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Growth kinetics of undercooled Fe–Co melts
Journal of Magnetism and Magnetic Materials 242–245 (2002) 285–287
Growth kinetics of undercooled Fe–Co melts . R. Hermann*, W. Loser Institute of Solid State and Materials Research, Helmholtzstr.20, D-01069 Dresden, Germany
Abstract The growth kinetics of undercooled melts of soft magnetic Fe 30 at% Co and Fe 50 at% Co alloys was investigated using the electromagnetic levitation technique. It could be shown that primary solidification of the metastable BCC phase is favoured by exceeding critical undercooling levels of 65–80 K and 140–160 K, respectively. The growth velocity of the metastable phase is considerably lowered compared to the stable one which agrees very well with the model calculations of the dendrite growth. r 2002 Elsevier Science B.V. All rights reserved. Keywords: FerromagnetsFsoft; Metastable phases; Cooling; Magnetic levitation
1. Introduction The non-equilibrium solidification of undercooled metallic melts offers the opportunity of improvement of the alloy parameters by the formation of metastable phases and grain refinement. The electromagnetic levitation technique allows the adjustment of high undercooling levels by containerless processing. Recently, much interest has been focused on microstructure evolution and metastable phase formation in undercooled Fe–Co melts [1]. Soft magnetic Fe–Co alloys display the primary FCC phase solidification for >18 at% Co in conventional near-equilibrium solidification processes. However, the transition to primary metastable BCC phase has been achieved in submicron powders processed by the electrohydrodynamic atomization technique over a wide range of compositions up to 90 at% Co [2]. It is known that the crystal habit influences the microstructure and the magnetic properties, an important factor referring to industrial application. The Fe–Co phase diagram exhibits a peritectic reaction in the Fe-rich part suggesting the competitive growth of the metastable d-phase (BCC) and the stable *Corresponding author. Institute of Solid State & Materials Research (IFW), Institute of Metallic Materials, mail box 270016, D-01171 Dresden, Germany. Tel.: +49-351-4659-646; fax: +49-351-4659-541. E-mail address: [email protected] (R. Hermann).
g-phase (FCC) from the undercooled state. Neither critical process parameters for the occurrence of the metastable phase nor growth and transformation kinetics have been specified so far. The aim of this work is the study of the growth kinetics and the metastable phase formation in undercooled Fe–Co melts by using the electromagnetic levitation technique.
2. Experimental Samples with the compositions of Fe 30 at% Co and Fe 50 at% Co, respectively were prepared from elemental Fe and Co by melting in a cold crucible induction furnace under argon atmosphere. Spheres of about 6 mm in diameter and about 1.4 g mass were melted containerlessly and solidified in an electromagnetic levitation facility under helium gas atmosphere. The molten samples were cooled by a gas stream with a cooling rate of about 10 K/s. The temperature of the sample was monitored by a two-colour pyrometer with a sampling rate of 50 Hz and a relative accuracy of 73 K. For resolving the fast solidification event, a silicon photodiode was used enabling sample rate of 500 MHz. The sampling rate of the transient recorder was 1.5 MHz. The growth velocity of the primary phase was measured in situ from the temperature–time characteristics.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 2 5 5 - 0
286
R. Hermann, W. Loser / Journal of Magnetism and Magnetic Materials 242–245 (2002) 285–287 .
3. Results and discussion The Fe–Co melts were undercooled up to 270 K below their liquidus temperature in the electromagnetic levitation device. In order to set the absolute temperature scale, a two colour pyrometer was used in combination with a silicon photodiode. The samples were melted and superheated for purification and then cooled till the onset of spontaneous solidification. The liquidus temperatures of the alloys were estimated from the heating cycles during the levitation experiments to 15051C for the Fe 30 at% Co alloy and 14941C for the Fe 50 at% Co alloy. Fig. 1a and b show typical temperature–timeprofiles of an Fe 30 at% Co and an Fe 50 at% Co alloy, respectively, solidified spontaneously during levitation at different undercooling levels below their liquidus temperature. The solidification process crosses from a single-step to a two-step recalescence if the undercooling
Fig. 1. Typical temperature–time characteristics of the recalescence process for different undercooling levels; (a) Fe 30 at% Co, (b) Fe 50 at% Co.
level exceeds a critical value. The critical undercooling levels could be determined from numerous temperature– time measurements to 65–80 K below the liquidus temperature of the Fe 30 at% Co alloy and 140–160 K below the liquidus temperature of the Fe 50 at% Co alloy, respectively. The two-step recalescence event indicates the interim formation of a metastable phase. Regarding the phase diagram of Fe–Co and considering the metastable extension of the liquidus line of the involved phases, it can be attributed to the primary solidification of the metastable d-Fe (BCC) followed by the formation of the stable g-Fe phase (FCC) in a very short time of few milliseconds. Finally, at about 1250 K, the transformation from g-Fe (FCC) to a-Fe phase (BCC) takes place. The small solidification interval between the solidus line and the liquidus line of the Fe– Co system just as the expected metastable extensions enabled the relative precise determination of the liquidus temperature of the metastable phase from the in situtemperature–time measurements. As visible in Fig. 1a and b, a plateau temperature is achieved if the solidification of the metastable phase is completed. This temperature corresponds to the metastable liquidus line of the BCC phase. The metastable liquidus temperatures were estimated to 14901C for the Fe 30 at% Co alloy and 14501C for the Fe 50 at% Co alloy, respectively, which is much smaller than the values calculated from Li et al. [1]. The length of the plateau region following the solidification of the metastable FCC phase shortens with increasing undercooling level. The reason is the increasing volume fraction of the formed metastable phase and the reduction of the volume fraction of the residual melt, transforming to stable FCC phase by simultaneous rise of the growth velocity. The growth velocities of the primary solidifying phases were calculated by estimating the time within the planar solidification front crosses the viewing field of the photodiode to several m/s. The dependence of the growth velocity on the undercooling level is shown in Fig. 2a and b. A distinct reduction of the growth velocity of the metastable phase compared to the stable one was detected. The growth velocity of the primary stable FCC phase increases with increasing undercooling up to the critical value. A sudden change of the growth velocity as function of undercooling is attributed to the primary formation of the metastable BCC phase. It is obvious that the growth velocity of the metastable BCC phase is lowered which is more distinct going to higher Co amounts due to the increase of the critical undercooling level and growth velocity. The experiments were completed with numerical results by calculating the dendrite tip velocity in dependence on the undercooling level using a Lipton–Kurz–Trivedi (LKT) theory of dendrite growth in undercooled melts which implies the velocity dependence of the partition coefficient and the liquidus slope. For detailed description of the theory it is referred to Ref. [3]. The liquidus
R. Hermann, W. Loser / Journal of Magnetism and Magnetic Materials 242–245 (2002) 285–287 .
287
experiments. The used parameter set is similar to that used in Ref. [1]. As visible in Fig. 2a and b, the calculated growth velocities agree very well with the experimental results. It clearly supports a primary dendrite growth mechanism in the Fe–Co system.
4. Conclusions *
*
*
*
*
Fe 30 at% Co and Fe 50 at% Co melt drops were undercooled up to 270 K below the liquidus temperature using the electromagnetic levitation technique. The recalescence curves show a clear transition from a single-step to a two-step recalescence by exceeding a critical undercooling level, indicating the transition to primary formation of the metastable d (BCC) phase. The critical melt undercoolings below the liquidus temperature amount to 65–80 K for the Fe 30 at% Co and 140–160 K for the Fe 50 at% Co alloy. The growth velocity as function of undercooling of the solidification front was estimated from temperature–time characteristics to several m/s. The calculated dendrite tip velocities from the (LKTtheory) agree well with the experimental data, supporting the primary dendrite growth mechanism in this type of alloys.
Acknowledgements The authors wish to thank Mr. H.-G. Lindenkreuz, . Mr. O. Filip and Mrs. M. Frommel for carrying out undercooling experiments and sample preparation.
References Fig. 2. Growth velocity vs. undercooling level; symbols: experimental, line: calculated dendrite growth velocity (LKTmodel); (a) Fe30 at% Co, (b) Fe50 at% Co.
temperatures for the metastable phase were taken from the temperature–time characteristics of the levitation
[1] Mingjun Li, Xin Lin, Guangsheng Song, Gencang Yang, Yahoe Zhou, Mater. Sci. Eng. A 268 (1999) 90. [2] Y.-W. Kim, T.F. Kelly, Acta metall. Mater. 39 (1991) 3237. [3] W. Kurz, D.J. Fischer, Fundamentals of Solidification, Trans Tech Publications, Aedermannsdorf, 1992.
Growth kinetics of undercooled Fe–Co melts . R. Hermann*, W. Loser Institute of Solid State and Materials Research, Helmholtzstr.20, D-01069 Dresden, Germany
Abstract The growth kinetics of undercooled melts of soft magnetic Fe 30 at% Co and Fe 50 at% Co alloys was investigated using the electromagnetic levitation technique. It could be shown that primary solidification of the metastable BCC phase is favoured by exceeding critical undercooling levels of 65–80 K and 140–160 K, respectively. The growth velocity of the metastable phase is considerably lowered compared to the stable one which agrees very well with the model calculations of the dendrite growth. r 2002 Elsevier Science B.V. All rights reserved. Keywords: FerromagnetsFsoft; Metastable phases; Cooling; Magnetic levitation
1. Introduction The non-equilibrium solidification of undercooled metallic melts offers the opportunity of improvement of the alloy parameters by the formation of metastable phases and grain refinement. The electromagnetic levitation technique allows the adjustment of high undercooling levels by containerless processing. Recently, much interest has been focused on microstructure evolution and metastable phase formation in undercooled Fe–Co melts [1]. Soft magnetic Fe–Co alloys display the primary FCC phase solidification for >18 at% Co in conventional near-equilibrium solidification processes. However, the transition to primary metastable BCC phase has been achieved in submicron powders processed by the electrohydrodynamic atomization technique over a wide range of compositions up to 90 at% Co [2]. It is known that the crystal habit influences the microstructure and the magnetic properties, an important factor referring to industrial application. The Fe–Co phase diagram exhibits a peritectic reaction in the Fe-rich part suggesting the competitive growth of the metastable d-phase (BCC) and the stable *Corresponding author. Institute of Solid State & Materials Research (IFW), Institute of Metallic Materials, mail box 270016, D-01171 Dresden, Germany. Tel.: +49-351-4659-646; fax: +49-351-4659-541. E-mail address: [email protected] (R. Hermann).
g-phase (FCC) from the undercooled state. Neither critical process parameters for the occurrence of the metastable phase nor growth and transformation kinetics have been specified so far. The aim of this work is the study of the growth kinetics and the metastable phase formation in undercooled Fe–Co melts by using the electromagnetic levitation technique.
2. Experimental Samples with the compositions of Fe 30 at% Co and Fe 50 at% Co, respectively were prepared from elemental Fe and Co by melting in a cold crucible induction furnace under argon atmosphere. Spheres of about 6 mm in diameter and about 1.4 g mass were melted containerlessly and solidified in an electromagnetic levitation facility under helium gas atmosphere. The molten samples were cooled by a gas stream with a cooling rate of about 10 K/s. The temperature of the sample was monitored by a two-colour pyrometer with a sampling rate of 50 Hz and a relative accuracy of 73 K. For resolving the fast solidification event, a silicon photodiode was used enabling sample rate of 500 MHz. The sampling rate of the transient recorder was 1.5 MHz. The growth velocity of the primary phase was measured in situ from the temperature–time characteristics.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 2 5 5 - 0
286
R. Hermann, W. Loser / Journal of Magnetism and Magnetic Materials 242–245 (2002) 285–287 .
3. Results and discussion The Fe–Co melts were undercooled up to 270 K below their liquidus temperature in the electromagnetic levitation device. In order to set the absolute temperature scale, a two colour pyrometer was used in combination with a silicon photodiode. The samples were melted and superheated for purification and then cooled till the onset of spontaneous solidification. The liquidus temperatures of the alloys were estimated from the heating cycles during the levitation experiments to 15051C for the Fe 30 at% Co alloy and 14941C for the Fe 50 at% Co alloy. Fig. 1a and b show typical temperature–timeprofiles of an Fe 30 at% Co and an Fe 50 at% Co alloy, respectively, solidified spontaneously during levitation at different undercooling levels below their liquidus temperature. The solidification process crosses from a single-step to a two-step recalescence if the undercooling
Fig. 1. Typical temperature–time characteristics of the recalescence process for different undercooling levels; (a) Fe 30 at% Co, (b) Fe 50 at% Co.
level exceeds a critical value. The critical undercooling levels could be determined from numerous temperature– time measurements to 65–80 K below the liquidus temperature of the Fe 30 at% Co alloy and 140–160 K below the liquidus temperature of the Fe 50 at% Co alloy, respectively. The two-step recalescence event indicates the interim formation of a metastable phase. Regarding the phase diagram of Fe–Co and considering the metastable extension of the liquidus line of the involved phases, it can be attributed to the primary solidification of the metastable d-Fe (BCC) followed by the formation of the stable g-Fe phase (FCC) in a very short time of few milliseconds. Finally, at about 1250 K, the transformation from g-Fe (FCC) to a-Fe phase (BCC) takes place. The small solidification interval between the solidus line and the liquidus line of the Fe– Co system just as the expected metastable extensions enabled the relative precise determination of the liquidus temperature of the metastable phase from the in situtemperature–time measurements. As visible in Fig. 1a and b, a plateau temperature is achieved if the solidification of the metastable phase is completed. This temperature corresponds to the metastable liquidus line of the BCC phase. The metastable liquidus temperatures were estimated to 14901C for the Fe 30 at% Co alloy and 14501C for the Fe 50 at% Co alloy, respectively, which is much smaller than the values calculated from Li et al. [1]. The length of the plateau region following the solidification of the metastable FCC phase shortens with increasing undercooling level. The reason is the increasing volume fraction of the formed metastable phase and the reduction of the volume fraction of the residual melt, transforming to stable FCC phase by simultaneous rise of the growth velocity. The growth velocities of the primary solidifying phases were calculated by estimating the time within the planar solidification front crosses the viewing field of the photodiode to several m/s. The dependence of the growth velocity on the undercooling level is shown in Fig. 2a and b. A distinct reduction of the growth velocity of the metastable phase compared to the stable one was detected. The growth velocity of the primary stable FCC phase increases with increasing undercooling up to the critical value. A sudden change of the growth velocity as function of undercooling is attributed to the primary formation of the metastable BCC phase. It is obvious that the growth velocity of the metastable BCC phase is lowered which is more distinct going to higher Co amounts due to the increase of the critical undercooling level and growth velocity. The experiments were completed with numerical results by calculating the dendrite tip velocity in dependence on the undercooling level using a Lipton–Kurz–Trivedi (LKT) theory of dendrite growth in undercooled melts which implies the velocity dependence of the partition coefficient and the liquidus slope. For detailed description of the theory it is referred to Ref. [3]. The liquidus
R. Hermann, W. Loser / Journal of Magnetism and Magnetic Materials 242–245 (2002) 285–287 .
287
experiments. The used parameter set is similar to that used in Ref. [1]. As visible in Fig. 2a and b, the calculated growth velocities agree very well with the experimental results. It clearly supports a primary dendrite growth mechanism in the Fe–Co system.
4. Conclusions *
*
*
*
*
Fe 30 at% Co and Fe 50 at% Co melt drops were undercooled up to 270 K below the liquidus temperature using the electromagnetic levitation technique. The recalescence curves show a clear transition from a single-step to a two-step recalescence by exceeding a critical undercooling level, indicating the transition to primary formation of the metastable d (BCC) phase. The critical melt undercoolings below the liquidus temperature amount to 65–80 K for the Fe 30 at% Co and 140–160 K for the Fe 50 at% Co alloy. The growth velocity as function of undercooling of the solidification front was estimated from temperature–time characteristics to several m/s. The calculated dendrite tip velocities from the (LKTtheory) agree well with the experimental data, supporting the primary dendrite growth mechanism in this type of alloys.
Acknowledgements The authors wish to thank Mr. H.-G. Lindenkreuz, . Mr. O. Filip and Mrs. M. Frommel for carrying out undercooling experiments and sample preparation.
References Fig. 2. Growth velocity vs. undercooling level; symbols: experimental, line: calculated dendrite growth velocity (LKTmodel); (a) Fe30 at% Co, (b) Fe50 at% Co.
temperatures for the metastable phase were taken from the temperature–time characteristics of the levitation
[1] Mingjun Li, Xin Lin, Guangsheng Song, Gencang Yang, Yahoe Zhou, Mater. Sci. Eng. A 268 (1999) 90. [2] Y.-W. Kim, T.F. Kelly, Acta metall. Mater. 39 (1991) 3237. [3] W. Kurz, D.J. Fischer, Fundamentals of Solidification, Trans Tech Publications, Aedermannsdorf, 1992.