equilibrium solidification conditions, melts of the investigated composition range solidify primarily in the ␥-Fe solid solution, which is followed by the peritectic formation of the chemically ordered Fe17 Nd2 -phase. In electromagnetic levitated Fe75 Nd25 alloys the primary crystallisation of ␥-Fe could be suppressed in favour of the peritectic phase with rising undercooling. In Fe-rich alloys the direct crystallisation of the Fe17 Nd2 -phase was only observed in drop tube solidified particles with diameters below 0.2 mm. © 2003 Elsevier B.V. All rights reserved. Keywords: Rare-earth magnetic alloys; Undercooling of melts; Containerless processing; Metastable phases
1. Introduction It is well known that undercooling of melts gives access to alternative solidification pathways [1]. The study of the Fe–Nd system is of relevance for an understanding of phase formation in ternary Fe–Nd–B alloy melts, which are used for the development of high-performance magnetic materials based on the hard magnetic compound Fe14 Nd2 B1 [2,3]. Recently, we reported on undercooling experiments on Fe–Nd–B alloy melts [4] showing that primary ␥-Fe solidification can be avoided and that the peritectic Fe14 Nd2 B1 -phase can be crystallized directly from undercooled melts. In order to obtain more insight in the crystallization behaviour and phase selection with respect to the competition of properitectic and peritectic phases in undercooled rare-earth-transition metal alloy melts, binary Fe–Nd alloys have been investigated by containerless processing of melts. Up to now, the intermetallic compounds Fe17 Nd2 [5], Fe17 Nd5 [6,7], and Fe2 Nd [8,9] have been proposed as phases that are formed by a peritectic reaction. The present investigations are focussed on Fe–Nd melts with concentrations between 10.5 and 25 at.% Nd. Under equilibrium solidification conditions the melt solidifies primarily in the ␥-Fe solid solution, which is followed by the formation of ∗ Corresponding author. Tel.: +49-2203-601-4583; fax: +49-2203-61768. E-mail address: [email protected] (J. Strohmenger).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.191
the peritectic Nd2 Fe17 -phase. Since the peritectic transformation is diffusion-controlled the dissolution of properitectic ␥-Fe is incomplete in conventional casting processes, leading to an significant amount of residual ␣-Fe at room temperature. It is shown that the primary crystallization of ␥-Fe is suppressed in favour of the peritectic Fe17 Nd2 -phase with rising undercooling of the melt prior to solidification.
2. Experimental details For both, electromagnetic levitation and drop tube experiments samples of about 1.5 g in mass were prepared from the elements (purity at least 99.9%) by arc melting under argon atmosphere (99.999% purity). The electromagnetic levitation facility consists of a conical coil that is connected to a radio-frequency generator for levitating and inductively heating of the metallic sample. Both, the sample and the coil are placed in an high vacuum chamber. After evacuation to a vacuum in the order of 10−7 mbar, the levitation chamber was backfilled with helium cooling gas (99.999% purity). Undercooling of the highly reactive Fe–Nd melts was achieved by levitation processing under He pressures below 50 mbar enabling an evaporization of Nd-oxides [4]. The temperature of the levitated samples was measured by a pyrometer with a relative accuracy of ±5 K during the entire experiment cycle in order to determine the undercooling level prior to solidification. The pyrometer was
562
J. Strohmenger et al. / Materials Science and Engineering A 375–377 (2004) 561–564
calibrated at the liquidus temperature, which is detected in the temperature–time profile during melting of the sample. In drop tube experiments, the melt is atomised in droplets with diameters in the range of 0.1–2 mm. During the free fall of nearly 8 m in a high purity He-gas atmosphere, the droplets were undercooled and solidified at cooling rates between 102 and 104 K/s. The disintegration of the melt into many particles led to an isolation of impurities, which act as heterogeneous nucleation sites so that deep undercoolings of the droplets can be achieved. Due to the high cooling rates, it is possible to retain metastable phases solidified from the undercooled melt. Phase constituents and microstructure of levitated and solidified samples and of drop tube solidified particles were analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The compositions of the different phases have been verified by energy-dispersive X-ray analysis (EDX).
3. Results and discussion Two different solidification pathways have been found as shown in Fig. 1 by the temperature–time profiles of levitated Fe75 Nd25 melts for different undercooling levels T. In Fig. 1(a) at T = 50 K, a two-step solidification is observed which is due to the primary crystallization of ␥-Fe and subsequent formation of the Fe17 Nd2 -phase by a peritectic reaction as expected from the equilibrium phase diagram. According to the microstructural analysis by SEM and EDX in Fig. 2(a), the sample consists of the Fe-phase with more than 99 at.% Fe (black) which is surrounded by the intermetallic Fe17 Nd2 -phase (grey). The intergranular phase (white) with a Nd concentration of 90 at.% is formed as soon as solidification is completed at low temperatures
1750
Temperature T(K)
(b) ∆T=75K
(a) ∆T=50K
1700
Fig. 2. Back-scattered SEM micrographs of levitated Fe75 Nd25 samples solidified at T = 50 K (a) and T = 75 K (b). The phases are marked as: ␣-Fe (black), Fe17 Nd2 (grey) and Nd-rich phase (white).
out of the range of the pyrometer. The corresponding X-ray diffraction diagram in Fig. 3(a) clearly reveals reflections of ␣-Fe with a bcc structure, the rhombohedral Fe17 Nd2 -phase according to Stadelmeier et al. [5] and dhcp-Nd. By contrast, if the undercooling is increased to T = 75 K, which is 25 K below the peritectic temperature TP , only a single recalescence can be detected in the temperature–time profile as shown in Fig. 1(b). The microstructure of the sample mainly exhibits grains of the
Fe75Nd25
1650 1600
Fe17Nd2
γ-Fe
1550
TL=1533K TP=1483K
1500
Fe17Nd2
1450 1400 1350 1300 0
10
20
30
40
50
60
70
80
90
100
Time t(s) Fig. 1. Temperature–time profiles of levitated Fe75 Nd25 samples with different bulk undercoolings T: (a) two-step solidification at T = 50 K due to primary ␥-Fe and subsequent peritectic formation Fe17 Nd2 and (b) single step solidification at T = 75 K due to primary solidification of the Fe17 Nd2 -phase. The liquidus temperature TL and the peritectic temperature TP are indicated.
Fig. 3. Powder X-ray diffraction patterns of levitated Fe75 Nd25 samples solidified at undercoolings of T = 50 K (a) and T = 75 K (b). The reflections of ␣-Fe (bcc), rhombohedral Fe17 Nd2 and dhcp-Nd are marked.
J. Strohmenger et al. / Materials Science and Engineering A 375–377 (2004) 561–564
intermetallic Fe17 Nd2 -phase instead of dendrites of ␣-Fe (Fig. 2(b)). Locally, a minor fraction of the Fe-phase was found. According to the X-ray diffraction pattern in Fig. 3(b) the amount of ␣-Fe is significantly reduced compared to the sample solidified in two steps at an undercooling of T = 50 K. It is concluded that the crystallization of properitectic ␥-Fe is suppressed, and solidification was initiated by the crystallization of the Fe17 Nd2 -phase. This transition of the solidification mode with rising undercooling is similar to direct crystallization of the peritectic Fe14 Nd2 B1 -phase observed in undercooled ternary Fe–Nd–B alloy melts [4]. The small amount of ␣-Fe in undercooled and solidified Fe–Nd samples is presumably formed during solidification of the residual liquid after recalescence. As can be seen in the temperature–time profile in Fig. 1(b), the temperature of the sample is raised above the peritectic temperature in the region of the two-phase equilibrium of ␥-Fe and the liquid phase, which may explain the secondary solidification of ␥-Fe. In Fe87.5 Nd12.5 alloys the primary crystallization of ␥-Fe could not be suppressed even at the largest undercooling of T = 100 K achieved in the electromagnetic levitation experiments. The temperature–time profile in Fig. 4 reveals two recalescences corresponding to primary ␥-Fe solidification and subsequent formation of the peritectic phase, which is confirmed by the microstructural investigations by SEM and by X-ray diffraction of the solidified samples. In Fe-rich alloys, the direct crystallization of the Fe17 Nd2 -phase on the expense of the properitectic ␥-phase was obtained in drop tube solidified particles as illustrated by the SEM micrographs of solidified particles of Fe89.5 Nd10.5 alloys in Fig. 5(a) and (b). Besides primary ␥ solidification and peritectic formation of the Fe17 Nd2 -phase (Fig. 5(a)), some droplets have been crystallized directly in the Fe17 Nd2 -phase (Fig. 5(b)), which predominantly occurs in the smaller particles with diameters below approximately 0.2 mm. Moreover, the formation of the ␥-phase was completely avoided in small particles, which is presumably caused by the high cool1750
∆T=100K
Temperature T(K)
1700 1650
TL=1638K
Fe87.5Nd12.5
γ-Fe
1600 1550 1500
Fe17Nd2
TP=1483K
1450 1400 1350 1300 1490
1500
1510
1520
1530
1540
1550
1560
Time t(s) Fig. 4. Temperature–time profiles of a levitated Fe87.5 Nd12.5 sample with undercooling of T = 100 K. The liquidus temperature TL and the peritectic temperature TP are indicated.
563
Fig. 5. Back-scattered SEM micrographs of drop tube solidified Fe89.5 Nd10.5 particles with different diameter d showing different solidification modes: (a) d = 0.25 mm: primary ␥-Fe (black), peritectic Fe17 Nd2 -phase (grey) and intergranular Nd-rich phase (white) and (b) d = 0.18 mm: primary solidification of Fe17 Nd2 -phase, whereas ␥-Fe is completely suppressed.
ing rates of the order of 104 K/s and presumptive high undercooling level. Obviously, the undercooling level required for the direct solidification of the peritectic Fe17 Nd2 -phase is increased with rising Fe concentration of the alloy. This behaviour can be explained by thermodynamic considerations. Primary solidification of the peritectic phase is only possible if the melts is undercooled at least below the liquidus temperature TL (Fe17 Nd2 ). The difference of the equilibrium liquidus temperature TL (␥) and TL (Fe17 Nd2 ) is increased with rising Fe content. Similarly, the same tendency is observed for the transition of primary ␥-Fe solidification to the direct crystallization of the Fe14 Nd2 B1 -phase in ternary Fe–Nd–B alloy melts [10]. A detailed understanding of phase selection as a function of undercooling and composition requires a modelling of nucleation and growth of competing properitectic and peritectic phases which are subject of further investigations. First experiments for the in situ observation of phase selection in levitated Fe–Nd samples by diffraction experiments with synchrotron radiation [11] have been performed successfully. The phase sequence consisting of primary ␥-Fe followed by peritectic Fe17 Nd2 -phase has been observed in Fe87.5 Nd12.5 during solidification at an undercooling of
564
J. Strohmenger et al. / Materials Science and Engineering A 375–377 (2004) 561–564
(a)
1750
∆T=50K
Temperature T(K)
1700 1650 1600
TL=1638K
Fe87.5Nd12.5
γ-Fe
1550 1500 1450
Fe17Nd2
TP=1483K
1400
(1)
1350
(2)
(3)
1300 1280 1290 1300 1310 1320 1330 1340 1350 1360 1370 1380 1390 1400
Time t(s)
electromagnetic levitation technique and by the drop tube technique. Undercooling of bulk samples up to 100 K prior to solidification have been achieved in the electromagnetic levitation experiments. In Fe75 Nd25 alloys solidified at undercoolings larger than 70 K, the amount of residual ␣-Fe was significantly reduced indicating that the crystallization of the properitectic ␥-Fe-phase is suppressed in favour of the peritectic Fe17 Nd2 -phase with rising undercooling. In Fe87.5 Nd12.5 and Fe89.5 Nd10.5 alloys, the crystallization of ␥-Fe could not be suppressed in levitated samples, but it was completely avoided in drop tube solidification experiments on small particles. It is concluded that the suppression of primary ␥-Fe requires larger undercooling levels by increasing the Fe concentration.
(b) 1,0
* Fe17Nd2 γ fcc-γ-Fe
Fe87,5Nd12,5 ∆T=50K
Intensity
0,8
0,6
γ
γ
period 1
γ 0,4
*γ
period 2
γ
0,2
** 0,0
* *
γ
*
period 3
Acknowledgements The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support under No. He 1601/14-1 (J.S.) and Dr. M. Kolbe for his kind help in SEM analysis. One of the authors, J. Gao, is grateful to the Alexander von Humboldt Foundation for a long-term research fellowship.
References
1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4 3,6 3,8 4,0 4,2 4,4 4,6 4,8 5,0
10
Scattering vector q(1/m)*10
Fig. 6. Temperature–time profile (a) of a levitated Fe87.5 Nd12.5 sample (T = 50 K) appertaining to the synchrotron radiation diffraction patterns, shown in (b). Patterns taken during three periods reveal (1) liquid state, (2) ␥-Fe solidification and (3) peritectic Fe17 Nd2 formation.
T = 50 K (Fig. 6(a)). The diffraction patterns (Fig. 6(b)) taken during the three periods ((1): liquid, (2): ␥-Fe solidification, (3): peritectic Fe17 Nd2 formation) proves the assumed phase formation, such as in Fig. 4.
4. Summary Phase formation in undercooled Fe–Nd alloy melts with different compositions have been investigated by the
[1] D.M. Herlach, Mater. Sci. Eng. R 12 (1994) 177. [2] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. [3] J.J. Croat, J.F. Herbst, R.W. Lee, F.E. Pinkerton, J. Appl. Phys. 55 (1984) 2078. [4] T. Volkmann, J. Gao, D.M. Herlach, Appl. Phys. Lett. 80 (2002) 1915. [5] H.H. Stadelmaier, G. Schneider, M. Ellner, J. Less-Common Met. 115 (1986) L11. [6] G. Schneider, F.J.G. Landgraf, F.P. Missel, J. Less-Common Met. 153 (1989) 169. [7] F.J.G. Landgraf, F.P. Missel, H.R. Rechenberg, G. Schneider, V. Villas-Boas, J.M. Moreau, L. Paccard, J.P. Nozieres, J. Appl. Phys. 70 (1991) 6125. [8] J.F. Cannon, D.L. Robertson, H.T. Hall, Mater. Res. Bull. 7 (1972) 5. [9] I.A. Santos, S. Gama, J. Appl. Phys. 86 (1999) 2334. [10] T. Volkmann, J. Gao, J. Strohmenger, D.M. Herlach, 375–377 (2004) 1153. [11] C. Notthoff, B. Feuerbacher, H. Franz, D.M. Herlach, D. Holland-Moritz, Phys Rev. Lett. 86 (2001) 1038.
Phase selection in undercooled Fe–Nd alloy melts J. Strohmenger a,∗ , T. Volkmann a , J. Gao a,b , D.M. Herlach a b
a Institute of Space Simulation, Cologne D-51170, Germany Department of Applied Physics, Northwestern Polytechnical University, Xian 710072, China
Abstract Undercooling experiments on Fe89.5 Nd10.5 , Fe87.5 Nd12.5 and Fe75 Nd25 alloy melts have been performed by electromagnetic levitation and drop tube experiments. Phase formation in Fe–Nd alloys is of interest due to the ternary compound Fe14 Nd2 B1, which exhibits extraordinary hard magnetic properties. Under equilibrium solidification conditions, melts of the investigated composition range solidify primarily in the ␥-Fe solid solution, which is followed by the peritectic formation of the chemically ordered Fe17 Nd2 -phase. In electromagnetic levitated Fe75 Nd25 alloys the primary crystallisation of ␥-Fe could be suppressed in favour of the peritectic phase with rising undercooling. In Fe-rich alloys the direct crystallisation of the Fe17 Nd2 -phase was only observed in drop tube solidified particles with diameters below 0.2 mm. © 2003 Elsevier B.V. All rights reserved. Keywords: Rare-earth magnetic alloys; Undercooling of melts; Containerless processing; Metastable phases
1. Introduction It is well known that undercooling of melts gives access to alternative solidification pathways [1]. The study of the Fe–Nd system is of relevance for an understanding of phase formation in ternary Fe–Nd–B alloy melts, which are used for the development of high-performance magnetic materials based on the hard magnetic compound Fe14 Nd2 B1 [2,3]. Recently, we reported on undercooling experiments on Fe–Nd–B alloy melts [4] showing that primary ␥-Fe solidification can be avoided and that the peritectic Fe14 Nd2 B1 -phase can be crystallized directly from undercooled melts. In order to obtain more insight in the crystallization behaviour and phase selection with respect to the competition of properitectic and peritectic phases in undercooled rare-earth-transition metal alloy melts, binary Fe–Nd alloys have been investigated by containerless processing of melts. Up to now, the intermetallic compounds Fe17 Nd2 [5], Fe17 Nd5 [6,7], and Fe2 Nd [8,9] have been proposed as phases that are formed by a peritectic reaction. The present investigations are focussed on Fe–Nd melts with concentrations between 10.5 and 25 at.% Nd. Under equilibrium solidification conditions the melt solidifies primarily in the ␥-Fe solid solution, which is followed by the formation of ∗ Corresponding author. Tel.: +49-2203-601-4583; fax: +49-2203-61768. E-mail address: [email protected] (J. Strohmenger).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.191
the peritectic Nd2 Fe17 -phase. Since the peritectic transformation is diffusion-controlled the dissolution of properitectic ␥-Fe is incomplete in conventional casting processes, leading to an significant amount of residual ␣-Fe at room temperature. It is shown that the primary crystallization of ␥-Fe is suppressed in favour of the peritectic Fe17 Nd2 -phase with rising undercooling of the melt prior to solidification.
2. Experimental details For both, electromagnetic levitation and drop tube experiments samples of about 1.5 g in mass were prepared from the elements (purity at least 99.9%) by arc melting under argon atmosphere (99.999% purity). The electromagnetic levitation facility consists of a conical coil that is connected to a radio-frequency generator for levitating and inductively heating of the metallic sample. Both, the sample and the coil are placed in an high vacuum chamber. After evacuation to a vacuum in the order of 10−7 mbar, the levitation chamber was backfilled with helium cooling gas (99.999% purity). Undercooling of the highly reactive Fe–Nd melts was achieved by levitation processing under He pressures below 50 mbar enabling an evaporization of Nd-oxides [4]. The temperature of the levitated samples was measured by a pyrometer with a relative accuracy of ±5 K during the entire experiment cycle in order to determine the undercooling level prior to solidification. The pyrometer was
562
J. Strohmenger et al. / Materials Science and Engineering A 375–377 (2004) 561–564
calibrated at the liquidus temperature, which is detected in the temperature–time profile during melting of the sample. In drop tube experiments, the melt is atomised in droplets with diameters in the range of 0.1–2 mm. During the free fall of nearly 8 m in a high purity He-gas atmosphere, the droplets were undercooled and solidified at cooling rates between 102 and 104 K/s. The disintegration of the melt into many particles led to an isolation of impurities, which act as heterogeneous nucleation sites so that deep undercoolings of the droplets can be achieved. Due to the high cooling rates, it is possible to retain metastable phases solidified from the undercooled melt. Phase constituents and microstructure of levitated and solidified samples and of drop tube solidified particles were analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The compositions of the different phases have been verified by energy-dispersive X-ray analysis (EDX).
3. Results and discussion Two different solidification pathways have been found as shown in Fig. 1 by the temperature–time profiles of levitated Fe75 Nd25 melts for different undercooling levels T. In Fig. 1(a) at T = 50 K, a two-step solidification is observed which is due to the primary crystallization of ␥-Fe and subsequent formation of the Fe17 Nd2 -phase by a peritectic reaction as expected from the equilibrium phase diagram. According to the microstructural analysis by SEM and EDX in Fig. 2(a), the sample consists of the Fe-phase with more than 99 at.% Fe (black) which is surrounded by the intermetallic Fe17 Nd2 -phase (grey). The intergranular phase (white) with a Nd concentration of 90 at.% is formed as soon as solidification is completed at low temperatures
1750
Temperature T(K)
(b) ∆T=75K
(a) ∆T=50K
1700
Fig. 2. Back-scattered SEM micrographs of levitated Fe75 Nd25 samples solidified at T = 50 K (a) and T = 75 K (b). The phases are marked as: ␣-Fe (black), Fe17 Nd2 (grey) and Nd-rich phase (white).
out of the range of the pyrometer. The corresponding X-ray diffraction diagram in Fig. 3(a) clearly reveals reflections of ␣-Fe with a bcc structure, the rhombohedral Fe17 Nd2 -phase according to Stadelmeier et al. [5] and dhcp-Nd. By contrast, if the undercooling is increased to T = 75 K, which is 25 K below the peritectic temperature TP , only a single recalescence can be detected in the temperature–time profile as shown in Fig. 1(b). The microstructure of the sample mainly exhibits grains of the
Fe75Nd25
1650 1600
Fe17Nd2
γ-Fe
1550
TL=1533K TP=1483K
1500
Fe17Nd2
1450 1400 1350 1300 0
10
20
30
40
50
60
70
80
90
100
Time t(s) Fig. 1. Temperature–time profiles of levitated Fe75 Nd25 samples with different bulk undercoolings T: (a) two-step solidification at T = 50 K due to primary ␥-Fe and subsequent peritectic formation Fe17 Nd2 and (b) single step solidification at T = 75 K due to primary solidification of the Fe17 Nd2 -phase. The liquidus temperature TL and the peritectic temperature TP are indicated.
Fig. 3. Powder X-ray diffraction patterns of levitated Fe75 Nd25 samples solidified at undercoolings of T = 50 K (a) and T = 75 K (b). The reflections of ␣-Fe (bcc), rhombohedral Fe17 Nd2 and dhcp-Nd are marked.
J. Strohmenger et al. / Materials Science and Engineering A 375–377 (2004) 561–564
intermetallic Fe17 Nd2 -phase instead of dendrites of ␣-Fe (Fig. 2(b)). Locally, a minor fraction of the Fe-phase was found. According to the X-ray diffraction pattern in Fig. 3(b) the amount of ␣-Fe is significantly reduced compared to the sample solidified in two steps at an undercooling of T = 50 K. It is concluded that the crystallization of properitectic ␥-Fe is suppressed, and solidification was initiated by the crystallization of the Fe17 Nd2 -phase. This transition of the solidification mode with rising undercooling is similar to direct crystallization of the peritectic Fe14 Nd2 B1 -phase observed in undercooled ternary Fe–Nd–B alloy melts [4]. The small amount of ␣-Fe in undercooled and solidified Fe–Nd samples is presumably formed during solidification of the residual liquid after recalescence. As can be seen in the temperature–time profile in Fig. 1(b), the temperature of the sample is raised above the peritectic temperature in the region of the two-phase equilibrium of ␥-Fe and the liquid phase, which may explain the secondary solidification of ␥-Fe. In Fe87.5 Nd12.5 alloys the primary crystallization of ␥-Fe could not be suppressed even at the largest undercooling of T = 100 K achieved in the electromagnetic levitation experiments. The temperature–time profile in Fig. 4 reveals two recalescences corresponding to primary ␥-Fe solidification and subsequent formation of the peritectic phase, which is confirmed by the microstructural investigations by SEM and by X-ray diffraction of the solidified samples. In Fe-rich alloys, the direct crystallization of the Fe17 Nd2 -phase on the expense of the properitectic ␥-phase was obtained in drop tube solidified particles as illustrated by the SEM micrographs of solidified particles of Fe89.5 Nd10.5 alloys in Fig. 5(a) and (b). Besides primary ␥ solidification and peritectic formation of the Fe17 Nd2 -phase (Fig. 5(a)), some droplets have been crystallized directly in the Fe17 Nd2 -phase (Fig. 5(b)), which predominantly occurs in the smaller particles with diameters below approximately 0.2 mm. Moreover, the formation of the ␥-phase was completely avoided in small particles, which is presumably caused by the high cool1750
∆T=100K
Temperature T(K)
1700 1650
TL=1638K
Fe87.5Nd12.5
γ-Fe
1600 1550 1500
Fe17Nd2
TP=1483K
1450 1400 1350 1300 1490
1500
1510
1520
1530
1540
1550
1560
Time t(s) Fig. 4. Temperature–time profiles of a levitated Fe87.5 Nd12.5 sample with undercooling of T = 100 K. The liquidus temperature TL and the peritectic temperature TP are indicated.
563
Fig. 5. Back-scattered SEM micrographs of drop tube solidified Fe89.5 Nd10.5 particles with different diameter d showing different solidification modes: (a) d = 0.25 mm: primary ␥-Fe (black), peritectic Fe17 Nd2 -phase (grey) and intergranular Nd-rich phase (white) and (b) d = 0.18 mm: primary solidification of Fe17 Nd2 -phase, whereas ␥-Fe is completely suppressed.
ing rates of the order of 104 K/s and presumptive high undercooling level. Obviously, the undercooling level required for the direct solidification of the peritectic Fe17 Nd2 -phase is increased with rising Fe concentration of the alloy. This behaviour can be explained by thermodynamic considerations. Primary solidification of the peritectic phase is only possible if the melts is undercooled at least below the liquidus temperature TL (Fe17 Nd2 ). The difference of the equilibrium liquidus temperature TL (␥) and TL (Fe17 Nd2 ) is increased with rising Fe content. Similarly, the same tendency is observed for the transition of primary ␥-Fe solidification to the direct crystallization of the Fe14 Nd2 B1 -phase in ternary Fe–Nd–B alloy melts [10]. A detailed understanding of phase selection as a function of undercooling and composition requires a modelling of nucleation and growth of competing properitectic and peritectic phases which are subject of further investigations. First experiments for the in situ observation of phase selection in levitated Fe–Nd samples by diffraction experiments with synchrotron radiation [11] have been performed successfully. The phase sequence consisting of primary ␥-Fe followed by peritectic Fe17 Nd2 -phase has been observed in Fe87.5 Nd12.5 during solidification at an undercooling of
564
J. Strohmenger et al. / Materials Science and Engineering A 375–377 (2004) 561–564
(a)
1750
∆T=50K
Temperature T(K)
1700 1650 1600
TL=1638K
Fe87.5Nd12.5
γ-Fe
1550 1500 1450
Fe17Nd2
TP=1483K
1400
(1)
1350
(2)
(3)
1300 1280 1290 1300 1310 1320 1330 1340 1350 1360 1370 1380 1390 1400
Time t(s)
electromagnetic levitation technique and by the drop tube technique. Undercooling of bulk samples up to 100 K prior to solidification have been achieved in the electromagnetic levitation experiments. In Fe75 Nd25 alloys solidified at undercoolings larger than 70 K, the amount of residual ␣-Fe was significantly reduced indicating that the crystallization of the properitectic ␥-Fe-phase is suppressed in favour of the peritectic Fe17 Nd2 -phase with rising undercooling. In Fe87.5 Nd12.5 and Fe89.5 Nd10.5 alloys, the crystallization of ␥-Fe could not be suppressed in levitated samples, but it was completely avoided in drop tube solidification experiments on small particles. It is concluded that the suppression of primary ␥-Fe requires larger undercooling levels by increasing the Fe concentration.
(b) 1,0
* Fe17Nd2 γ fcc-γ-Fe
Fe87,5Nd12,5 ∆T=50K
Intensity
0,8
0,6
γ
γ
period 1
γ 0,4
*γ
period 2
γ
0,2
** 0,0
* *
γ
*
period 3
Acknowledgements The authors thank the Deutsche Forschungsgemeinschaft (DFG) for financial support under No. He 1601/14-1 (J.S.) and Dr. M. Kolbe for his kind help in SEM analysis. One of the authors, J. Gao, is grateful to the Alexander von Humboldt Foundation for a long-term research fellowship.
References
1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4 3,6 3,8 4,0 4,2 4,4 4,6 4,8 5,0
10
Scattering vector q(1/m)*10
Fig. 6. Temperature–time profile (a) of a levitated Fe87.5 Nd12.5 sample (T = 50 K) appertaining to the synchrotron radiation diffraction patterns, shown in (b). Patterns taken during three periods reveal (1) liquid state, (2) ␥-Fe solidification and (3) peritectic Fe17 Nd2 formation.
T = 50 K (Fig. 6(a)). The diffraction patterns (Fig. 6(b)) taken during the three periods ((1): liquid, (2): ␥-Fe solidification, (3): peritectic Fe17 Nd2 formation) proves the assumed phase formation, such as in Fig. 4.
4. Summary Phase formation in undercooled Fe–Nd alloy melts with different compositions have been investigated by the
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