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Microstructure evolution and magnetic properties of hard magnetic FeCr22Co15 alloy subjected to tension combined with torsion
Journal of Alloys and Compounds xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructure evolution and magnetic properties of hard magnetic FeCr22Co15 alloy subjected to tension combined with torsion Anna Korneva a,⇑, Galija Korznikova b, Katarzyna Berent a, Aleksandr Korznikov b, Rishat Kashaev b, Joanna Bogucka a, Krzysztof Sztwiertnia a a b
Polish Academy of Sciences, Institute of Metallurgy and Materials Science, Cracow, Poland Russian Academy of Sciences, Institute for Metal Superplasticity Problems, Ufa, Russia
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Permanent magnets High temperature deformation Microstructure Scanning electron microscopy Transmission electron microscopy Magnetic measurements
a b s t r a c t Axially symmetric magnets with good mechanical properties in the surface layer and good magnetic properties in the volume of material are essential in the production of increasing number of modern electrical devices. In order to obtain this type of gradient structure, two phase (a + c) hard magnetic FeCr22Co15 alloy was subjected to tension and torsion in temperature range 725–850 °C. The microstructure observations of deformed samples showed the formation of gradient microstructure with minimum grain size (about 1 lm) in the surface layer of material. On the basis of TEM observations, it was found that the deformation resulted in the martensitic transformation of c into ac phase. The microstructure observations also showed the presence of intermetallic r phase (Fe–Cr) with the maximum precipitates amount in the surface layer of material. The measurements of magnetic properties showed decreasing coercive force of FeCr22Co15 alloy after deformation due to preservation of ac phase (about of 35%) in the microstructure after magnetic treatment. On the other hand, ac phase might have appeared as a plastic constituent among brittle grains of a phase, which had a positive influence on mechanical properties of the material. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The development of new generation of high-speed electrical devices requires high strength characteristics of magnetic materials used. Most magnetic materials of today reveal high magnetic characteristics but they are brittle and have low ultimate rupture strength. The highest level of mechanical properties is realized in magnetically hard alloys of Fe–Cr–Co system. The Fe–Cr–Co based alloys belong to the deformable magnetic materials of the precipitation-hardening class. Due to their good ductility, excellent magnetic properties and low cost, they are used for the production of permanent magnets of various sizes and shapes, such as wire, tube, bar and strip magnets [1–3]. A high-coercive state is obtained by a magnetic treatment and multistage tempering. This leads to the decomposition of a solid solution into the isomorphous a1 and a2 phases, containing ordered and coherent precipitates [4,5]. The formation of such structures, in which each precipitate of the a1 phase is a single magnetic domain, provides superior magnetic properties. However, internal stress fields, which originate from
⇑ Corresponding author. E-mail address: [email protected] (A. Korneva).
the formation of coherent boundaries between the precipitates of the a1 and a2 phases, cause a reduction in ductility and strength. It is known, that the structure of material and its mechanical properties can be changed using severe deformation techniques [6,7]. Complex loading by compression, tension and torsion at an elevated temperature is rather a new method of severe plastic deformation [8]. It ensures a substantially refined microstructure without changing the shape of the specimen. Depending on the mode of the deformation chosen, this method allows localizing strain in specific regions and ensures the formation of gradient microstructure with different combination of magnetic and mechanical properties [9,10]. The aim of the present work is to present the results of the microstructure and magnetic properties of the FeCr22Co15 alloy deformed by tension combined with torsion under isothermal conditions at different temperatures.
2. Experimental details In order to obtain the a solid solution the alloy was subjected to homogenization and water-quench from 1240 °C. The chemical composition of the examined alloy was 23.0 wt.% Cr, 15.8 wt.% Co, 0.9 wt.% Ti, 0.9 wt.% V, balance Fe.
http://dx.doi.org/10.1016/j.jallcom.2014.01.207 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Korneva et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.207
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A. Korneva et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
The FeCr22Co15 alloy was subjected to plastic deformation in a modernized ‘‘Instron’’ machine, which allowed deformation at single or simultaneous actions of axial loading and torsion under superplasticity conditions [8]. Cylindrical samples, 8 mm in diameter and 44 mm of length, were deformed simultaneously by tension applied to the upper part of sample and torsion applied to the bottom part of sample. The samples were subjected to tension at the rate of 6 04 s1 to obtain the deformation of 20% and torsion at the rate of 8.5 104 s1 in 7 rotations. The deformation temperatures 725, 750, 800, 850 °C corresponded to the appearance range of the a + c phases. The deformation temperatures as well as deformation rates followed the superplasticity conditions of the examined alloy. The total strain of deformation, which takes into account the contribution of torsion e1 and tension e2, can be estimated by [11],
pffiffiffi e ¼ e1 þ e2 ¼ u R= 3 l0 þ Inðl=l0 Þ;
ð1Þ
where l0 (mm) and l (mm) are the sample length before and after deformation; u (rad) is the angle of torsion deformation and R (mm) is the distance from the axis of rotation. The total degree of deformation, at a distance of half the radius (R = 2 mm) was about 1.34, while at the surface (R = 3.5 mm) about 2.21. The microstructure was examined by means of the scanning electron microscope (SEM) XL 30 ESEM, Philips and the transmission electron microscope (TEM) Tecnai G2 F20 (200 kV). The maps of orientations were measured with the use of EBSD method for the analysis of grain boundaries. The crystallographic orientation was determined by means of convergent beam electron diffraction (CBED) analysis in TEM (Philips CM 20). The microhardness was measured with the Vickers method at the load of 98 mN by means of CSM Instrument. Magnetic treatment was carried out according to the Russian Standard [12]: heating at 620–650 °C in a magnetic field with an intensity of at least 100 kA m1 for 1–2 h; and tempering at 610 °C for 2 h, at 580 °C for 3 h, at 560 °C for 4 h and at 540 °C for 6 h. The magnetic field direction was parallel to the tension direction. Magnetic measurements were carried out by means of a magnetometer Permagraph C-300, while the external magnetic field was parallel to the tension direction. The measurements were carried out on a cuboidal sample with dimensions 3 1.7 15 mm.
3. Results and discussion The microstructure of the pre-deformation state (after quenching from high temperatures and subsequent annealing within the temperature range of 725–850 °C), consists of large (700 lm) grains of the a phase which contains lamellar, 20 lm thick, precipitates of c phase. Fig. 1a shows an example of such a microstructure for the annealing temperature of 800 °C. The SEM observations of longitudinal sections of deformed samples show a gradient microstructure with the highest grain refinement in the zone of intensive deformation (i.e. in the samples surface layer). Fig. 2 presents, for comparison, the examples of the microstructure of the samples deformed at 725 and 850 °C taken from the surface layer and from inside of the samples (as shown in scheme Fig. 2c). The increasing of the deformation temperature from 725 to 850 °C slightly increased the grain size of a phase and the thickness of the fine-grained surface layer from 2.5 to 3.5 mm (for the sample thickness of 8 mm).
Fig. 1. Lamellar structure (a + c) of FeCr25Co15 alloy, obtained by heating the solid a solution at 800 °C.
On the basis of X-ray phase analysis, it can be concluded that the c phase disappears, and the intermetallic r phase is formed. Usually, the appearance of non magnetic r phase is undesirable because it is brittle and hard, so it worsens the mechanical and magnetic properties of material. The process of r-phase precipitation as a rule is very time-consuming [13]. During annealing at temperatures at which the r-phase is stable (it is for the examined alloy in the temperature range 562–750 °C) its precipitation can take several days. However, the intensive deformation of Fe–Cr– Co alloys stimulates the precipitation of r phase by the activation of diffusion processes [9]. It seems that the r-phase precipitation promotes the refinement of the material’s microstructure. The r phase usually precipitates at the grain boundaries of the a phase. Therefore, the movement of dislocations and the transfer of deformation from one grain to another one become more difficult. Consequently, the density of dislocations inside the a grains increases and brings about their refinement. From this point of view the process of precipitation of r phase can be seen as a positive phenomenon. The EBSD/SEM measurements of deformed FeCr22Co15 alloy confirm that more r phase precipitates form in the regions of intense deformation (at the surface of the samples) (Fig. 3a). The fraction of r phase precipitates could reach there up to 42% and inside the material up to 25% (depending on the deformation temperature). The grain sizes of a phase in the surface layer of samples reaches 0.8 and 1.6 lm for the deformation temperatures 725 and 850 °C, respectively. The EBSD/SEM measurement show also, that in the volume of material in the areas of former c phase the grains of a phase develop with a high density of low angle grain boundaries (Fig. 3b). The TEM investigations of such areas show thin plates with low angles of disorientation and very high density of dislocations (Fig. 3d). Some of these plates show a strong deformed lattice as evidenced by diffused lines in diffraction patterns. Electron diffractions obtained from separate (single) plates exhibit a good solution for both a phase and the tetragonal martensite. The martensitic transformation of c phase into ac after quenching from high temperature (800–1100 °C) was already observed in the FeCr23Co15 alloy [14]. For the investigated alloy, the strong deformation and cooling from high temperature could probably cause the transformation of c phase into the ac phase. The TEM observations show that inside the material the r grains mainly precipitated on the boundaries between the a grains and former c phase (now ac),while at the surface layer the r phase particles were more uniformly distributed among a grains (Fig. 3c). Before magnetic treatment, the deformed samples were subjected to annealing at 1000 °C for 10 min in order to dissolve non-magnetic r phase. The magnetic measurements were carried out on cuboidal samples with dimensions 3 1.7 15 mm, which contained both the material from the interior and from the surface layer of the deformed alloy. The SEM observations of microstructure of all deformed samples after the magnetic treatment demonstrate about 29–35% of ac phase in the a matrix (depending on the deformation temperature) (Fig. 4). The a phase decomposed into the isomorphous a1 and a2 phases, containing ordered and coherent precipitates of sizes about 100 nm. Table 1 presents the magnetic properties (coercive force – Hc, residual flux density – Br, magnetic flux saturation density – Bs) of the alloy after deformation and subsequent magnetic treatment. The table shows that magnetic properties of the deformed alloy are worse than these in the high-coercive state (without deformation). The deterioration of magnetic properties is related primarily to the presence of soft magnetic ac phase [14]. The microstructure refinement after deformation may also have a negative impact on magnetic properties. This is related to the increased area of grain boundaries. The decomposition of the a solid solution near the
Please cite this article in press as: A. Korneva et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.207
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A. Korneva et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
Fig. 2. Microstructure of deformed FeCr22Co15 alloy at surface (location a) and in the volume (location b) of samples deformed at 725 (a) and 850 °C (b) according to the scheme (c).
Fig. 3. EBSD image of phase topography and TEM observation of microstructure at the surface layer (a and c) and inside at the areas of former c phase (b and d) of the FeCr22Co15 alloy deformed at 750 °C.
Table 1 Magnetic properties of FeCr22Co15 alloy subjected to tension combined with torsion: the coercive force Hc (kA m1), the saturation flux density Bs (T), the residual flux density Br (T). The alloy properties in the high-coercive state (HCS) are presented for comparison.
Fig. 4. Microstructure of FeCr22Co15 alloy after deformation at 750 °C and subsequent magnetic treatment.
Temperature of deformation
Hc (kA/v)
Br (T)
Bs (T)
725 °C 750 °C 800 °C 850 °C HCS without deformation
34.58 34.30 34.90 34.83 40
0.84 0.78 0.86 0.86 1.2
1.12 1.08 1.15 1.1 1.3
grain boundary differs from the decomposition inside the grain, as the precipitates of a1 and a2 phases have different modulation (size), shape and orientation. The changes of precipitate orientation close to the grain boundary reduce the material magnetization and coercive force [9,10].
Please cite this article in press as: A. Korneva et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.207
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A. Korneva et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
On one hand, the ac phase impairs magnetic properties. On the other hand, it may also appear as a plastic constituent among brittle grains of a phase, which has a positive influence on mechanical properties of the material. The measurement of microhardness conformed that the ac phase is soft in comparison to the (a1 + a2) structure. The microhardness HV0.01 of the (a1 + a2) structure is about 420, while this of ac phase is 350. The next part of the work will be devoted to the investigation of mechanical properties of deformed FeCr23Co15 alloy after magnetic treatment. 4. Conclusions The deformation by tension combined with torsion resulted in the formation of gradient microstructure with the minimum grain size in the surface layer of the material. The grain size and thickness of surface layer with fine grains slightly increased with the change of deformation temperature from 725 to 850 °C. The deformation stimulated the precipitation of intermetallic r phase. The highest amount of r phase was observed in the most deformed surface layer. Precipitation of the r-phase promotes the refinement of the material microstructure. The deformation caused the martensitic transformation of c phase into ac. The presence of soft magnetic ac phase in the FeCr22Co15 alloy after magnetic treatment resulted in the reduction of coercivity and saturation magnetization. It should be noted that the here obtained coercive forces are higher about 30% than that obtained for single phase (a) Fe30Cr8Co alloy in which a similar plastic deformation did not caused the formation of a soft magnetic ac phase [15]. The ac phase may be seen as a plastic constituent among brittle grains of a phase, what has a positive impact on mechanical properties of the material.
Acknowledgements This work has been supported by the Ministry of Science and Higher Education of Poland within the Project No. 507 530539. The SEM and TEM studies were performed in the Accredited Testing Laboratories at the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences. References [1] A.V. Korznikov, Structure and Mechanical Properties of Metals and Alloys Subjected to Severe Plastic Deformation, Habilitation Thesis, IPMS Ufa, Russia, 2000. [2] Z. Ahmad, A. Hag, S.W. Husain, T. Abbas, J. Magn. Magn. Mater. 257 (2003) 397. [3] X.Y. Sun, C.Y. Xu, L. Zhen, R.S. Gao, R.G. Xu, J. Magn. Magn. Mater. 283 (2004) 231–237. [4] Z. Ahmad, A. Hag, S.W. Husain, T. Abbas, Physica B 321 (2002) 96–103. [5] X.Y. Sun, C.Y. Xu, L. Zhen, L.X. Lv, L. Jang, J. Magn. Magn. Mater. 312 (2007) 342– 346. [6] H. Gleiter, Acta Mater. 48 (2000) 1–29. [7] R.Z. Valiev, I.V. Aleksandrov, R.K. Islamgaleev, Prog. Mater. Sci. 46 (2000) 103– 189. [8] O.A. Kajbyshev, R.A. Vasin, V.K. Berdin, R.M. Kashaev, Kuznechno Shtampowoe Proizwodstwo 4 (1999) 8 (in Russian). [9] A. Korneva, M. Bieda-Niemiec, G. Korznikova, A. Korznikov, K. Sztwiertnia, Int. J. Mater. Res. 1 (2011) 32–40. [10] A. Korneva, M. Bieda-Niemiec, G. Korznikova, K. Sztwiertnia, Int. J. Mater. Res. 9 (2008) 991–998. [11] R.M. Kashaev, G.V. Korznikova, A.V. Korznikov, Perspektivnye Materaly 12 (2011) 213–217 (in Russian). [12] Yu.K. Kovneristyj, S.P. Efimenko, G.F. Korznikova, A.I. Milyaev, Metalloved. Term. Obrab. Met. 5 (2004) 15–17 (in Russian). [13] K. Racka-Dzietko, Struktura i włas´ciwos´ci magnetyczne nanocza˛stek Fe–Cr w funkcji zawartos´ci chromu, Thesis, Warszawa, 2007 25 (in Polish). [14] V.S. Shubakov, D.G. Zhukov, O.A. Ushakova, Metalloved. Term. Obrab. Met. 3 (2012) 16–19 (in Russian). [15] A. Korneva, et al., in preparation.
Please cite this article in press as: A. Korneva et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.207
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructure evolution and magnetic properties of hard magnetic FeCr22Co15 alloy subjected to tension combined with torsion Anna Korneva a,⇑, Galija Korznikova b, Katarzyna Berent a, Aleksandr Korznikov b, Rishat Kashaev b, Joanna Bogucka a, Krzysztof Sztwiertnia a a b
Polish Academy of Sciences, Institute of Metallurgy and Materials Science, Cracow, Poland Russian Academy of Sciences, Institute for Metal Superplasticity Problems, Ufa, Russia
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Permanent magnets High temperature deformation Microstructure Scanning electron microscopy Transmission electron microscopy Magnetic measurements
a b s t r a c t Axially symmetric magnets with good mechanical properties in the surface layer and good magnetic properties in the volume of material are essential in the production of increasing number of modern electrical devices. In order to obtain this type of gradient structure, two phase (a + c) hard magnetic FeCr22Co15 alloy was subjected to tension and torsion in temperature range 725–850 °C. The microstructure observations of deformed samples showed the formation of gradient microstructure with minimum grain size (about 1 lm) in the surface layer of material. On the basis of TEM observations, it was found that the deformation resulted in the martensitic transformation of c into ac phase. The microstructure observations also showed the presence of intermetallic r phase (Fe–Cr) with the maximum precipitates amount in the surface layer of material. The measurements of magnetic properties showed decreasing coercive force of FeCr22Co15 alloy after deformation due to preservation of ac phase (about of 35%) in the microstructure after magnetic treatment. On the other hand, ac phase might have appeared as a plastic constituent among brittle grains of a phase, which had a positive influence on mechanical properties of the material. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The development of new generation of high-speed electrical devices requires high strength characteristics of magnetic materials used. Most magnetic materials of today reveal high magnetic characteristics but they are brittle and have low ultimate rupture strength. The highest level of mechanical properties is realized in magnetically hard alloys of Fe–Cr–Co system. The Fe–Cr–Co based alloys belong to the deformable magnetic materials of the precipitation-hardening class. Due to their good ductility, excellent magnetic properties and low cost, they are used for the production of permanent magnets of various sizes and shapes, such as wire, tube, bar and strip magnets [1–3]. A high-coercive state is obtained by a magnetic treatment and multistage tempering. This leads to the decomposition of a solid solution into the isomorphous a1 and a2 phases, containing ordered and coherent precipitates [4,5]. The formation of such structures, in which each precipitate of the a1 phase is a single magnetic domain, provides superior magnetic properties. However, internal stress fields, which originate from
⇑ Corresponding author. E-mail address: [email protected] (A. Korneva).
the formation of coherent boundaries between the precipitates of the a1 and a2 phases, cause a reduction in ductility and strength. It is known, that the structure of material and its mechanical properties can be changed using severe deformation techniques [6,7]. Complex loading by compression, tension and torsion at an elevated temperature is rather a new method of severe plastic deformation [8]. It ensures a substantially refined microstructure without changing the shape of the specimen. Depending on the mode of the deformation chosen, this method allows localizing strain in specific regions and ensures the formation of gradient microstructure with different combination of magnetic and mechanical properties [9,10]. The aim of the present work is to present the results of the microstructure and magnetic properties of the FeCr22Co15 alloy deformed by tension combined with torsion under isothermal conditions at different temperatures.
2. Experimental details In order to obtain the a solid solution the alloy was subjected to homogenization and water-quench from 1240 °C. The chemical composition of the examined alloy was 23.0 wt.% Cr, 15.8 wt.% Co, 0.9 wt.% Ti, 0.9 wt.% V, balance Fe.
http://dx.doi.org/10.1016/j.jallcom.2014.01.207 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Korneva et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.207
2
A. Korneva et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
The FeCr22Co15 alloy was subjected to plastic deformation in a modernized ‘‘Instron’’ machine, which allowed deformation at single or simultaneous actions of axial loading and torsion under superplasticity conditions [8]. Cylindrical samples, 8 mm in diameter and 44 mm of length, were deformed simultaneously by tension applied to the upper part of sample and torsion applied to the bottom part of sample. The samples were subjected to tension at the rate of 6 04 s1 to obtain the deformation of 20% and torsion at the rate of 8.5 104 s1 in 7 rotations. The deformation temperatures 725, 750, 800, 850 °C corresponded to the appearance range of the a + c phases. The deformation temperatures as well as deformation rates followed the superplasticity conditions of the examined alloy. The total strain of deformation, which takes into account the contribution of torsion e1 and tension e2, can be estimated by [11],
pffiffiffi e ¼ e1 þ e2 ¼ u R= 3 l0 þ Inðl=l0 Þ;
ð1Þ
where l0 (mm) and l (mm) are the sample length before and after deformation; u (rad) is the angle of torsion deformation and R (mm) is the distance from the axis of rotation. The total degree of deformation, at a distance of half the radius (R = 2 mm) was about 1.34, while at the surface (R = 3.5 mm) about 2.21. The microstructure was examined by means of the scanning electron microscope (SEM) XL 30 ESEM, Philips and the transmission electron microscope (TEM) Tecnai G2 F20 (200 kV). The maps of orientations were measured with the use of EBSD method for the analysis of grain boundaries. The crystallographic orientation was determined by means of convergent beam electron diffraction (CBED) analysis in TEM (Philips CM 20). The microhardness was measured with the Vickers method at the load of 98 mN by means of CSM Instrument. Magnetic treatment was carried out according to the Russian Standard [12]: heating at 620–650 °C in a magnetic field with an intensity of at least 100 kA m1 for 1–2 h; and tempering at 610 °C for 2 h, at 580 °C for 3 h, at 560 °C for 4 h and at 540 °C for 6 h. The magnetic field direction was parallel to the tension direction. Magnetic measurements were carried out by means of a magnetometer Permagraph C-300, while the external magnetic field was parallel to the tension direction. The measurements were carried out on a cuboidal sample with dimensions 3 1.7 15 mm.
3. Results and discussion The microstructure of the pre-deformation state (after quenching from high temperatures and subsequent annealing within the temperature range of 725–850 °C), consists of large (700 lm) grains of the a phase which contains lamellar, 20 lm thick, precipitates of c phase. Fig. 1a shows an example of such a microstructure for the annealing temperature of 800 °C. The SEM observations of longitudinal sections of deformed samples show a gradient microstructure with the highest grain refinement in the zone of intensive deformation (i.e. in the samples surface layer). Fig. 2 presents, for comparison, the examples of the microstructure of the samples deformed at 725 and 850 °C taken from the surface layer and from inside of the samples (as shown in scheme Fig. 2c). The increasing of the deformation temperature from 725 to 850 °C slightly increased the grain size of a phase and the thickness of the fine-grained surface layer from 2.5 to 3.5 mm (for the sample thickness of 8 mm).
Fig. 1. Lamellar structure (a + c) of FeCr25Co15 alloy, obtained by heating the solid a solution at 800 °C.
On the basis of X-ray phase analysis, it can be concluded that the c phase disappears, and the intermetallic r phase is formed. Usually, the appearance of non magnetic r phase is undesirable because it is brittle and hard, so it worsens the mechanical and magnetic properties of material. The process of r-phase precipitation as a rule is very time-consuming [13]. During annealing at temperatures at which the r-phase is stable (it is for the examined alloy in the temperature range 562–750 °C) its precipitation can take several days. However, the intensive deformation of Fe–Cr– Co alloys stimulates the precipitation of r phase by the activation of diffusion processes [9]. It seems that the r-phase precipitation promotes the refinement of the material’s microstructure. The r phase usually precipitates at the grain boundaries of the a phase. Therefore, the movement of dislocations and the transfer of deformation from one grain to another one become more difficult. Consequently, the density of dislocations inside the a grains increases and brings about their refinement. From this point of view the process of precipitation of r phase can be seen as a positive phenomenon. The EBSD/SEM measurements of deformed FeCr22Co15 alloy confirm that more r phase precipitates form in the regions of intense deformation (at the surface of the samples) (Fig. 3a). The fraction of r phase precipitates could reach there up to 42% and inside the material up to 25% (depending on the deformation temperature). The grain sizes of a phase in the surface layer of samples reaches 0.8 and 1.6 lm for the deformation temperatures 725 and 850 °C, respectively. The EBSD/SEM measurement show also, that in the volume of material in the areas of former c phase the grains of a phase develop with a high density of low angle grain boundaries (Fig. 3b). The TEM investigations of such areas show thin plates with low angles of disorientation and very high density of dislocations (Fig. 3d). Some of these plates show a strong deformed lattice as evidenced by diffused lines in diffraction patterns. Electron diffractions obtained from separate (single) plates exhibit a good solution for both a phase and the tetragonal martensite. The martensitic transformation of c phase into ac after quenching from high temperature (800–1100 °C) was already observed in the FeCr23Co15 alloy [14]. For the investigated alloy, the strong deformation and cooling from high temperature could probably cause the transformation of c phase into the ac phase. The TEM observations show that inside the material the r grains mainly precipitated on the boundaries between the a grains and former c phase (now ac),while at the surface layer the r phase particles were more uniformly distributed among a grains (Fig. 3c). Before magnetic treatment, the deformed samples were subjected to annealing at 1000 °C for 10 min in order to dissolve non-magnetic r phase. The magnetic measurements were carried out on cuboidal samples with dimensions 3 1.7 15 mm, which contained both the material from the interior and from the surface layer of the deformed alloy. The SEM observations of microstructure of all deformed samples after the magnetic treatment demonstrate about 29–35% of ac phase in the a matrix (depending on the deformation temperature) (Fig. 4). The a phase decomposed into the isomorphous a1 and a2 phases, containing ordered and coherent precipitates of sizes about 100 nm. Table 1 presents the magnetic properties (coercive force – Hc, residual flux density – Br, magnetic flux saturation density – Bs) of the alloy after deformation and subsequent magnetic treatment. The table shows that magnetic properties of the deformed alloy are worse than these in the high-coercive state (without deformation). The deterioration of magnetic properties is related primarily to the presence of soft magnetic ac phase [14]. The microstructure refinement after deformation may also have a negative impact on magnetic properties. This is related to the increased area of grain boundaries. The decomposition of the a solid solution near the
Please cite this article in press as: A. Korneva et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.207
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A. Korneva et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
Fig. 2. Microstructure of deformed FeCr22Co15 alloy at surface (location a) and in the volume (location b) of samples deformed at 725 (a) and 850 °C (b) according to the scheme (c).
Fig. 3. EBSD image of phase topography and TEM observation of microstructure at the surface layer (a and c) and inside at the areas of former c phase (b and d) of the FeCr22Co15 alloy deformed at 750 °C.
Table 1 Magnetic properties of FeCr22Co15 alloy subjected to tension combined with torsion: the coercive force Hc (kA m1), the saturation flux density Bs (T), the residual flux density Br (T). The alloy properties in the high-coercive state (HCS) are presented for comparison.
Fig. 4. Microstructure of FeCr22Co15 alloy after deformation at 750 °C and subsequent magnetic treatment.
Temperature of deformation
Hc (kA/v)
Br (T)
Bs (T)
725 °C 750 °C 800 °C 850 °C HCS without deformation
34.58 34.30 34.90 34.83 40
0.84 0.78 0.86 0.86 1.2
1.12 1.08 1.15 1.1 1.3
grain boundary differs from the decomposition inside the grain, as the precipitates of a1 and a2 phases have different modulation (size), shape and orientation. The changes of precipitate orientation close to the grain boundary reduce the material magnetization and coercive force [9,10].
Please cite this article in press as: A. Korneva et al., J. Alloys Comp. (2014), http://dx.doi.org/10.1016/j.jallcom.2014.01.207
4
A. Korneva et al. / Journal of Alloys and Compounds xxx (2014) xxx–xxx
On one hand, the ac phase impairs magnetic properties. On the other hand, it may also appear as a plastic constituent among brittle grains of a phase, which has a positive influence on mechanical properties of the material. The measurement of microhardness conformed that the ac phase is soft in comparison to the (a1 + a2) structure. The microhardness HV0.01 of the (a1 + a2) structure is about 420, while this of ac phase is 350. The next part of the work will be devoted to the investigation of mechanical properties of deformed FeCr23Co15 alloy after magnetic treatment. 4. Conclusions The deformation by tension combined with torsion resulted in the formation of gradient microstructure with the minimum grain size in the surface layer of the material. The grain size and thickness of surface layer with fine grains slightly increased with the change of deformation temperature from 725 to 850 °C. The deformation stimulated the precipitation of intermetallic r phase. The highest amount of r phase was observed in the most deformed surface layer. Precipitation of the r-phase promotes the refinement of the material microstructure. The deformation caused the martensitic transformation of c phase into ac. The presence of soft magnetic ac phase in the FeCr22Co15 alloy after magnetic treatment resulted in the reduction of coercivity and saturation magnetization. It should be noted that the here obtained coercive forces are higher about 30% than that obtained for single phase (a) Fe30Cr8Co alloy in which a similar plastic deformation did not caused the formation of a soft magnetic ac phase [15]. The ac phase may be seen as a plastic constituent among brittle grains of a phase, what has a positive impact on mechanical properties of the material.
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