- Email: [email protected]
Formation of structure and crystallographic texture in Fe-50%Ni thin tapes under high magnetic field annealing
Physica B 468-469 (2015) 66–71
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Formation of structure and crystallographic texture in Fe-50%Ni thin tapes under high magnetic field annealing I.V. Gervasyeva a,n, E. Beaugnon b, V.A. Milyutin a, E.G. Volkova a, D.P. Rodionov a, Yu.V. Khlebnikova a, D.A. Shishkin a,c a
Institute of Metal Physics, Ural Division, Russian Academy of Sciences, 620990 Ekaterinburg, Russia National Laboratory of High Magnetic Field, CNRS, B.P. 166 38042, Grenoble cedex 9, France c Institute of Natural Sciences, Ural Federal University, 620083 Ekaterinburg, Russia b
art ic l e i nf o
a b s t r a c t
Article history: Received 4 March 2015 Accepted 14 April 2015 Available online 15 April 2015
For the first time the influence of a high direct current (DC) magnetic field on the structure and texture formation in tapes of Ni48.8Fe51.2 alloy was investigated. Cube texture formation was studied in tapes after cold rolling to 98.8% and annealing in a high magnetic field up to 20 T. Samples were subjected to magnetic annealing below and above the Curie point. Structure and texture were studied using electron backscatter diffraction (EBSD) technique, transmission electron microscopy, and magnetic hysteresis loops measurements. The experiments show a significant increase of the {001}7 10° volume fraction in the recrystallization texture with the main orientation {100}〈001〉, when the magnetic annealing is carried out at a temperature below the Curie point. & 2015 Elsevier B.V. All rights reserved.
Keywords: Iron–nickel alloy Cold rolling DC high magnetic field annealing Recrystallization Cube texture
1. Introduction Iron–nickel alloys have long been used as soft magnet materials. In recent years, Fe–Ni alloy tapes have been used as substrates for high temperature superconductors (HTS) [1]. Epitaxial deposition of consecutive layers on a substrate for HTS compositions requires the presence of a very sharp {100} 〈001〉 texture, which is formed during primary recrystallization of a tape cold rolled to a high degree of deformation (98–99%). The formation of a cube texture of fcc materials is determined by the chemical composition of alloys and depends on a number of processing conditions, in particular, the fine initial grain is necessary and a high degree of cold rolling deformation is required. An important condition is the type of the deformation texture. The crystallographic orientations S {123}〈634〉 and С {112}〈111〉dominate in the deformation texture of tapes from the alloy Ni48.8Fe51.2, which, after primary recrystallization, leads to the formation of a single-component sharp cube texture {100}〈001〉 [2]. For Fe–Ni alloy compositions, in which 〈001〉 is the direction of easy magnetization, the degree of sharpness of the cube texture to a great extent determines the level of magnetic properties. Over recent years, research interest has grown to the studies of high magnetic field causing effect on the processes of grain n
Corresponding author: fax: þ 7 343 374 52 44. E-mail address: [email protected] (I.V. Gervasyeva).
http://dx.doi.org/10.1016/j.physb.2015.04.016 0921-4526/& 2015 Elsevier B.V. All rights reserved.
nucleation and growth during structural and phase transformations. The increased research interest can be attributed largely to the designing of powerful installations generating DC magnetic fields of 20–30 T and higher and to the feasibility of maintaining high temperatures inside the electric magnets. A number of works have demonstrated effect of high magnetic field on grain boundary mobility in nonmagnetic materials. For instance, Molodov et al. have studied the motion of specific planar boundaries in Bi-bicrystals under magnetic driving forces [3]. Experimental data have been gathered demonstrating the effect of magnetic field on the structure of materials, which have undergone recrystallization or a phase transformation under magnetic field, or which have crystallized from the amorphous state. Thus, Watanabe et al. [4] have demonstrated that crystallization of an amorphous tape in a magnetic field of 6 T results in a sharp crystallographic texture {110}. Bacaltchuk et al. in [5] have evaluated the effect of magnetic annealing at 19 T on the increased volume fraction of Goss-oriented grains {110} in the grain non-oriented electric steel. Zhang et al. in [6] and other researchers in their works have established that, under the effect of high DC magnetic field, phase stability of steels changes, eutectic carbon concentration increases, the critical temperatures Ac1 and Ac3 grow, carbon solubility in the ferrite phase increases, and a unique ferrite-pearlite microstructure is formed having magnetic field-induced grain elongation in the direction parallel to the magnetic field. It has been shown in [7] that in a binary alloy of Co–B that has the eutectic transformation
I.V. Gervasyeva et al. / Physica B 468-469 (2015) 66–71
temperature of around 1100 °C, which exceeds the Curie point of the cobalt-enriched phase by 20 °C only, crystallization in a 16 T magnetic field near the eutectic alloy under the conditions of sufficient cooling results in well-pronounced anisotropy of microstructure. It should be noted that there are not many studies that explore the effect of high magnetic fields on processes in solids; and each of these studies contains, mostly, experimental data, with efforts to interpret them being different in every study. No common theory has yet been developed about the essence of the effect that a high magnetic field causes on the grain boundary structure formation, on the orientation of grains nucleating and growing in ferromagnetic and nonmagnetic materials. In this work, the formation of structure and texture in Fe–Ni tapes upon annealing high DC magnetic field has been investigated.
2. Experimental The Ni48.8Fe51.2 (50 wt%) alloys were used as the material for studying. Nickel of 99.99% purity and carbonyl iron (99.97%) were smelted in alundum crucibles in an argon atmosphere in a laboratory vacuum induction furnace. The ingots were forged at a temperature of 1000–800 °С to bars of 10 10 mm2 in cross section. Then they were warm-forged at 650 °С to a size of 7 7 mm2. After grinding the blanks with dimensions of 6 6 150 mm3 were annealed at 800 °С for 1.5 h. The average grain size before the cold rolling operation was 40 μm. After cold rolling bands about 80 μm thick were formed using polished rolls. The degree of cold deformation was about 99%. Tapes from the Ni48.8Fe51.2 alloy after cold rolling to 99% were subjected to magnetic annealing in the French National High Magnetic Field Laboratory at its Grenoble site. A 50-mm-internaldiameter resistive magnet was used, which allows a DC magnetic field of 20 T to be generated. A double spiral SiC heater of 200 mm in length was inserted into the magnet. Samples 20 mm long and 5 mm wide were vacuumed in sealed ampoules and positioned in a narrow sealed tube made of stainless steel, and then were put down to a quartz tube inside the heater. A field of 10–20 T was used. Annealing without magnetic field was carried out in the same conditions. The samples were annealed in this setup at a temperature of 490 °C for 15 min and at a temperature of 600 °C for 30 min. Then the samples, subjected to pre-recrystallization annealing at 490 °C with and without magnetic field, were put in a laboratory furnace without field at the Institute of Metal Physics and annealed at a temperature of 600 °C for 30 min and at a 800 °C for 30 min. For carrying out the annealing at 800 °С, the samples were heated up from 500 °C at a rate of 100°/h and then were kept at 800 °С for 1 h. The structure and orientations in the annealed samples were analyzed simultaneously using the method of electron backscatter diffraction (EBSD), which involved the interpretation of Kikuchi patterns with an EDAX attachment in a FEI Quanta 200 scanning electron microscope. Depending on the treatment, scanning was carried out at a step of 1–4 μm, and at a magnification of 200 or 400 . The samples were electrolytically etched before this investigation. The EBSD software package was used to estimate the volume fraction of the cube planes {100} ( 710°) in the surface of the samples. The samples after low temperature annealing were further investigated by transmission electron microscopy using a JEM 200CX microscope. The selected-area electron diffraction patterns were obtained using diaphragms with a diameter of 500 nm. Average grain size after primary recrystallization was determined using an optical microscope.
67
The microhardness of the tape samples after annealing in the magnetic field of a variable strength was measured under a load of 70 g. The value of the microhardness for each sample was determined by averaging the results over at least fifteen measurements. The magnetization measurements were performed using a vibrating sample magnetometer Lake Shore 7407 in magnetic fields of 17 kOe. Relative error amounted to 1%.
3. Results and discussion Tapes of the alloy Ni48.8Fe51.2 after cold rolling 99% were subjected to magnetic annealing at a temperature of 490 or 600 °С, which is either above or below the Curie point (500 °С). This also corresponds to the temperature below or above the onset temperature of primary recrystallization for this alloy (570 °С, [8]). The sample subjected to magnetic annealing at 490 °С were then annealed in a vacuum furnace without magnetic field at 600 °С and 800 °С with slow heating. All samples were investigated using a scanning electron microscope by the EBSD technique. Orientation maps and image quality patterns obtained for the tapes annealed at a temperature below the onset temperature of primary recrystallization at 490 °С provide evidence that, as the strength of magnetic field grows in the course of annealing, identification of structure and orientations degrade. This means that the strain relieving processes are less intense in the sample annealed under magnetic field 20 T. Table 1 represents the mean values of microhardness in these samples along with the average image quality value. The samples annealed without field and under the effect of 10 T magnetic field show nearly the same level of mean value of microhardness, while the sample annealed at 20 T magnetic field exhibits a considerably greater value. The average image quality value in the samples decreases as the magnetic field strength at annealing grows. Figs. 1 and 2 display microstructure patterns obtained in a transmission electron microscope for the samples annealed at 490 °C without field and with 20 Т field, respectively. Both samples have a similar structure pattern. At high reductions, in the tapes there form, as a result of multiple slip, a cellular structure. Cell walls consist of volume dislocation pile-ups. In the course of heating, the three-dimensional walls flatten and turn into flat lowangle subboundaries, whereas cells become subgrains. Part of dislocations pass from the cell volume into the subboundaries formed. Evidently, after the pre-recrystallization annealings in magnetic field and without it these processes proceed with different rates. After the magnetic field annealing the cell walls are broad and smeared. In the selected-area electron diffraction pattern of the sample annealed in a magnetic field (Fig. 2a), the poles are more smeared in compared with the pattern of the sample annealed without field (Fig. 1a). Since the selected-area electron diffraction patterns were obtained from the same size area it can be concluded that the processes of coarsening the cells and the decrease in the magnitude of microdistortions in the lattice occur more slowly upon annealing in a field. In the dark-filed images (Figs.1b and 2b) it is seen that in the structure of the alloy annealed without field one can find areas containing subgrains, which are separated from the matrix by quite clear boundaries, Table 1 Mean values of microhardness (Нμ70) and average image quality values of Kikuchi patterns (IQ) in samples after magnetic annealing at 490 °C. 490 °C,15 min at H¼0 T
490 °C,15 min at H ¼10 T 490 °C,15 min at H¼20 T
Нμ70,МПа
Нμ70,МПа 2146
2192
IQ 2288
IQ 2012
Нμ70,МПа 2509
IQ 1941
68
I.V. Gervasyeva et al. / Physica B 468-469 (2015) 66–71
Fig. 1. TEM images taken from a sample annealed at 490 °C without field: (a) a bright-field image and a selected-area electron diffraction pattern (zone axis [110]) obtained for the area indicated with a circle; (b) a dark-field image in reflection [111] and an electron diffraction pattern of the whole image area (diaphragm with a diameter of 2 μm).
whereas in the sample annealed in the field of 20 T, no such areas are observed, and its structure is like the structure of a deformed metal. Using the EBSD technique, the samples subjected to annealing at temperatures above the primary recrystallization temperature at 600 and 800 °C were compared with the samples subjected to prior annealing at 490 °C under the magnetic field (20 Т) and without field. In all of them the main orientation was the cube component {100}〈001〉. Fig. 3 displays fragments of EBSD orientation maps for samples annealed at 600 °C for 30 min without field, for samples annealed at 600 °C in the magnetic field of 20 Т (above the Curie point), and for samples subjected to prior annealing in the magnetic field of 20 T at 490 °C (below the Curie point) and then annealed at 600 °C without field. The temperature 600 °C is only slightly above the region of realization of primary recrystallization in the alloy Ni48.8Fe51.2 [2]; the main processes related to the replacement of deformation structure by recrystallized grains have already ended, but, yet, small areas with the deformation-texture orientation are still retained. For all the samples, the volume fractions of cube orientation was determined, which are summarized in Table 2. Attention is drawn to an observation that, if magnetic annealing was carried out directly at 600 °C (above the Curie point), then the fraction of cube orientation had little difference from that for the sample annealed without field (87 and 86.9%, respectively). A different result is obtained, if before conducting the recrystallization annealing at 600 or 800 °C, a preliminary magnetic annealing is carried out at a temperature below the Curie point (490 °C). It should be noted that in fcc nickel-based alloys, which are prone to the formation of cube texture during recrystallization (which is also true for the alloy of Ni48.8Fe51.2), the step-by-step annealing, i.e., annealing
with an additional exposure to a lower temperature (which at the same time is above the onset temperature of primary recrystallization) prior to the final high temperature annealing, leads to an increased sharpness of the cube texture [2]. It has been established in the present work that sharpness of cube texture is also increased, if the preliminary annealing is carried out below the onset temperature of primary recrystallization. Preliminary annealing at 490 °C without field results in a 3.8% increase in the sharpness of cube texture in the alloy annealed at 600 °C. If such preliminary annealing is carried out in a magnetic field of 20 Т, then sharpness of cube texture increases by 7.2% (from 86.9 to 94.1%). For the alloy annealed at 800 °C, the preliminary annealing at a temperature below the Curie point with and without field 20 T leads to the sharpness of cube texture of 94.7 and 98.7%, respectively. For three samples recrystallized following various regimes, the magnetic hysteresis loops were measured (Fig. 4a). Saturation induction is approximately the same for all three versions and is given in Table 3. Induction values in the fields prior to saturation differ significantly. Fig. 4b shows a scaled up fragment of the hysteresis loop. It can be seen that for the samples annealed in the magnetic field the magnetization values are approximately the same, while the sample annealed without field the magnetization value is lower considerably. Table 3 demonstrates the values of magnetic induction in the field of 600 Oe (47600 A/m). It can be seen that, while measuring the hysteresis loop, the magnetization process runs at a higher rate for the samples subjected to the treatment in high magnetic field. That appears to support the assumption that the samples contain more grains having crystallographic direction approaching the easy magnetization direction 〈001〉.
Fig. 2. TEM images taken from a sample annealed at 490 °C in the magnetic field of 20 T: (a) a bright-field image and a selected-area electron diffraction pattern (zone axis [123]) obtained for the area indicated with a circle; (b) a dark-field image in reflection [111] and an electron diffraction pattern of the whole image area (diaphragm with a diameter of 2 μm).
I.V. Gervasyeva et al. / Physica B 468-469 (2015) 66–71
69
Fig. 3. Orientation maps of Ni48.8Fe51.2 alloy samples annealed during 30 min (a) at 600 °С without field; (b) at 600 °С in the field of 20 T; (c) at 600 °С without field of the sample preliminary subjected to pre-recrystallization annealing at 490 °С, 15 min in magnetic field 20 T and (d) the interpretation of orientations (colors) in the inverted pole figure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 2 {001} volume fraction (7 10°), % after different treatments of the samples 600 °C, 30 min
490 °C,15 minþ 600 °C, 30 min
Without field At 20 T Without field 86.9
87.0
90.7
490 °C at 20 T 94.1
490 °C,15 min þ800 °C, 30 min Without field 94.7
490 °C at 20 T 98.7
It is essential to note that in order to obtain a perfect cube texture, an elevated temperature and a longer exposure are used for the primary recrystallization process, namely, 1000–1100 °С for 1–1.5 h. The lower temperature in this study was selected in order for more pronounced effect of treatment in the magnetic field After all the treatments of samples in the high magnetic field applied, the average grain size was measured metallographically. The average values of grain size are given in Table 4. It can be noted that in the samples treated without magnetic annealing the grains grow to a larger size in the process of primary recrystallization. On the contrary, Bacaltchuk et al. [5] found that in the process of primary recrystallization the grain size is increased
in the isotropic electric steel Fe-0.75%Si as the strength of magnetic field increases up to 19 T under magnetic annealing at 800 °С for 10 min. In the present work, annealing under magnetic field was carried out preferably at temperatures below the onset temperature of primary recrystallization; and it was positively noted that softening processes are slowed down as compared to annealing at the same temperature but without magnetic field. As a result, during the following high temperature annealing of the samples annealed under the magnetic field, the grains seem to have little time to grow up to the same size as the grains in the samples annealed without field. Other works also report on the primary recrystallization process slowing down under the annealing treatments in a high magnetic field. Thus, the authors of [9] found for non-oriented 3% Si steel that the primary recrystallization process in the specimens 87% cold rolled and magnetically annealed at 10 T was slower than that in the specimens annealed without magnetic field. Marticainen and Lindroos reported in [10] that annealing in a magnetic field retards the early stage of recovery and recrystallization. They suppose this effect to be orientation-dependent because of the magnetostriction anisotropy. The same explanation is given in [11]
70
I.V. Gervasyeva et al. / Physica B 468-469 (2015) 66–71
Fig. 4. Hysteresis loop for the samples annealed at different regimes (a) and fragment of the hysteresis loop for the same samples (b).
as a possible reason for the retardation of recovery and recrystallization and for growth of grains with certain orientations; however, the authors have to report on the absence of any evidence of this. Thus, no unified opinion on the mechanisms of influence of magnetic field on the processes of recovery and recrystallization has been worked out so far. The work presented shows the effect of external magnetic field on the processes of structure transformation upon recovery. To
understand the reasons for changes in the microstructure of a deformed material upon magnetic annealing, let us pay attention to works which consider motion of dislocation in magnetic field and their interaction with domain walls, under magnetic field. The energy of interaction of dislocations with domain walls is characterized by their mutual hindering [12–14]. The authors of [12] represent the energy of interaction of domain walls with edge dislocations Фw as follows:
I.V. Gervasyeva et al. / Physica B 468-469 (2015) 66–71
Table 3 Magnetic induction in the field of 600 Oe (47600 A/m) and saturation induction in the samples subjected to various treatments. 600 °C, 30 min Н¼ 0 Т В 47600 Вs, Т
a/m,
Т 1.56 1.66
490 °C, 15 min Н ¼ 20 T þ 600 °C, 30 min
490 °C, 15 min Н ¼ 20 T þ800 °C, 1 h
1.62 1.68
1.61 1.67
Table 4 Grain size in the studied tapes after various annealing treatments. Treatment regime
Grain size, μm
600 °С, 30 min without field 600 °С, 30 min under magnetic field Н ¼ 10 T 600 °С, 30 min under magnetic field Н ¼ 20 T 490 °С, 15 min without field þ600 °С, 30 min 490 °С, 15 min under field Н ¼ 10 Tþ 600 °С, 30 min 490 °С, 15 min under field Н ¼ 20 T þ600 °С, 30 min 490 °С, 15 min without field þ800 °С, 1 h 490 °С, 15 min under field Н ¼ 10 Tþ 800 °С, 1 h 490 °С, 15 min under field Н ¼ 20 T þ800 °С, 1 h
38,2 32,8 32,2 42,0 36,0 35,0 59,6 54,8 53,1
71
magnetic field grow to larger sizes. On the other hand, by the onset of recrystallization annealing, the stored energy of deformation in the samples preliminary annealed in magnetic field remains at a higher level. The energy stimulus for the grain growth is quite high and the amount of nuclei with directions close to 〈001〉 is enhanced owing to the aligning influence of the field on the recovery stages (because of the tendency for minimizing the energy of crystallographic anisotropy) and, therefore, the sharpness of cube texture in the samples preliminary annealed in magnetic field is higher than in the samples annealed without field.
4. Conclusions
Φw = fd (
L 1/2 1/2 ) Nd = αNd1/2 2ld S
Upon primary recrystallization of the Fe–Ni samples pre-an-
(1)
where L is the average domain size, fd is the average force acting upon a domain wall from dislocations, ld is the average dislocation length, S is the area a domain wall, Nd is the dislocation density. The existence of magnetic moment of dislocations [12,13] creates an additional energy Фм of interaction with magnetization that hampers the dislocation motion
ΦM =
(λ⁎bG sin γ)2 Nd = βNd K
(2)
where λ is a function of magnetostriction constants, b is the Burgers vector, G is the shear modulus, γ is the angle between magnetization and the nearest easy-magnetization axis, K is the anisotropy constant. In large fields, the processes of domain wall movement cease because of the absence of domain walls; therefore, the energy of interaction of dislocations with domain walls Фw is absent as well. The energy of interaction of dislocations with magnetization Фм grows with increasing the magnetic field strength, since in large fields the dislocation motion is additionally hampered by the necessity for energy on overcoming the aligning effect of the magnetic field on magnetic moments. This serves in [15] as an explanation of the effect of decreasing the maximal elongation upon tension of steel in a magnetic field of 50 kOe. In the current work, samples under study were deformed to 99%; in their structure prior to annealing there are not many solitary dislocations, whereas mostly the dislocations are present in pile-ups located in the grid walls of the cellular structure. This circumstance significantly complicates the model of description of processes occurring upon simultaneous action of strong magnetic field and heating and do not allow taking into account considerations underlying formula (2); however, in the authors opinion, the main tendencies can retain themselves, which is supported by the experimental data. It is the retardation of dislocation motion and, possibly, of the diffusion processes on the whole [16] that results after a pre-recrystallization annealing in a more complete realization of the recovery processes upon annealing without field compared to the case of magnetic-field annealing. Due to this fact, upon subsequent recrystallization annealing, recrystallized grains in the samples preliminary annealed without n
nealed in a high magnetic field at a temperature below the Curie point, the volume fraction of grains with the easy magnetization direction 〈001〉 is increased in the texture; The recrystallized samples subjected to preliminary treatment in a high magnetic field have the values of magnetic induction in pre-saturation fields greater by approximately 4% than the magnetic induction values for the samples annealed without field; Applying a high magnetic field hinders the softening processes in the cold-rolled alloy during heating. The facts supporting this are that the samples annealed in magnetic field at a temperature below the recrystallization temperature are in a more stressed condition, and that in the samples preliminary annealed in magnetic field the grain size upon the primary recrystallization is less than the grain size in the samples annealed without field.
Acknowledgment We acknowledge the support of the LNCMI-CNRS, member of the European Magnetic Field Laboratory (EMFL). The work was performed within the State Program “Magnit” no. 01201463328 (Grant no 15-9-2-33) and was in part supported by the Russian Foundation for Basic Research (Grant no14-02-31143).
References [1] R.I. Tomov, A. Kurzumovic, M. Majoros, D.-J. Kang, B.A. Glowacki, J.E. Evetts, Supercond. Sci. Technol. 15 (2002) 598. [2] I.V. Gervas’eva, D.P. Rodionov, Yu.V. Khlebnikova, V.A. Kazantsev, Phys. Met. Metallogr. 111 (2011) 271. [3] D.A. Molodov, G. Gottstein, F. Heringhaus, L.S. Shvindlerman, Acta Mater. 46 (1998) 5627. [4] T. Watanabe, S. Tsurekawa, X. Zhao, L. Zuo, C.J. Esling, J. Mater. Sci. 41 (2006) 7747. [5] C.M.B. Bacaltchuk, G.A. Castello-Branco, H. Garmestani. Proceedings of the 14th International Confernce on Textures of Materials. Leuven, Belgium. Material Science Forum 702–703 (2005) 1165. [6] Y.D. Zhang, C. Esling, X. Zhao, L. Zuo. Proceedings of the14th International Confernce on Textures of Materials. Leuven, Belgium. Material Science Forum 702–703 (2005) 1131. [7] F. Gaucherand, E. Beaugnon, Phisica B 346-347 (2004) 262. [8] I.V. Gervasyeva, Yu.V. Khlebnikova, D.P. Rodionov, E.S. Belosludtseva, V. A. Milyutin, T.R. Suaridze, Phys. Met. and Metallogr. 114 (2013) 171. [9] Y. Xu, H. Ohtsuka, K. Itoh, H. Wada, J. Magn. Soc. Jpn. 24 (2000) 651. [10] H.O. Marticainen, V.K. Lindroos, Scan. J. Metals. 10 (1981) 3. [11] N. Masahashi, M. Matsuo, K. Watanabe, J. Mater. Res. 13 (1998) 457. [12] F. Vicena., Czech. J. Phys. 5 (1955) 480. [13] A. Seeger, H. Kronmuller, H. Rieger, R. Traubler, J.Appl. Phys. 35 (1964) 740. [14] R. Traubler, Z. Metallk. 53 (1962) 211. [15] T.V. Efimova, V.V. Polotnyuk, Ukraine Phys. J. 26 (1981) 1149 (in Russian). [16] H. Nakajima, S. Maekawa, Y. Aoki, et al., Trans. Jpn. Instit. Metals. 26 (1985) 1.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Formation of structure and crystallographic texture in Fe-50%Ni thin tapes under high magnetic field annealing I.V. Gervasyeva a,n, E. Beaugnon b, V.A. Milyutin a, E.G. Volkova a, D.P. Rodionov a, Yu.V. Khlebnikova a, D.A. Shishkin a,c a
Institute of Metal Physics, Ural Division, Russian Academy of Sciences, 620990 Ekaterinburg, Russia National Laboratory of High Magnetic Field, CNRS, B.P. 166 38042, Grenoble cedex 9, France c Institute of Natural Sciences, Ural Federal University, 620083 Ekaterinburg, Russia b
art ic l e i nf o
a b s t r a c t
Article history: Received 4 March 2015 Accepted 14 April 2015 Available online 15 April 2015
For the first time the influence of a high direct current (DC) magnetic field on the structure and texture formation in tapes of Ni48.8Fe51.2 alloy was investigated. Cube texture formation was studied in tapes after cold rolling to 98.8% and annealing in a high magnetic field up to 20 T. Samples were subjected to magnetic annealing below and above the Curie point. Structure and texture were studied using electron backscatter diffraction (EBSD) technique, transmission electron microscopy, and magnetic hysteresis loops measurements. The experiments show a significant increase of the {001}7 10° volume fraction in the recrystallization texture with the main orientation {100}〈001〉, when the magnetic annealing is carried out at a temperature below the Curie point. & 2015 Elsevier B.V. All rights reserved.
Keywords: Iron–nickel alloy Cold rolling DC high magnetic field annealing Recrystallization Cube texture
1. Introduction Iron–nickel alloys have long been used as soft magnet materials. In recent years, Fe–Ni alloy tapes have been used as substrates for high temperature superconductors (HTS) [1]. Epitaxial deposition of consecutive layers on a substrate for HTS compositions requires the presence of a very sharp {100} 〈001〉 texture, which is formed during primary recrystallization of a tape cold rolled to a high degree of deformation (98–99%). The formation of a cube texture of fcc materials is determined by the chemical composition of alloys and depends on a number of processing conditions, in particular, the fine initial grain is necessary and a high degree of cold rolling deformation is required. An important condition is the type of the deformation texture. The crystallographic orientations S {123}〈634〉 and С {112}〈111〉dominate in the deformation texture of tapes from the alloy Ni48.8Fe51.2, which, after primary recrystallization, leads to the formation of a single-component sharp cube texture {100}〈001〉 [2]. For Fe–Ni alloy compositions, in which 〈001〉 is the direction of easy magnetization, the degree of sharpness of the cube texture to a great extent determines the level of magnetic properties. Over recent years, research interest has grown to the studies of high magnetic field causing effect on the processes of grain n
Corresponding author: fax: þ 7 343 374 52 44. E-mail address: [email protected] (I.V. Gervasyeva).
http://dx.doi.org/10.1016/j.physb.2015.04.016 0921-4526/& 2015 Elsevier B.V. All rights reserved.
nucleation and growth during structural and phase transformations. The increased research interest can be attributed largely to the designing of powerful installations generating DC magnetic fields of 20–30 T and higher and to the feasibility of maintaining high temperatures inside the electric magnets. A number of works have demonstrated effect of high magnetic field on grain boundary mobility in nonmagnetic materials. For instance, Molodov et al. have studied the motion of specific planar boundaries in Bi-bicrystals under magnetic driving forces [3]. Experimental data have been gathered demonstrating the effect of magnetic field on the structure of materials, which have undergone recrystallization or a phase transformation under magnetic field, or which have crystallized from the amorphous state. Thus, Watanabe et al. [4] have demonstrated that crystallization of an amorphous tape in a magnetic field of 6 T results in a sharp crystallographic texture {110}. Bacaltchuk et al. in [5] have evaluated the effect of magnetic annealing at 19 T on the increased volume fraction of Goss-oriented grains {110} in the grain non-oriented electric steel. Zhang et al. in [6] and other researchers in their works have established that, under the effect of high DC magnetic field, phase stability of steels changes, eutectic carbon concentration increases, the critical temperatures Ac1 and Ac3 grow, carbon solubility in the ferrite phase increases, and a unique ferrite-pearlite microstructure is formed having magnetic field-induced grain elongation in the direction parallel to the magnetic field. It has been shown in [7] that in a binary alloy of Co–B that has the eutectic transformation
I.V. Gervasyeva et al. / Physica B 468-469 (2015) 66–71
temperature of around 1100 °C, which exceeds the Curie point of the cobalt-enriched phase by 20 °C only, crystallization in a 16 T magnetic field near the eutectic alloy under the conditions of sufficient cooling results in well-pronounced anisotropy of microstructure. It should be noted that there are not many studies that explore the effect of high magnetic fields on processes in solids; and each of these studies contains, mostly, experimental data, with efforts to interpret them being different in every study. No common theory has yet been developed about the essence of the effect that a high magnetic field causes on the grain boundary structure formation, on the orientation of grains nucleating and growing in ferromagnetic and nonmagnetic materials. In this work, the formation of structure and texture in Fe–Ni tapes upon annealing high DC magnetic field has been investigated.
2. Experimental The Ni48.8Fe51.2 (50 wt%) alloys were used as the material for studying. Nickel of 99.99% purity and carbonyl iron (99.97%) were smelted in alundum crucibles in an argon atmosphere in a laboratory vacuum induction furnace. The ingots were forged at a temperature of 1000–800 °С to bars of 10 10 mm2 in cross section. Then they were warm-forged at 650 °С to a size of 7 7 mm2. After grinding the blanks with dimensions of 6 6 150 mm3 were annealed at 800 °С for 1.5 h. The average grain size before the cold rolling operation was 40 μm. After cold rolling bands about 80 μm thick were formed using polished rolls. The degree of cold deformation was about 99%. Tapes from the Ni48.8Fe51.2 alloy after cold rolling to 99% were subjected to magnetic annealing in the French National High Magnetic Field Laboratory at its Grenoble site. A 50-mm-internaldiameter resistive magnet was used, which allows a DC magnetic field of 20 T to be generated. A double spiral SiC heater of 200 mm in length was inserted into the magnet. Samples 20 mm long and 5 mm wide were vacuumed in sealed ampoules and positioned in a narrow sealed tube made of stainless steel, and then were put down to a quartz tube inside the heater. A field of 10–20 T was used. Annealing without magnetic field was carried out in the same conditions. The samples were annealed in this setup at a temperature of 490 °C for 15 min and at a temperature of 600 °C for 30 min. Then the samples, subjected to pre-recrystallization annealing at 490 °C with and without magnetic field, were put in a laboratory furnace without field at the Institute of Metal Physics and annealed at a temperature of 600 °C for 30 min and at a 800 °C for 30 min. For carrying out the annealing at 800 °С, the samples were heated up from 500 °C at a rate of 100°/h and then were kept at 800 °С for 1 h. The structure and orientations in the annealed samples were analyzed simultaneously using the method of electron backscatter diffraction (EBSD), which involved the interpretation of Kikuchi patterns with an EDAX attachment in a FEI Quanta 200 scanning electron microscope. Depending on the treatment, scanning was carried out at a step of 1–4 μm, and at a magnification of 200 or 400 . The samples were electrolytically etched before this investigation. The EBSD software package was used to estimate the volume fraction of the cube planes {100} ( 710°) in the surface of the samples. The samples after low temperature annealing were further investigated by transmission electron microscopy using a JEM 200CX microscope. The selected-area electron diffraction patterns were obtained using diaphragms with a diameter of 500 nm. Average grain size after primary recrystallization was determined using an optical microscope.
67
The microhardness of the tape samples after annealing in the magnetic field of a variable strength was measured under a load of 70 g. The value of the microhardness for each sample was determined by averaging the results over at least fifteen measurements. The magnetization measurements were performed using a vibrating sample magnetometer Lake Shore 7407 in magnetic fields of 17 kOe. Relative error amounted to 1%.
3. Results and discussion Tapes of the alloy Ni48.8Fe51.2 after cold rolling 99% were subjected to magnetic annealing at a temperature of 490 or 600 °С, which is either above or below the Curie point (500 °С). This also corresponds to the temperature below or above the onset temperature of primary recrystallization for this alloy (570 °С, [8]). The sample subjected to magnetic annealing at 490 °С were then annealed in a vacuum furnace without magnetic field at 600 °С and 800 °С with slow heating. All samples were investigated using a scanning electron microscope by the EBSD technique. Orientation maps and image quality patterns obtained for the tapes annealed at a temperature below the onset temperature of primary recrystallization at 490 °С provide evidence that, as the strength of magnetic field grows in the course of annealing, identification of structure and orientations degrade. This means that the strain relieving processes are less intense in the sample annealed under magnetic field 20 T. Table 1 represents the mean values of microhardness in these samples along with the average image quality value. The samples annealed without field and under the effect of 10 T magnetic field show nearly the same level of mean value of microhardness, while the sample annealed at 20 T magnetic field exhibits a considerably greater value. The average image quality value in the samples decreases as the magnetic field strength at annealing grows. Figs. 1 and 2 display microstructure patterns obtained in a transmission electron microscope for the samples annealed at 490 °C without field and with 20 Т field, respectively. Both samples have a similar structure pattern. At high reductions, in the tapes there form, as a result of multiple slip, a cellular structure. Cell walls consist of volume dislocation pile-ups. In the course of heating, the three-dimensional walls flatten and turn into flat lowangle subboundaries, whereas cells become subgrains. Part of dislocations pass from the cell volume into the subboundaries formed. Evidently, after the pre-recrystallization annealings in magnetic field and without it these processes proceed with different rates. After the magnetic field annealing the cell walls are broad and smeared. In the selected-area electron diffraction pattern of the sample annealed in a magnetic field (Fig. 2a), the poles are more smeared in compared with the pattern of the sample annealed without field (Fig. 1a). Since the selected-area electron diffraction patterns were obtained from the same size area it can be concluded that the processes of coarsening the cells and the decrease in the magnitude of microdistortions in the lattice occur more slowly upon annealing in a field. In the dark-filed images (Figs.1b and 2b) it is seen that in the structure of the alloy annealed without field one can find areas containing subgrains, which are separated from the matrix by quite clear boundaries, Table 1 Mean values of microhardness (Нμ70) and average image quality values of Kikuchi patterns (IQ) in samples after magnetic annealing at 490 °C. 490 °C,15 min at H¼0 T
490 °C,15 min at H ¼10 T 490 °C,15 min at H¼20 T
Нμ70,МПа
Нμ70,МПа 2146
2192
IQ 2288
IQ 2012
Нμ70,МПа 2509
IQ 1941
68
I.V. Gervasyeva et al. / Physica B 468-469 (2015) 66–71
Fig. 1. TEM images taken from a sample annealed at 490 °C without field: (a) a bright-field image and a selected-area electron diffraction pattern (zone axis [110]) obtained for the area indicated with a circle; (b) a dark-field image in reflection [111] and an electron diffraction pattern of the whole image area (diaphragm with a diameter of 2 μm).
whereas in the sample annealed in the field of 20 T, no such areas are observed, and its structure is like the structure of a deformed metal. Using the EBSD technique, the samples subjected to annealing at temperatures above the primary recrystallization temperature at 600 and 800 °C were compared with the samples subjected to prior annealing at 490 °C under the magnetic field (20 Т) and without field. In all of them the main orientation was the cube component {100}〈001〉. Fig. 3 displays fragments of EBSD orientation maps for samples annealed at 600 °C for 30 min without field, for samples annealed at 600 °C in the magnetic field of 20 Т (above the Curie point), and for samples subjected to prior annealing in the magnetic field of 20 T at 490 °C (below the Curie point) and then annealed at 600 °C without field. The temperature 600 °C is only slightly above the region of realization of primary recrystallization in the alloy Ni48.8Fe51.2 [2]; the main processes related to the replacement of deformation structure by recrystallized grains have already ended, but, yet, small areas with the deformation-texture orientation are still retained. For all the samples, the volume fractions of cube orientation was determined, which are summarized in Table 2. Attention is drawn to an observation that, if magnetic annealing was carried out directly at 600 °C (above the Curie point), then the fraction of cube orientation had little difference from that for the sample annealed without field (87 and 86.9%, respectively). A different result is obtained, if before conducting the recrystallization annealing at 600 or 800 °C, a preliminary magnetic annealing is carried out at a temperature below the Curie point (490 °C). It should be noted that in fcc nickel-based alloys, which are prone to the formation of cube texture during recrystallization (which is also true for the alloy of Ni48.8Fe51.2), the step-by-step annealing, i.e., annealing
with an additional exposure to a lower temperature (which at the same time is above the onset temperature of primary recrystallization) prior to the final high temperature annealing, leads to an increased sharpness of the cube texture [2]. It has been established in the present work that sharpness of cube texture is also increased, if the preliminary annealing is carried out below the onset temperature of primary recrystallization. Preliminary annealing at 490 °C without field results in a 3.8% increase in the sharpness of cube texture in the alloy annealed at 600 °C. If such preliminary annealing is carried out in a magnetic field of 20 Т, then sharpness of cube texture increases by 7.2% (from 86.9 to 94.1%). For the alloy annealed at 800 °C, the preliminary annealing at a temperature below the Curie point with and without field 20 T leads to the sharpness of cube texture of 94.7 and 98.7%, respectively. For three samples recrystallized following various regimes, the magnetic hysteresis loops were measured (Fig. 4a). Saturation induction is approximately the same for all three versions and is given in Table 3. Induction values in the fields prior to saturation differ significantly. Fig. 4b shows a scaled up fragment of the hysteresis loop. It can be seen that for the samples annealed in the magnetic field the magnetization values are approximately the same, while the sample annealed without field the magnetization value is lower considerably. Table 3 demonstrates the values of magnetic induction in the field of 600 Oe (47600 A/m). It can be seen that, while measuring the hysteresis loop, the magnetization process runs at a higher rate for the samples subjected to the treatment in high magnetic field. That appears to support the assumption that the samples contain more grains having crystallographic direction approaching the easy magnetization direction 〈001〉.
Fig. 2. TEM images taken from a sample annealed at 490 °C in the magnetic field of 20 T: (a) a bright-field image and a selected-area electron diffraction pattern (zone axis [123]) obtained for the area indicated with a circle; (b) a dark-field image in reflection [111] and an electron diffraction pattern of the whole image area (diaphragm with a diameter of 2 μm).
I.V. Gervasyeva et al. / Physica B 468-469 (2015) 66–71
69
Fig. 3. Orientation maps of Ni48.8Fe51.2 alloy samples annealed during 30 min (a) at 600 °С without field; (b) at 600 °С in the field of 20 T; (c) at 600 °С without field of the sample preliminary subjected to pre-recrystallization annealing at 490 °С, 15 min in magnetic field 20 T and (d) the interpretation of orientations (colors) in the inverted pole figure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 2 {001} volume fraction (7 10°), % after different treatments of the samples 600 °C, 30 min
490 °C,15 minþ 600 °C, 30 min
Without field At 20 T Without field 86.9
87.0
90.7
490 °C at 20 T 94.1
490 °C,15 min þ800 °C, 30 min Without field 94.7
490 °C at 20 T 98.7
It is essential to note that in order to obtain a perfect cube texture, an elevated temperature and a longer exposure are used for the primary recrystallization process, namely, 1000–1100 °С for 1–1.5 h. The lower temperature in this study was selected in order for more pronounced effect of treatment in the magnetic field After all the treatments of samples in the high magnetic field applied, the average grain size was measured metallographically. The average values of grain size are given in Table 4. It can be noted that in the samples treated without magnetic annealing the grains grow to a larger size in the process of primary recrystallization. On the contrary, Bacaltchuk et al. [5] found that in the process of primary recrystallization the grain size is increased
in the isotropic electric steel Fe-0.75%Si as the strength of magnetic field increases up to 19 T under magnetic annealing at 800 °С for 10 min. In the present work, annealing under magnetic field was carried out preferably at temperatures below the onset temperature of primary recrystallization; and it was positively noted that softening processes are slowed down as compared to annealing at the same temperature but without magnetic field. As a result, during the following high temperature annealing of the samples annealed under the magnetic field, the grains seem to have little time to grow up to the same size as the grains in the samples annealed without field. Other works also report on the primary recrystallization process slowing down under the annealing treatments in a high magnetic field. Thus, the authors of [9] found for non-oriented 3% Si steel that the primary recrystallization process in the specimens 87% cold rolled and magnetically annealed at 10 T was slower than that in the specimens annealed without magnetic field. Marticainen and Lindroos reported in [10] that annealing in a magnetic field retards the early stage of recovery and recrystallization. They suppose this effect to be orientation-dependent because of the magnetostriction anisotropy. The same explanation is given in [11]
70
I.V. Gervasyeva et al. / Physica B 468-469 (2015) 66–71
Fig. 4. Hysteresis loop for the samples annealed at different regimes (a) and fragment of the hysteresis loop for the same samples (b).
as a possible reason for the retardation of recovery and recrystallization and for growth of grains with certain orientations; however, the authors have to report on the absence of any evidence of this. Thus, no unified opinion on the mechanisms of influence of magnetic field on the processes of recovery and recrystallization has been worked out so far. The work presented shows the effect of external magnetic field on the processes of structure transformation upon recovery. To
understand the reasons for changes in the microstructure of a deformed material upon magnetic annealing, let us pay attention to works which consider motion of dislocation in magnetic field and their interaction with domain walls, under magnetic field. The energy of interaction of dislocations with domain walls is characterized by their mutual hindering [12–14]. The authors of [12] represent the energy of interaction of domain walls with edge dislocations Фw as follows:
I.V. Gervasyeva et al. / Physica B 468-469 (2015) 66–71
Table 3 Magnetic induction in the field of 600 Oe (47600 A/m) and saturation induction in the samples subjected to various treatments. 600 °C, 30 min Н¼ 0 Т В 47600 Вs, Т
a/m,
Т 1.56 1.66
490 °C, 15 min Н ¼ 20 T þ 600 °C, 30 min
490 °C, 15 min Н ¼ 20 T þ800 °C, 1 h
1.62 1.68
1.61 1.67
Table 4 Grain size in the studied tapes after various annealing treatments. Treatment regime
Grain size, μm
600 °С, 30 min without field 600 °С, 30 min under magnetic field Н ¼ 10 T 600 °С, 30 min under magnetic field Н ¼ 20 T 490 °С, 15 min without field þ600 °С, 30 min 490 °С, 15 min under field Н ¼ 10 Tþ 600 °С, 30 min 490 °С, 15 min under field Н ¼ 20 T þ600 °С, 30 min 490 °С, 15 min without field þ800 °С, 1 h 490 °С, 15 min under field Н ¼ 10 Tþ 800 °С, 1 h 490 °С, 15 min under field Н ¼ 20 T þ800 °С, 1 h
38,2 32,8 32,2 42,0 36,0 35,0 59,6 54,8 53,1
71
magnetic field grow to larger sizes. On the other hand, by the onset of recrystallization annealing, the stored energy of deformation in the samples preliminary annealed in magnetic field remains at a higher level. The energy stimulus for the grain growth is quite high and the amount of nuclei with directions close to 〈001〉 is enhanced owing to the aligning influence of the field on the recovery stages (because of the tendency for minimizing the energy of crystallographic anisotropy) and, therefore, the sharpness of cube texture in the samples preliminary annealed in magnetic field is higher than in the samples annealed without field.
4. Conclusions
Φw = fd (
L 1/2 1/2 ) Nd = αNd1/2 2ld S
Upon primary recrystallization of the Fe–Ni samples pre-an-
(1)
where L is the average domain size, fd is the average force acting upon a domain wall from dislocations, ld is the average dislocation length, S is the area a domain wall, Nd is the dislocation density. The existence of magnetic moment of dislocations [12,13] creates an additional energy Фм of interaction with magnetization that hampers the dislocation motion
ΦM =
(λ⁎bG sin γ)2 Nd = βNd K
(2)
where λ is a function of magnetostriction constants, b is the Burgers vector, G is the shear modulus, γ is the angle between magnetization and the nearest easy-magnetization axis, K is the anisotropy constant. In large fields, the processes of domain wall movement cease because of the absence of domain walls; therefore, the energy of interaction of dislocations with domain walls Фw is absent as well. The energy of interaction of dislocations with magnetization Фм grows with increasing the magnetic field strength, since in large fields the dislocation motion is additionally hampered by the necessity for energy on overcoming the aligning effect of the magnetic field on magnetic moments. This serves in [15] as an explanation of the effect of decreasing the maximal elongation upon tension of steel in a magnetic field of 50 kOe. In the current work, samples under study were deformed to 99%; in their structure prior to annealing there are not many solitary dislocations, whereas mostly the dislocations are present in pile-ups located in the grid walls of the cellular structure. This circumstance significantly complicates the model of description of processes occurring upon simultaneous action of strong magnetic field and heating and do not allow taking into account considerations underlying formula (2); however, in the authors opinion, the main tendencies can retain themselves, which is supported by the experimental data. It is the retardation of dislocation motion and, possibly, of the diffusion processes on the whole [16] that results after a pre-recrystallization annealing in a more complete realization of the recovery processes upon annealing without field compared to the case of magnetic-field annealing. Due to this fact, upon subsequent recrystallization annealing, recrystallized grains in the samples preliminary annealed without n
nealed in a high magnetic field at a temperature below the Curie point, the volume fraction of grains with the easy magnetization direction 〈001〉 is increased in the texture; The recrystallized samples subjected to preliminary treatment in a high magnetic field have the values of magnetic induction in pre-saturation fields greater by approximately 4% than the magnetic induction values for the samples annealed without field; Applying a high magnetic field hinders the softening processes in the cold-rolled alloy during heating. The facts supporting this are that the samples annealed in magnetic field at a temperature below the recrystallization temperature are in a more stressed condition, and that in the samples preliminary annealed in magnetic field the grain size upon the primary recrystallization is less than the grain size in the samples annealed without field.
Acknowledgment We acknowledge the support of the LNCMI-CNRS, member of the European Magnetic Field Laboratory (EMFL). The work was performed within the State Program “Magnit” no. 01201463328 (Grant no 15-9-2-33) and was in part supported by the Russian Foundation for Basic Research (Grant no14-02-31143).
References [1] R.I. Tomov, A. Kurzumovic, M. Majoros, D.-J. Kang, B.A. Glowacki, J.E. Evetts, Supercond. Sci. Technol. 15 (2002) 598. [2] I.V. Gervas’eva, D.P. Rodionov, Yu.V. Khlebnikova, V.A. Kazantsev, Phys. Met. Metallogr. 111 (2011) 271. [3] D.A. Molodov, G. Gottstein, F. Heringhaus, L.S. Shvindlerman, Acta Mater. 46 (1998) 5627. [4] T. Watanabe, S. Tsurekawa, X. Zhao, L. Zuo, C.J. Esling, J. Mater. Sci. 41 (2006) 7747. [5] C.M.B. Bacaltchuk, G.A. Castello-Branco, H. Garmestani. Proceedings of the 14th International Confernce on Textures of Materials. Leuven, Belgium. Material Science Forum 702–703 (2005) 1165. [6] Y.D. Zhang, C. Esling, X. Zhao, L. Zuo. Proceedings of the14th International Confernce on Textures of Materials. Leuven, Belgium. Material Science Forum 702–703 (2005) 1131. [7] F. Gaucherand, E. Beaugnon, Phisica B 346-347 (2004) 262. [8] I.V. Gervasyeva, Yu.V. Khlebnikova, D.P. Rodionov, E.S. Belosludtseva, V. A. Milyutin, T.R. Suaridze, Phys. Met. and Metallogr. 114 (2013) 171. [9] Y. Xu, H. Ohtsuka, K. Itoh, H. Wada, J. Magn. Soc. Jpn. 24 (2000) 651. [10] H.O. Marticainen, V.K. Lindroos, Scan. J. Metals. 10 (1981) 3. [11] N. Masahashi, M. Matsuo, K. Watanabe, J. Mater. Res. 13 (1998) 457. [12] F. Vicena., Czech. J. Phys. 5 (1955) 480. [13] A. Seeger, H. Kronmuller, H. Rieger, R. Traubler, J.Appl. Phys. 35 (1964) 740. [14] R. Traubler, Z. Metallk. 53 (1962) 211. [15] T.V. Efimova, V.V. Polotnyuk, Ukraine Phys. J. 26 (1981) 1149 (in Russian). [16] H. Nakajima, S. Maekawa, Y. Aoki, et al., Trans. Jpn. Instit. Metals. 26 (1985) 1.