Effect of a High Magnetic Field on The Formation of Widmanstätten Ferrite in Fe-0.52C

Transcription

1 Materials Transactions, Vol. 48, No. 11 (2007) pp to 2820 Special Issue on Structural and Functional Control of Materials through Solid-Solid Phase Transformations in High Magnetic Fields #2007 The Japan Institute of Metals Effect of a High Magnetic Field on The Formation of Widmanstätten Ferrite in Fe-0.52C Shou Jing Wang 1; * 1, Xiang Zhao 1; * 2, Yu Dong Zhang 1;2, Liang Zuo 1 and Claude Esling 2 1 Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang , P.R. China 2 LETAM, CNRS-UMR 7078, University of Metz, Ile du Saulcy, Metz, France The influence of a high magnetic field on the formation of ferrite in high purity Fe-0.52C (mass%) during austenite to ferrite transformation was studied. Results show that magnetic field can reduce the amount of Widmanstätten ferrite by enhancing the transformation driving force and make the ferrite grains elongate and align along the field direction. [doi: /matertrans.mi200706] (Received May 1, 2007; Accepted July 19, 2007; Published October 25, 2007) Keywords: high magnetic field, diffusion phase transformation, microstructure, Widmanstätten structure 1. Introduction Since 1990s, many new phenomena induced by a high magnetic field in Fe-C and Fe-C-X (alloy elements) systems during! transformation have been revealed. Large amount of scientific reports surged in academic journals and Conference proceedings within a short period of time, which shows that this topic has drawn much attention in the domain of solid-state phase transformation in a high magnetic field. Ohishi et al. 1) found that a 7 T magnetic field enhances the pearlitic transformation, reduces the sizes of pearlite colonies and enlarges the lamellar spacing in Fe-0.82C alloy in Furthermore they reported that a 10 T magnetic field suppresses the formation of Fe 3 C. Xu et al. 2) studied the isothermal! transformation in a 10 T field in Fe-1.5Mn- 0.1C-0.05Nb in 1999 and found that the magnetic field accelerates the nucleation of ferrite and enhances the formation of ferrite. In 2001, Enomoto 3) reported that a 7.5 T magnetic field increased the amount of ferrite in Fe- 0.39C during! transformation. Shimotomai et al. 4) found in 2000 that during the inverse phase transformation from martensite to austenite in the Fe-0.1C and Fe-0.6C, the obtained austenite grains were elongated and aligned in the field direction. Almost at the same time, Ohtsuka et al. 5,6) found that in the! transformation of Fe-0.4C and Fe- 0.95C-0.20Si-1.25Mn in a 10 T field, the proeutectoid ferrite grains showed the similar phenomena. Later, Hao and Ohtsuka 7 9) investigated the! transformation in Fe 0.41C 0.08Si 0.003Al under different treatment conditions in a 10 T field. They found that the ferrite grains elongated and aligned in the field direction in both isothermal cooling and continuous cooling and the elongation degree increased with the increase of the magnetic field, the elevation of the heating temperature and prolongation of soaking time. Shimotomai et al. 10) found the similar phenomenon in Fe- 0.6C. Zhang et al. 11,12) also reported the similar result in a medium carbon low alloyed steel. Moreover, they observed that a 12 T magnetic field increases the amount of low CSL boundaries and enhances h001i fiber component in the * 1 Ph. D Student, Northeastern University * 2 Corresponding author, zhaox@mail.neu.edu.cn transverse field direction in Fe 0.49C 0.24Cr during! transformation. Based on these experimental results, they conducted theoretical analyses and proposed quantitative equations to describe grain elongation and preferential nucleating and growth of specifically orientated grains to explain the enhancement of the above mentioned texture component. 13,14) The above experimental observation and theoretical explanation is significant in revealing the effect of a magnetic field on! diffusional transformation in Fe-C alloy system. However, the studied materials in the above investigations contain certain amount of alloying elements or impurities. They surely exert influence on the formation and the microstructure of the product phases. For instance, Ohtsuka et al. 15) reported that in a Nb alloyed steel, there is no morphology change observed in ferrite grains whenever a magnetic field was applied or not. Moreover, so far there have not been any reports on the influence of a high magnetic field on formation of proeutectoid Widmanstätten ferrite in steels. Therefore, high purity Fe-0.52C was selected in the present work. The effect of a high magnetic field on the formation of proeutectoid Widmanstätten ferrite during! transformation was studied. 2. Experimental An ingot of Fe-0.52C (mass%) was prepared by repeated melting of the high-purity constituent elements in a vacuum arc furnace. It was then multi-directionally forged (more than 4 cycles) and homogeneously annealed at 1373 K for 10 hours to homogenize composition and microstructure. The ingot was further fully annealed to refine the microstructure. The corresponding microstructure is shown in Fig. 1. It is seen that the ferrite grains and pearlite colonies are distributed isotropically and no bended structure is observed. The chemical composition of the ingot analyzed after the full annealing is shown in Table 1. The Ae 3 temperature calculated with Thermo-Calc is 1037 K. Specimens of 7 mm 7 mm 1 mm were cut out from the ingot and heat treated without and with a magnetic field. Heat treatments were carried out in the furnace set in a 12 T cryocooled superconducting magnet 100 mm in bore size. Specimens were kept in the central (zero magnetic force) region

2 Effect of a High Magnetic Field on The Formation of Widmanstätten Ferrite in Fe-0.52C µ m Fig. 1 Optical microstructure of Fe 0.52C ingot after full annealing at 1073 K in vacuum. Table 1 Chemical composition of Fe 0.52C alloy (mass%). C S P Mn Cu O N Fe <0: Bal. Pt-Rh Thermocouple Cooling Water Sample 1mm MD 7mm Ar Cooling Water Pt Heater Magnet Field Center Sample Holder Water-Cooled Jacket Fig. 2 Schematic of heat-treating equipment installed in the high magnetic field and sample arrangement. with their longitudinal direction parallel to the axis of the magnet, as shown in Fig. 2. Specimens were sorted into two groups according to the heat treatment conditions. For group one, specimens were austenitized at 1067 K, 1092 K and 1117 K, respectively, for 30 minutes and then cooled at 0.5 K/min from 1067 K to 873 K and then cooled naturally within the furnace until 473 K. For specimens austenitized at 1092 K and 1117 K, they were first cooled at 5 K/min from the austenitization temperature to 1067 K. A 12 T magnetic field was applied for the field treated specimen. For group two, specimens were austenitized at 1067 K for 30 minutes and cooled at 0.5 K/min from 1067 K to 873 K and then cooled naturally within the furnace until 473 K. For this group, a respective 4 T, 8 T and 12 T field was applied. The corresponding heat treatment charts for the two groups of specimens are shown in Fig. 3. The comparative heat Ar Fig. 3 (a) Temperature, T/K (b) 1200 Temperature, T/K treatments without and with the magnetic field were performed under the same heating and cooling conditions for the two groups. When the magnetic field was applied, it was applied to the whole heating, soaking and cooling process. The transformed microstructures of the above treated specimens were etched out and analyzed by optical microscopy. The micrographs were taken from the longitudinal cross sections of the treated specimens. The area percentage of Widmanstätten ferrite was analyzed with an image analyzer. 3. Results 1117K 30min 1092K 30min 1067K 30min K Cooling <473K switch down magnetic field and devacuum Time, t/min 1067K30min 0. Cooling 873K <473K switch down magnetic field and devacuum Time, t/min Heat treatment charts of specimen group one (a) and group two (b). Figure 4 shows the microstructures of the specimens (group one) austenitized at various temperatures without and with a 12 T magnetic field, where the white areas are proeutectoid ferrite and the dark ones are pearlite. It is seen that proeutectoid ferrite is present in two kinds of morphologies: allotriomorphic and acicular. The latter is the wellknown Widmanstätten ferrite. The area percentage of Widmanstätten ferrite in both the non-field and field treated specimens was measured and is shown in Fig. 5. From Fig. 4 and Fig. 5, it is seen that although the amount of Widmanstätten ferrite increases with the elevation of austenitization temperature in both non-field and field treated specimens, the 12 T magnetic field greatly reduces the amount of Widmanstätten ferrite. In addition, under the magnetic field the non

3 2818 S. J. Wang, X. Zhao, Y. D. Zhang, L. Zuo and C. Esling H Fig. 4 Optical microstructures of Fe 0.52C obtained after being austenitized at various temperatures and cooled at 0.5 K/min without and with a 12 T magnetic field. (a) 1067 K 0 T; (b) 1067 K 12 T; (c) 1092 K 0 T; (d) 1092 K 12 T; (e) 1117 K 0 T and (f) 1117 K 12 T. The field direction is vertical in the pictures. Area percentage of Widmanstätten ferrite( 100%) T 12T Temperature,T/K Fig. 5 Amount of Widmanstätten ferrite vs. austenitization temperature in non-field and field treated specimens. Widmanstätten ferrite obtained obviously elongates and aligns along the field direction. However, the tendencies of the alignment of the ferrite grains decreases with the elevation of the austenitization temperature. To further reveal the effects of the magnetic field on the amount of Widmanstätten ferrite and ferrite elongation and alignment, specimens treated under various magnetic field inductions (group two) were investigated. Figure 6 and Figure 7 show the microstructures obtained under various field inductions and the corresponding amounts of the Widmanstätten ferrite measured. It is seen from the figures that the amount of Widmanstätten ferrite decreases with the increase of the field intensity. When the field applied reaches 12 T, there is almost no Widmanstätten ferrite appearing (Fig. 6(d)). Also the degrees of alignment and elongation of ferrite grains increase with the increase of the field intensity. 4. Discussion According to the solid-state phase transformation theory, the Gibbs free energy change related to uniform nucleation of a new phase is as follows: G ¼ VGvþSþV" ð1þ where V is the volume of the newly formed phase, Gv is the volume Gibbs free energy difference between the product phase and the parent phase, S is the surface area of the nuclei, is the interfacial energy between the parent phase and the product phase and " is the elastic strain energy of the new phase. In eq. (1), V Gv is the driving force of the

4 Effect of a High Magnetic Field on The Formation of Widmanstätten Ferrite in Fe-0.52C 2819 H Fig. 6 Optical microstructure of Fe 0.52C obtained after being austenitized at 1067 K for 30 min. and cooled 0.5 K/min under various field inductions. (a) 0 T; (b) 4 T; (c) 8 T and (d) 12 T. The field direction is vertical in the pictures. Area percentage of Widmanstätten ferrite( 100%) Magnetic field strength, H/T Fig. 7 Amount of Widmanstätten ferrite obtained after being austenitized at 1067 K for 30 min. and cooled at 0.5 K/min. vs. induction of magnetic field. transformation, and S is the total interfacial energy and V" is the total elastic strain energy. The latter two items form the energy barrier of the transformation. It has been reported that there are two mechanisms that govern the formation of Widmanstätten ferrite. One is that Widmanstätten ferrite forms from the boundary between allotriomorphic ferrite and the original austenite and grows into the austenite grain following the K-S orientation relationship, f110g bcc == f111g fcc, h111i bcc == h110i fcc. The second one is that the Widmanstätten ferrite forms directly in the original austenite grain and grows in the austenite grain according the K-S orientation relationship. In both cases, the K-S orientation relationship guarantees a low energy semi-coherent interface between the parent austenite and the product ferrite hence the transformation barrier related to the interfacial energy is greatly lowered. The misfit between f110g bcc and f111g fcc 0:025. The modulus of the Burger s vector of the misfit dislocations on the semi-coherent interface b 0:2 nm. The interspacing between the misfit dislocations is 8 nm. To

5 2820 S. J. Wang, X. Zhao, Y. D. Zhang, L. Zuo and C. Esling further increase the matching level on the interfaces, atomic steps perpendicular to the interface form. The existence of the interfacial steps render the interface 10 away from the f111g fcc. Therefore, boundary migration in the direction normal to the interface by dislocation climbing is very difficult and requires large activation energy. However, boundary migration through the movement of steps in the direction parallel to the interface is very easy. In this way, ferrite with acicular shape forms. So the Widmanstätten ferrite usually forms when the driving force of the transformation is low (or undercooling degree is small) and the reduction of energy barrier by forming coherent or semicoherent interface is required. It is known that both parent austenite and product ferrite can be magnetized to some extent in a high magnetic field. The corresponding Gibbs free energy is lowered by the application of the field. As the magnetization of ferrite is much higher than that of austenite, a negative Gibbs free energy difference Gm between the product and parent phases - with the same sign as Gv - is created. In this case, the total Gibbs free energy corresponds to the formation of ferrite under a magnetic field should be: G ¼ VðGvþGmÞþS þ V" ð2þ As Gv and Gm have the same sign, the driving force of the transformation from austenite to ferrite increases, there is less need for a low energy interfaces. As consequence, the chance for the formation of Widmanstätten ferrite reduces. There may well exist the influence of the magnetic field on S, the interfacial energy, and V", the strain energy in eq. (2). The magnetic field applied may increase the interfacial energy, as the magnetization of interfacial area is much lower due to the high magnetic disordered degree and that of grain interior is much higher owing to the high magnetic ordering (as described in detail in Ref. 16)). Thus the magnetic free energy difference between the boundary area and the grain interior should be increased by the magnetic field. As the interfacial energy has a positive sign, the increase of the interfacial energy cancels out part of the influence of the magnetic field on volume free energy term in eq. (2). However, from the result obtained from the present study that magnetic field reduces the amount of Widmanstätten ferrite, it can be deduced that this counter-balance is neglectable. As for the strain energy, since the transformation happens at relatively high temperature and at very slow cooling rate with small volume change and the degree of magnetic anisotropy of the material is also very small, the effect of the magnetic field on strain energy may also be neglectable. The elongation and alignment of ferrite grains along the field direction have been found during the austenite to ferrite transformation in various steels. 4 12) Both may be related to the dipolar interaction between magnetic dipoles. In the former case, the Fe atoms in the ferrite grains are taken as magnetic dipoles while in the latter, the ferrite grains are considered as the magnetic dipoles. The elongation or the alignment can reduce the demagnetization factor and thus lower the demagnetization energy. Therefore, this microstructural configuration is energetically favored by the magnetic field. It should also be noted that the alignment degree decreases with the increase of austenitization temperature. It is known that the magnetic dipolar interaction reduces dramatically with the distance between the dipoles. As the elevation of the austenitization temperature surely results in grain growth of austenite and the dominant grain nucleation site is austenite grain boundaries, especially triple boundary junctions, the distance between ferrite nuclei that act as magnetic dipoles increases. Thus the alignment degree decreases. As the occurrence of grain elongation and alignment is conditional on composition and heat treatment parameters, 13,17) there may be some other reasons additional to the dipolar interaction effect. Further study is needed. 5. Summary High magnetic field shows strong influence on the formation of proeutectoid ferrite in the Fe 0.52C. It greatly reduces the amount of Widmanstätten ferrite by introducing the additional transformation driving force. It also makes the ferrite grains elongate and align along the field direction through dipolar magnetic interaction. Acknowledgement This work was supported by the National Science Found for Distinguished Young Scholars (Grant No ), the key project of National Natural Science Foundation of China (Grant No ),the National Natural Science Foundation of China (Grant No ), and the 111 Project (Grant No. B07015). The authors would like to appreciate the High Magnetic Field Laboratory of Northeastern University for providing the facilities. REFERENCES 1) Y. Ohishi, T. Murai, H. Ohtsuka, K. Itoh and H. Wada: CAMP-ISIJ. 11 (1998) ) Y. Xu, H. Ohtsuka and H. Wada: CAMP-ISIJ. 12 (1999) ) M. Enomoto: Materia Japan. 40 (2001) ) M. Shimotomai and K. Maruta: Scripta Mater. 42 (2000) ) H. Ohtsuka, Y. Xu and H. Wada: Mat. Trans. JIM. 41 (2000) ) J. K. Choi, H. Ohtsuka, Y. Xu and W. Y. Choo: Scripta Mater. 43 (2000) ) X. J. Hao, H. Ohtsuka and H. Wada: Mater. Trans. 44 (2003) ) X. J. Hao and H. Ohtsuka: Materials Transactions. 45 (2004) ) X. J. Hao and H. Ohtsuka: Journal of Materials and Metallurgy. 4 (2005) ) M. 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