Structural Elongation and Alignment in an Fe-0.4C Alloy by Isothermal Ferrite Transformation in High Magnetic Fields

Transcription

1 Materials Transactions, Vol. 44, No. 12 (2003) pp to 2536 Special Issue on Structural and Functional Control of Materials through Solid-Solid Phase Transformations in High Magnetic Field #2003 The Japan Institute of Metals Structural Elongation and Alignment in an Fe-0.4C Alloy by Isothermal Ferrite Transformation in High Magnetic Fields Xin Jiang Hao, Hideyuki Ohtsuka* and Hitoshi Wada Tsukuba Magnet Laboratory, National Institute for Materials Science, Tsukuba , Japan Structural elongation and alignment in an Fe-0.4C alloy transformed in high magnetic fields has been studied by quantitative microscopy analysis. An elongated and aligned two-phase structure is formed in high magnetic fields by isothermal ferrite transformation both below and above Curie temperature. Equiaxed ferrite grains nucleate randomly at austenite grain boundaries and they become elongated by preferred growth along the direction of applied magnetic field. Below Curie temperature, the degree of elongation increases with increasing transformation temperature, whereas it decreases above Curie temperature. Small chemical driving force for ferrite precipitation and large magnetization of ferrite in high magnetic fields favor the formation of elongated structure. (Received July 15, 2003; Accepted October 16, 2003) Keywords: structural elongation and alignment, ferrite transformation, high magnetic field, quantitative microscopy, isothermal transformation 1. Introduction Recently with the rapid development of high field magnets, it has been shown that high magnetic field has influences on the solid/solid phase transformations such as martensite, 1) ferrite 2 11) and pearlite 12) transformation behavior and recrystallization behavior and texture 13 17) of Fe-based alloys. The elongated and aligned structure is a unique feature of transformed structure in magnetic fields. It was first reported by Ohtsuka et al. that an elongated and aligned structure is formed by austenite () to ferrite () transformation during slow cooling in a high magnetic field. 4) A method of quantitative characterization of the degree of structural elongation has been developed in our previous paper. 6) It was found that the degree of structural elongation formed by slow cooling increases continuously with increasing magnetic field strength up to 10 T. The structural alignment of ferrite was also reported by Maruta et al. 10) and Shimotomai et al., 11) and they concluded that a combination of prior rolling and transformation in an external field is essential to yield a two-phase microstructure with elongated ferrite grains, but elongated structure is obtained without any deformation as shown in this paper and previous papers. 4,6,7) The purpose of this study is to clarify: (1) whether ferrite grains are elongated and aligned by isothermal transformation, (2) whether ferrite grains are elongated and aligned above Curie temperature of -Fe, (3) whether the elongated structure is formed in nucleation stage or growth stage, and (4) the dependence of the degree of elongation on transformation temperature. 2. Experimental Procedures The alloy used in the present study was Fe-0.4C alloy prepared by vacuum induction melting and its chemical composition was Fe-0.41C-0.08Si-0.003Al (in mass%). The Ae 3 temperature of this alloy was calculated by Thermo-Calc to be about 785 C. After hot rolling specimens were *Corresponding author, ohtsuka.hideyuki@nims.go.jp machined to 5 mm 5 mm 1 mm, and then set in a vacuum furnace, which was installed in a helium-free type superconducting magnet with a bore size of 100 mm. A magnetic field perpendicular to the 5 mm 5 mm specimen s surface was increased to 10 T in 27 min before austenization and kept constant during austenization, isothermal holding and subsequent cooling, and then decreased to 0 T. Specimens were fixed at the center of magnetic field and the magnetic force to the specimen is negligible. Specimens were austenitized at 1000 C for 15 min and cooled to C at a cooling rate of 50 C/min, then isothermally held for ferrite transformation, followed by rapid cooling down to room temperature by helium gas. The specimen temperature can be decreased from 1000 to 400 C in about 40 seconds by helium gas quenching. Microstructure observation was performed on the plane parallel to the direction of magnetic field by optical microscope after polishing and 3% nital etching, and the direction of applied magnetic field is shown by an arrow in each picture. 3. Results and Discussion 3.1 Effects of magnetic field on isothermal ferrite transformation Figure 1 shows the micrographs of specimens transformed at 740 C, which is lower than the Curie temperature (T c ¼ 770 C) of -Fe, for one hour without (a) and with a magnetic field of 10 T (b). The white grains are precipitated from during isothermal holding. The dark regions are pearlite transformed from during quenching by helium gas. Without an applied magnetic field, most of ferrite grains are equiaxed. With an applied magnetic field, the microstructure is quite different. First, the volume fraction of transformed ferrite is higher than that without magnetic field. Second, the shape of many ferrite grains is rod or needle-like, and its long axis mostly aligns with the direction of magnetic field. Figure 2 shows the microstructures of specimens heat treated at 785 C (higher than T c ) for 240 min. Without magnetic field (a), even after isothermal holding at 785 C (Ae 3 temperature of this alloy) for 240 min, there is no ferrite precipitation,

2 Structural Elongation and Alignment in an Fe-0.4C Alloy by Isothermal Ferrite Transformation in High Magnetic Fields 2533 Fig. 1 Microstructures of Fe-0.4C alloy heat treated below Curie temperature of ferrite. Specimens were austenitized at 1000 C for 15 min and transformed at 740 C for 1 hour without (a) or with a magnetic field of 10 T (b), followed by helium gas quenching. Fig. 2 Microstructures of Fe-0.4C alloy heat treated above Curie temperature of ferrite. Specimens were austenitized at 1000 C for 15 min and transformed at 785 C for 240 min without (a) or with a magnetic field of 10 T (b), followed by helium gas quenching. since transformation driving force is zero. The austenite grain boundaries are clearly shown by thin films of ferrite precipitated along austenite grain boundaries during quenching by helium gas both in (a) and (b). With magnetic field (b), big ferrite grains precipitated along austenite grain boundaries, and the ferrite grains are elongated along the direction of magnetic field. It was noticed that, with or without a magnetic field of 10 T, there is no much difference for ferrite transformation during quenching by helium gas, and only a small amount of ferrite precipitates along the austenite grain boundaries as thin films. Figures 1 and 2 show clearly that high magnetic field has strong effects on the ferrite transformation behavior and microstructures both below and above T c. Ferrite transformation is accelerated and the volume fraction of ferrite is increased by a magnetic field. 2,3,5,8,9) This is because the Ae 3 temperature increases with magnetic field as shown by thermodynamic calculation, 5,8,9) and magnetic field provides the driving force for ferrite precipitation. The elongation and alignment of ferrite grains during slow cooling has been already reported by the authors, 4,6,7) and now confirmed also for isothermal transformation both below and above T c. The elongation and alignment of ferrite grains is a unique feature obtained by transformation in a high magnetic field, however, its formation mechanism is not well understood. The mechanism is discussed in the next section based on the nucleation and growth of ferrite grains in a magnetic field. 3.2 Nucleation and growth of ferrite in a magnetic field Figure 3 shows the micrographs of specimens transformed at 785 C for (a) 10 min, (b) 20 min, (c) 30 min and (d) 240 min with a magnetic field of 10 T. It was found that during transformation, most of ferrite grains precipitate at austenite grain boundaries and triple junctions. At the initial stage (a), grains are equiaxed and distributed randomly at austenite grain boundaries. With increasing holding time to 20 min (b) and 30 min (c), the amount of transformed ferrite increases and some ferrite grains start to elongate along the magnetic field. Ferrite grains grow preferably along the magnetic field. After 240 min holding (d), most of ferrite grains are elongated and some of them are distributed head to tail and connected with each other along the direction of applied magnetic field. The degree of structural elongation can be evaluated by measuring the number of intersections between test lines and ferrite/austenite phase boundaries. 6) The degree of elonga-

3 2534 X. J. Hao, H. Ohtsuka and H. Wada Fig. 3 Microstructures of Fe-0.4C alloy heat treated in a magnetic field of 10 T. Specimens were austenitized at 1000 C for 15 min and transformed at 785 C for (a) 10 min, (b) 20 min, (c) 30 min and (d) 240 min. tion! is calculated from the following equation, 18)! ¼ N? N k N? þ 0:571N k Fig. 4 Degree of elongation as a function of transformation time. in which N? and N k are the intersection number when test lines are vertical or parallel to the direction of magnetic field respectively. Figure 4 shows the measured degree of elongation as a function of transformation time. During transformation, the degree of elongation increases continuously. At initial stage, the degree of elongation is very weak. Then, the degree of elongation quickly increases. Finally, the degree of elongation increases slowly. The elongated and aligned structure is mostly formed during the grain growth stage, not the nucleation stage. 3.3 Effects of transformation temperature on the degree of elongation Figure 5 shows the microstructures of specimens heat treated at (a) T c and (b) 750 C for 30 minutes with a magnetic field 10 T. At T c, ferrite grains are elongated and many of them are almost aligned with the direction of applied magnetic field as shown in (a). Decreasing transformation temperature to 750 C, the volume faction of ferrite increases and ferrite grains become smaller. Besides austenite grain boundaries, the interiors of austenite grains also become possible nucleation sites because nucleation driving force increases. Rod or needle-like ferrite grains appear and their long axis is not always aligned along the magnetic field. The measured degree of elongation! at different temperature is shown in Fig. 6. The degree of elongation increases with decreasing temperature and reaches a peak at T c and then decreases gradually. The reason for elongation of ferrite grains is not well understood. It is speculated that grain elongated along the direction of magnetic field reduces the demagnetization field in ferrite grains and therefore decreases the free energy of

4 Structural Elongation and Alignment in an Fe-0.4C Alloy by Isothermal Ferrite Transformation in High Magnetic Fields 2535 Fig. 5 Microstructures of Fe-0.4C alloy heat treated in a magnetic field of 10 T. Specimens were austenitized at 1000 C for 15 min and transformed at (a) 770 C and (b) 750 C for 30 min. Fig. 6 Degree of elongation as a function of transformation temperature. at 0 K). At T c it increases to about 0:287M 0. Below T c, the magnetization composes of spontaneous magnetization and field-induced magnetization. At 760 C, the magnetization was calculated to be 0:313M 0 and it increased to 0:367M 0 at 740 C. Therefore the degree of elongation increases with decreasing temperature. As for the chemical driving force, small chemical driving force favors the elongation of ferrite as shown by the experimental data that the degree of elongation increases with decreasing cooling rate, 4,7) that is, decreasing chemical driving force. With decreasing transformation temperature, the chemical driving force for ferrite precipitation increases, which promotes isotropic growth. The degree of elongation of ferrite grain is determined by the competition between demagnetization effect and chemical driving force, and their combined effects make the degree of elongation reach peak value at around T c. system. 4) As is already shown above, ferrite grains are elongated as a result of preferred growth along the magnetic field. However, it is very hard to explain the microscopic mechanism of elongation because growth rate depends on several factors, such as = phase boundary energy, chemical driving force for ferrite transformation, demagnetization effect and diffusion of carbon. There is no report about the effects of magnetic field on = phase boundary energy and diffusion of carbon, so only the demagnetization effect and the driving force will be considered as follows. The demagnetizing field H d in ferrite grain is expressed as follows. H d ¼ðN d MÞ= 0 Where N d is the demagnetizing factor which depends on the shape of ferrite grain, M is the magnetization of ferrite and 0 is the permeability of vacuum. High magnetization strength of ferrite, that is, large M promotes the preferred growth of ferrite along the direction of magnetic field to reduce N d and thereby the demagnetization energy. The magnetization strength of pure Fe in a magnetic field of 10 T was calculated by molecular field theory. 5) It was found that magnetic moment exists even above Curie temperature 5,8,9) and the field induced magnetization at 785 C is about 0:245M 0 (M 0 ¼ 1: A/m, saturation magnetization of pure Fe 4. Conclusions (1) Structural elongation and alignment was found in Fe- 0.4mass%C alloy by isothermal ferrite transformation both below and above Curie temperature in a high magnetic field. (2) Equiaxed ferrite grains nucleated randomly at austenite grain boundaries and they become elongated during growth along the direction of applied magnetic field. (3) The degree of elongation depends on the transformation temperature. Below Curie temperature, the degree of elongation increases with increasing transformation temperature, whereas it decreases above Curie temperature. (4) Small chemical driving force for ferrite precipitation and large magnetization of ferrite in high magnetic field favor the formation of elongated and aligned structure. The dependence of the degree of elongation on temperature is determined by the competition between demagnetization effect and chemical driving force. REFERENCES 1) T. Kakeshita, K. Kuroiwa, K. Shimizu, T. Ikeda, A. Yamagishi and M. Date: Mater. Trans., JIM 34 (1993)

5 2536 X. J. Hao, H. Ohtsuka and H. Wada 2) Y. Xu, H. Ohtsuka and H. Wada: Journal of the Magnetics Society of Japan 24 (2000) ) Y. Xu, H. Ohtsuka and H. Wada: Trans. Mater. Res. Soc. Japan 25 (2000) ) H. Ohtsuka, Y. Xu and H. Wada: Mater. Trans., JIM 41 (2000) ) Jong-Kyo Choi, H. Ohtsuka, Y. Xu and Wung-Yong Choo: Scr. Mater. 43 (2000) ) X. J. Hao, H. Ohtsuka, P. DE Rango and H. Wada: Mater. Trans. 44 (2003) ) P. DE Rango, X. J. Hao, H. Ohtsuka and H. Wada: Trans. Mater. Res. Soc. Japan 28 (2003) ) H. Guo and M. Enomoto: Mater. Trans., JIM 41 (2000) ) M. Enomoto, H. Guo, Y. Tazuke, Y. R. Abe and M. Shimotomai: Metall. Mater. Trans. 32A (2001) ) K. Maruta and M. Shimotomai: J. Cryst. Growth (2002) ) M. Shimotomai, K. Maruta, K. Mine and M. Matsui: Acta Mater. 51 (2003) ) Y. Xu, H. Ohtsuka and H. Wada: Trans. Mater. Res. Soc. Japan 25 (2000) ) T. Watanabe, Y. Suzuki, S. Tanii and H. Oikawa: Philos. Mag. Lett. 62 (1990) ) N. Masahashi, M. Matsuo and K. Watanabe: J. Mater. Res. 13 (1998) ) Y. Xu, H. Ohtsuka, S. Umezawa, K. Anak, S. Miyazaki and H. Wada: Proc. of the Fourth Int. Conf. on Recrystallization and Related Phenomena (ReX 99), Tsukuba, July (1999) ) Y. Xu, H. Ohtsuka, K. Itoh and H. Wada: Journal of the Magnetics Society of Japan 24 (2000) ) Y. Xu, H. Ohtsuka and H. Wada: Trans. Mater. Res. Soc. Japan 25 (2000) ) E. E. Underwood: Quantitative Microscopy, ed. by R. T. Dehoff and F. N. Rhines, (McGraw-Hill, New York, 1968) pp