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Character of transformations in Fe–Co system
Materials Science and Engineering A248 (1998) 238 – 244
Character of transformations in Fe–Co system Y. Ustinovshikov *, S. Tresheva Physical-Technical Institute, 132 Kiro6 Str., 426000 Izhe6sk, Russia Received 28 April 1997; received in revised form 12 November 1997
Abstract The phase transformations of Fe–(35–65%)Co alloys and ‘Permendur’ (48.4%Fe – 49.8%Co – 1.8wt.%V) have been studied. It was found that the B2 phase is formed solely during heat treatment in air atmosphere and in a thin surface layer, but not in the bulk and not in vacuum. At high temperatures, the Fe – Co alloys separate into the pure components, but also only in a thin surface layer. A ‘550°C anomaly’ arises as a result of changes of magnetic configuration in the alloys. The causes and consequences of these transformations in Fe–Co alloys are analysed. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Ordering; Phase transformations; Separation
1. Introduction The nearly equiatomic Fe – Co alloys, which have wide technological application, are known as soft magnetic materials with maximum saturation magnetization and Curie temperature and a high magnetostriction. It is believed that all these excellent magnetic properties can manifest themselves if a long-range ordering of A2B2 type takes place in the bulk. In this case, only a rearrangement of atoms on the same lattice occurs, with a very small change in volume (0.2% expansion) and no local composition changes. A previous experimental study [1] of ordering in the Fe – Co system showed that the A2 B2 transformation was of the homogeneous type, i.e. the degree of order within the alloys remained uniform during transformation. However, a network of antiphase domains was formed which coalesced on prolonged annealing [1]. The order–disorder transitions that occur in Fe–Co solid solutions have attracted considerable attention because they are expected to behave in the same way mathematically near the order – disorder transition temperature as the Ising model [1]. In recent years, many studies of the critical behavior of the order –disorder transition in Fe–Co alloys have been made [1–8]. * Corresponding author. Tel.: +7 3412 216633; fax: + 7 3412 250614; e-mail: [email protected] 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00506-1
In order to achieve a long-range ordering of the B2 type, the standard heat treatment (annealing at 850°C for 2 h and then furnace cooling down to 400°C at a rate of 100° C h − 1) is commonly used [2]. It is supposed that such a prolonged heat treatment, especially below the A2B2 phase transition temperature, transfers the alloy to the ordered state with the degree of order close to the equilibrium value of 0.9 [2]. The peaks of specific heat capacity and specific electrical resistivity associated with the order–disorder transformations have been observed over the composition range Fe–(30–70)%Co [3–5], and a second maximum in the heat-capacity curves at 550°C has been reported for alloys in the range Fe–(35–65)%Co [3]. The second peak is very sensitive to heat treatment and becomes much smaller in the furnace-cooled specimens than in the rapidly cooled material. This peak is interpreted as being due to kinetic effects [6]. Most of the investigations have employed X-ray and neutron diffraction techniques to determine the degree of long-range order as a function of annealing temperature and time [7,8]. Unfortunately, accurate measurement of the degree of long-range order is difficult and generally relative, rather than absolute. Therefore, many details of the B2 superstructure formation, as well as the nature of exceptional magnetic properties as a function of atomic ordering, remain unknown. It is thought that the rate of long-range ordering is so high that any quenching in water can not, as a rule, suppress
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the disorder–order transformation. This was overcome by quenching the 0.6 mm thick specimens from 850°C in iced brine to retain the disordered state [8]. But even quenching at a rate of 104 K s − 1, used to obtain amorphous alloys, can not completely suppress the long-range ordering process, although the degree of order is low (0.2– 0.3) [2]. The b.c.c. Fe–Co alloys are stable only in a limited temperature and composition range as shown in Fig. 1 [6]. Chemical ordering reactions lead to a B2-type ordered structure, which is experimentally confirmed for Co contents 0.3 5 CCo 50.7. The b.c.c. alloys are ferromagnetic with a Curie temperature which for most compositions (CCo \0.15) is higher than that of the b.c.cf.c.c. transition. The aim of the present work was to examine the conditions of the B2-phase formation in the Fe–(35– 65)%Co alloys as well as to study the influence of the B2 phase on embrittlement, hardness, magnetic and other properties of the alloys mentioned.
2. Experimental procedure The alloys 50%Fe – 50%Co, 65%Fe – 35%Co and 35%Fe–65%Co were examined, as well as the commercial alloy ‘Permendur’ (48.4%Fe – 49.8%Co – 1.8wt.%V) for comparison. The ingots were made by vacuummelting high-purity electrolytic iron (99.86%) and cobalt (99.87%) in an alumina crucible and casting into a steel mold 20 mm in diameter. Chemical analysis of the resulting alloys showed them to contain 90.35 wt.% of the desired Co content. The ingots obtained were cut, hot-forged and then machined to the required size (rectangular plates of 15 mm×10 mm × 3 mm).
Fig. 1. Phase diagram of Fe–Co [6].
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The specimens were heat treated in furnaces controlled within 9 1°C to obtain either the fully ordered or disordered states. It was expected that the ordered state would be obtained by the standard heat treatment— heating at 850°C for 2 h and then furnace cooling to 400°C at a rate of 100°C h − 1 —as well as by isothermal aging at 700°C. It also was expected that the disordered state would be obtained by heating to 800–1200°C with subsequent quenching in iced brine. During the process of heat treatment in air atmosphere very thin oxide films form in the surface layer of the specimen. The films peeled immediately during water quenching. X-ray examination showed the films are complex oxides. X-ray measurements at room and high temperatures were made using an X-ray diffractometer fitted with a diffracted-beam monochromator set for cobalt Ka radiation. In some cases it was necessary to carry out the X-ray measurements both at the surface and in the bulk of the specimens. For this purpose the surface layer was removed by sanding. It could be supposed that this mechanical surface treatment may produce disordering of the alloys studied. That is why for comparison the surface layer of some specimens was removed by electrolytic polishing. But the results in both cases were the same. The microhardness was measured under 500 g loading. The saturation magnetic induction was measured for different structure states of the alloys in an electric field equal to 15000 A m − 1.
3. Results and discussion We consider first the Fe–50Co alloy aged isothermally at 700°C in vacuum and air atmosphere. X-ray phase analysis was made at the surface and in the bulk (after removing not less than 0.1 mm of surface layer). Fig. 2 shows a portion near the (110) line of the X-ray patterns of specimens heat treated in air atmosphere. Interpretation of X-ray phase analysis in the bulk and at the surface showed that the long-range ordering process resulting in the B2-structure formation takes place only in a thin surface layer, not in the bulk. The percentage of B2 phase at the surface never exceeded 90. Removing the surface layers lowers the quantity of B2 phase. Fig. 3 shows the depth of B2-phase penetration into the bulk (i.e. the depth at which the X-ray lines of B2 phase can still be recorded) as a function of the 700°C aging duration. The rate of penetration during aging up to 72 h is logarithmic. After the standard heat treatment (850°C for 0.5 h, cooling down to 400°C at a rate of 100°C h − 1), carried out in air atmosphere, 55% of B2 phase is formed at the surface of the Fe–50Co alloy and the depth of B2-phase penetration is 0.03–0.04 mm; at the surface of ‘Permendur’ 80% of B2 phase is formed, its depth being the same. Thus it can be concluded that vanadium addition in-
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Fig. 2. Portions of the X-ray pattern near (110)a line, obtained in the bulk (a) and from the surface (b) of the Fe – 50Co alloy aged at 700°C in air atmosphere.
creases the quantity of B2 phase at the surface of the Fe–50Co alloy, but does not change the depth of B2 penetration into the bulk. If isothermal 700°C aging or the standard heat treatment are carried out in vacuum,
X-ray phase analysis shows that the B2 phase does not form either in the bulk or in the surface layer. (X-ray patterns in these two cases coincide with that shown in Fig. 2(a).) This indicates that air atmosphere is apparently acting as a catalyst in the A2B2 transformation. The B2-type long-range ordering of Fe–Co solid solutions, as is usually believed, results in intergranular brittle fracture, but addition of 1.5%V suppresses the brittle fracture [9]. Because the B2 phase is formed only Table 1 Microhardness of the microstructures (HV) Alloy
Place of measurement
Structure
Microhardness
Fe – 50Co
In the bulk At surface In the bulk At surface
A2 40%B2 A2 60%B2
320 240 260 210
‘Permendur’ Fig. 3. Growth rate of the B2 phase surface layer of the Fe – 50Co alloy during 700°C aging in air atmosphere.
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Fig. 4. Portion of the X-ray pattern near (110)a line, obtained from the surface of the Fe – 50Co alloy, water quenched from 1000°C for 4 h.
in a thin surface layer and only during air heat treatment, this phase cannot be responsible for the intergranular brittle fracture in the bulk. In an attempt to assess the tendency to brittleness of the alloys from their hardness, microhardness measurements were made at the surface and in the bulk of the Fe – 50Co and ‘Permendur’ alloys heat treated (850 – 400°C, at 100°C h − 1) in air atmosphere. The results are presented in Table 1. Two conclusions can clearly be made from Table 1: (1) The 1.5%V addition to the Fe – 50Co alloy considerably decreases the hardness level both in the bulk and at the surface. (2) Formation of the B2 phase in the thin surface layer decreases the surface hardness in comparison with the bulk hardness in the alloys both with and without vanadium addition. The surface microhardness of the specimens after the standard heat treatment in vacuum (the A2 B2 phase transformation does not occur) is the same as the microhardness in the bulk. Thus, B2 phase decreasing the hardness of the Fe– Co alloys as well as forming only at the free surface and in air atmosphere can not be the cause of intergranular brittle fracture of the Fe – Co alloys. Since the surface layer, where the B2 phase forms upon standard heat treatment in air atmosphere, is rather thin as compared to the thickness of sheets, bars and forgings, the influence of the layer on magnetic properties such as saturation magnetic induction and magnetostriction is of no importance. This is particu-
larly obvious because the standard heat treatment now accepted is carried out in vacuum resulting in no B2 phase formation at all. Therefore, the B2 phase does not define particular values of magnetic properties of Fe–Co alloys and does not result in their embrittlement. As mentioned above, at about 730°C, high maxima of specific heat capacity [3,4] and specific electrical resistivity [5] have been observed. (The studies [3–5] were carried out in air atmosphere.) Discovering B2 phase at the surface, the authors of these papers [3–5] considered the effects to be a result of A2 B2 phase transformation in all the bulk of Fe–Co alloys. However, since this transformation actually takes place only in a very thin surface layer (volume percentage of the converted material is almost always less than 1%), this transformation can not have such a profound effect on both specific heat capacity and specific electrical resistivity as they stated [3–5]. The question therefore arises: what is the cause of the changes in physical properties observed in those studies [3–5]? Precise measurements of the unit cell parameter in the bulk of the Fe–50Co alloy heat treated at different temperatures (from 500 to 1200°C at 100°C intervals) in air atmosphere were made. The time of exposure at every temperature was 1 h and then the specimens were Table 3 Kinetics of A2 l transformation Heat treatment
Table 2 Unit cell parameter of B2 phase Alloy
Fe–35Co
Fe–50Co
Permendur
Fe – 65Co
Parameter (nm)
0.2902
0.2905
0.2905
0.2909
700°C, water quenched 500°C A2 l transformation time at 500°C aging
Unit cell parameter (nm) Fe – 50Co
Fe –65Co
0.28585 0.28610 5th minute
0.28510 0.28535 10th minute
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Table 4 Induction of magnetic saturation of ‘Permendur’ as a function of the temperature NN
Temperature (°C)
State
Induction of magnetic saturation (Tl)
1 2 3 4
1000 700 700 500
Separation at surface B2 phase at the surface B2 phase was removed from the surface l-structure
0.76 0.47 0.50 2.20
quenched in water. After heat treatment at all temperatures from 600 to 1200°C, the unit cell parameter at room temperature in the bulk was the same and equal to 0.2859 nm, but after 500°C it was equal to 0.2861 nm. For comparison, these experiments were repeated in the vacuum chamber of the X-ray diffractometer, successively raising the temperature at 100°C intervals from 500°C to 1000°C, and then reducing the temperature down to 500°C at the same intervals. A 0.00025 nm jump of the unit cell parameter was observed between 500 and 600°C as in the case of heat treatment in air atmosphere. Hence, two conclusions can be made: (1) No phase transformations in the bulk occur in the temperature interval 600–1000°C, i.e. below the f.c.c. b.c.c. transition line (Fig. 1); this signifies that the long-range ordering of A2B2 type takes place only in the specific conditions—in the thin layer at the free surface and at air atmosphere. (2) Some phase transformation occurs in all the bulk at about 550°C (apparently, this is the so-called ‘550°C anomaly’). This transformation is discussed later. Because the specific heat capacity (cP) and the specific electrical resistivity (r) are physical properties characteristic for all the bulk of the alloy, their high maxima at about 730°C can not be a result of A2 B2 phase transformation, the volume share of which is too little. Because cP and r maxima, as well as B2 phase at the surface, appear at the same temperature, it can be supposed that there is some common cause for their appearance. We hope to try and find this cause. The specimens of all the alloys studied were heat treated at 800°C for 4 h and 1000°C for 4 h as well as at 1200°C for 1 h and then water quenched. Lines of the Fe–Co solid solution were absent in the X-ray patterns obtained from the surface after these heat treatments; instead, two systems of lines from pure Fe (unit cell parameter of 0.2848 nm) and pure Co (unit cell parameter of 0.3564 nm) were observed. The X-ray pattern of the Fe–50Co specimen heat treated at 1000°C for 4 h shows the line (110) of b.c.c. Fe and the line (111) of f.c.c. Co (Fig. 4). Separation of the Fe – Co solid solution into pure Fe regions and pure Co regions takes place in a thin surface layer (the depth is less than 0.15 mm). The hardness of the surface layer that underwent separation (HV 180) is essentially less than that of the
Fe–Co solid solution (HV 290) and B2 phase at the surface (HV 240). Analogous results were obtained in high temperature X-ray studies in vacuum: the separation in a thin surface layer of the Fe–50Co alloy into pure Fe and pure Co regions was observed at 900 and 1000°C. Separation of the Fe–Co solid solution into pure components above 730°C is possible if repulsive interaction between Fe and Co atoms takes place (Emix \0). The tendency for separation appears to exist in all the bulk of the alloy above 730°C, but for some reason it is manifest only at the surface. Long-range ordering of the Fe–Co solid solution by B2 type below 730°C is possible if the attractive interaction between Fe and Co atoms takes place (Emix \ 0). Although this tendency exists in all the bulk below 730°C, it shows up only at the surface and only during heating in the air atmosphere. Therefore, at 730°C the sign of the chemical interaction between Fe and Co atoms, i.e. the sign of mixing energy, reverses. The latter is possible if the electron structure of the alloy rearranges in some way. Analogous reconstruction of the electron structure was observed earlier in the Fe–Cr alloys [10]. It was found that the mixing energy reverses its sign at the upper and lower boundary of the temperature interval of s-phase existence. As in the case of Fe–Co alloys, the s-phase in Fe–Cr alloys was formed in a thin surface layer, although the thickness of the s-phase layer was somewhat more than in Fe–Co alloys. The separation process in Fe–Cr alloys took place in all the bulk above 830°C and below 600°C [10]. In our opinion, it is reasonable to call the alloys of Fe–Cr and Fe–Co systems as the alloys display reversible chemical orientation of components. Apparently, this property is characteristic not only for alloys of Fe–Cr and Fe–Co systems but for some alloys of other transition metals. Thus, the alloys of Fe–Co system are not exceptional in changing the sign of mixing energy with temperature, but it is not understood why the separation and longrange ordering processes in the Fe–Co system take place in thin surface layers only. True enough, in the latter case it can be explained by air atmosphere which can act as a catalyst of the A2 B2 phase transformation at the surface, but this explanation is not plausible for separations that are observed both in air atmosphere and vacuum but always in a thin surface layer only.
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Table 2 shows the results of the unit cell parameter measurements of the B2 phase formed at the surface of the alloys heat treated in air atmosphere. It can be seen that as the Co content in the alloy increases the unit cell parameter of the B2 phase also increases. A good correlation between the composition of the alloys and the B2 unit cell parameter is observed. This correlation may mean that the A2 B2 transforma-
Fig. 5. The lattice parameter of Fe–50Co solid solution measured at high temperatures. Time of holding at every temperature was about 0.5 h, except 820°C at which the X-ray measurement was carried out immediately in order to avoid measurement of the structure after separation.
Fig. 6. The Fe – Co phase diagram in terms of results obtained in the present paper. 1, critical temperature of change of chemical orientation; 2, critical temperature of change of magnetic configuration.
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tion occurs without (or with very short) a preliminary stage of up-hill diffusion. Usually that diffusion takes place in the alloys of non-stoichiometric composition and leads to cluster formation, the composition of which strives to achieve stoichiometric composition of the forming phase [11]. In non-stoichiometric Fe– 35Co and Fe–65Co alloys the B2 phase has a composition corresponding to the composition of the alloy and, therefore, clusters do not form in these alloys. At uniform long-range ordering of the nonstoichiometric alloys, the path of diffusion (especially, at the surface) is too short in comparison with the path during up-hill diffusion cluster formation and, therefore, the B2 phase at the surface (in air atmosphere) forms even in the process of water quenching. Hence some conclusions can be made: (1) The composition of B2 phase is subject to variation according to the composition of the alloy in which it forms. (2) In the alloys of non-stoichiometric composition the B2 phase forms uniformly over the whole surface but not at the local points. In all the preceding experiments the structures resulting from the separation or ordering were formed at the surfaces which before the corresponding heat treatment represented solid solutions. Naturally, it would be interesting to study the process of transformation of the structure after separation, for example, into the B2 phase structure. For this purpose the ‘Permendur’ alloy was heat treated first at 800°C for 2 h, and then at 650°C for different intervals of time. The structure after separation formed at 800°C is transformed gradually into solid solution during 2 h at 650°C, and the B2 phase begins to form at the surface only after the solid solution is formed in full. Now, the nature of a 0.00025 nm jump of the unit cell parameter will be discussed. The structural state of the solid solution with lattice parameter 0.00025 nm higher than its usual value will be designated as l-structure. Table 3 shows the conditions of the l-structure formation for the Fe–50Co and Fe–65Co alloys. The specimens were heated at 700°C, then water quenched and the unit cell parameters were measured. The subsequent heat treatment was carried out at 500°C for some time. The unit cell parameter increased in all the bulk of both alloys studied by the same value, namely 0.00025 nm, but the b.c.c. lattice is not changed or distorted. As mentioned above, a small maximum of the specific heat capacity-curves is observed at 550°C (‘the 550°C anomaly’) [3]. Earlier this extra peak in the cP curve at this temperature was shown to depend on the heating rate and was interpreted as being due to kinetic effects and not connected with any phase transitions at atomic or
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electron level [12]. But increasing the lattice parameter at this transformation lets us consider this transition as the phase transformation, especially as the magnetic properties of the alloys are grossly changed. For example, the saturation magnetic induction was measured for different structural states. The results are shown in Table 4, and allow us to reach the following conclusions: (1) high values of the saturation magnetic induction are characteristic for a structural state, named by us the l-structure, which is formed upon heat treatment at 550°C and below, and for which the parameter of the b.c.c. unit cell increases a little in comparison with the state after heat treatment at higher temperature; (2) other structural states, including those with B2 phase formed at the surface, are of no interest for use as a magnetically soft material. Therefore, A2 l transition is phase transformation in the whole bulk, accompaniing by a small peak of specific heat capacity, a small increase of lattice parameter and formation of a new magnetic configuration leading to high increase of the saturation magnetic induction. Thus, there are three temperature regions in which atomic interactions between Fe and Co atoms considerably differ and which are characterized by different slopes of the curve of the lattice parameter measured at different temperatures (Fig. 5): (1) the region above about 730°C (for both b.c.c. and f.c.c. lattices) where the tendency for separation exists and manifests itself structurally only at the surface; (2) the 550–730°C region where the tendency for ordering exists and manifests itself structurally as the B2 phase formation only at the surface and only in air atmosphere; (3) the region below about 550°C where the b.c.c. unit cell parameter increases a little in all the bulk (l-structure) and where a new magnetic configuration forms. X-ray phase analysis of Fe – 35Co and Fe – 65Co alloys showed that the curve of critical temperatures of chemical ordering (T B2 – A2), presented in Fig. 1, is in reality the curve of reversal of the mixing energy sign and just by way of the latter the curve can be drawn on the Fe–Co phase diagram (Fig. 6). The structures both above and below this curve can be different in dependence on proximity to surface of the specimen as well as on an atmosphere at heat treatment. Moreover, it is necessary to draw the line of the change of magnetic configuration on the phase diagram (Fig. 6) because this line determines the region of the l-structure whose existence is very important in technology as it has a high saturation magnetic induction.
4. Conclusion The B2 phase formation in Fe–Co alloys occurs only in a very thin surface layer and only during heat treatment below 730°C in air atmosphere. The composition of the B2 phase in non-stoichiometric alloys does not correspond to the Fe–Co stoichiometric composition. The B2 phase does not affect significantly the level of magnetic properties of the alloys studied and does not result in their embrittlement. No evidence of B2 phase formation was found in the bulk of the alloys heat treated either in vacuum or in air atmosphere, or in the surface layer after vacuum heat treatment. At temperatures above 730°C in the Fe–Co alloys heat treated in both vacuum and air atmosphere the separation of solid solution into its components takes place, and this separation occurs in a thin surface layer only. The maxima of specific heat capacity cP and electrical resistivity r observed at 730°C are apparently the result of transition from the tendency for ordering (below 730°C) to that for separation (above 730°C), i.e. the result of change of the mixing energy sign in Fe–Co solid solutions. Thus, the alloys of the Fe–Co system, together with those of the Fe–Cr system, belong to the alloys with reversible chemical orientation of components. The so-called ‘550°C anomaly’ is a phase transformation at which a new magnetic configuration forms and the b.c.c. lattice parameter increases.
Acknowledgements The authors thank Drs. S. Lenkov, Y. Tarukhan and A. Ruts for their assistance.
References [1] R.A. Buckley, Met. Sci. 9 (1975) 243. [2] M.U. Glazyrina, et al., Fiz. Met. Metalloved. 56 (1983) 733. [3] A.S. Normanton, P.E. Bloomfield, F.R. Sale, B.B. Argent, Met. Sci. 9 (1975) 510. [4] J. Orehotsky, K. Shroder, Phys. F, Met. Phys. 4 (1974) 196. [5] M.S. Seehra, P. Silinsky, Phys. Rev. B 13 (1976) 5183. [6] G. Inden, Z. Metallkd. 68 (1977) 529. [7] D.W. Clegg, R.A. Buckley, Met. Sci. 7 (1973) 48. [8] A.W. Smith, R.D. Rawlings, Phys. Status Solidi A 34 (1976) 117. [9] A. Glezer, I. Maleeva, Fiz. Met. Metalloved. 68 (1989) 901. [10] Y. Ustinovshikov, B. Pushkarev, Mater. Sci. Eng. A241 (1998) 159. [11] Y. Ustinovshikov, J. Mater. Sci. 27 (1992) 3993. [12] T. Yokoyama, T. Takezawa, H. Higashida, Trans. Jpn. Inst. Met. 12 (1971) 3.
Character of transformations in Fe–Co system Y. Ustinovshikov *, S. Tresheva Physical-Technical Institute, 132 Kiro6 Str., 426000 Izhe6sk, Russia Received 28 April 1997; received in revised form 12 November 1997
Abstract The phase transformations of Fe–(35–65%)Co alloys and ‘Permendur’ (48.4%Fe – 49.8%Co – 1.8wt.%V) have been studied. It was found that the B2 phase is formed solely during heat treatment in air atmosphere and in a thin surface layer, but not in the bulk and not in vacuum. At high temperatures, the Fe – Co alloys separate into the pure components, but also only in a thin surface layer. A ‘550°C anomaly’ arises as a result of changes of magnetic configuration in the alloys. The causes and consequences of these transformations in Fe–Co alloys are analysed. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Ordering; Phase transformations; Separation
1. Introduction The nearly equiatomic Fe – Co alloys, which have wide technological application, are known as soft magnetic materials with maximum saturation magnetization and Curie temperature and a high magnetostriction. It is believed that all these excellent magnetic properties can manifest themselves if a long-range ordering of A2B2 type takes place in the bulk. In this case, only a rearrangement of atoms on the same lattice occurs, with a very small change in volume (0.2% expansion) and no local composition changes. A previous experimental study [1] of ordering in the Fe – Co system showed that the A2 B2 transformation was of the homogeneous type, i.e. the degree of order within the alloys remained uniform during transformation. However, a network of antiphase domains was formed which coalesced on prolonged annealing [1]. The order–disorder transitions that occur in Fe–Co solid solutions have attracted considerable attention because they are expected to behave in the same way mathematically near the order – disorder transition temperature as the Ising model [1]. In recent years, many studies of the critical behavior of the order –disorder transition in Fe–Co alloys have been made [1–8]. * Corresponding author. Tel.: +7 3412 216633; fax: + 7 3412 250614; e-mail: [email protected] 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00506-1
In order to achieve a long-range ordering of the B2 type, the standard heat treatment (annealing at 850°C for 2 h and then furnace cooling down to 400°C at a rate of 100° C h − 1) is commonly used [2]. It is supposed that such a prolonged heat treatment, especially below the A2B2 phase transition temperature, transfers the alloy to the ordered state with the degree of order close to the equilibrium value of 0.9 [2]. The peaks of specific heat capacity and specific electrical resistivity associated with the order–disorder transformations have been observed over the composition range Fe–(30–70)%Co [3–5], and a second maximum in the heat-capacity curves at 550°C has been reported for alloys in the range Fe–(35–65)%Co [3]. The second peak is very sensitive to heat treatment and becomes much smaller in the furnace-cooled specimens than in the rapidly cooled material. This peak is interpreted as being due to kinetic effects [6]. Most of the investigations have employed X-ray and neutron diffraction techniques to determine the degree of long-range order as a function of annealing temperature and time [7,8]. Unfortunately, accurate measurement of the degree of long-range order is difficult and generally relative, rather than absolute. Therefore, many details of the B2 superstructure formation, as well as the nature of exceptional magnetic properties as a function of atomic ordering, remain unknown. It is thought that the rate of long-range ordering is so high that any quenching in water can not, as a rule, suppress
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the disorder–order transformation. This was overcome by quenching the 0.6 mm thick specimens from 850°C in iced brine to retain the disordered state [8]. But even quenching at a rate of 104 K s − 1, used to obtain amorphous alloys, can not completely suppress the long-range ordering process, although the degree of order is low (0.2– 0.3) [2]. The b.c.c. Fe–Co alloys are stable only in a limited temperature and composition range as shown in Fig. 1 [6]. Chemical ordering reactions lead to a B2-type ordered structure, which is experimentally confirmed for Co contents 0.3 5 CCo 50.7. The b.c.c. alloys are ferromagnetic with a Curie temperature which for most compositions (CCo \0.15) is higher than that of the b.c.cf.c.c. transition. The aim of the present work was to examine the conditions of the B2-phase formation in the Fe–(35– 65)%Co alloys as well as to study the influence of the B2 phase on embrittlement, hardness, magnetic and other properties of the alloys mentioned.
2. Experimental procedure The alloys 50%Fe – 50%Co, 65%Fe – 35%Co and 35%Fe–65%Co were examined, as well as the commercial alloy ‘Permendur’ (48.4%Fe – 49.8%Co – 1.8wt.%V) for comparison. The ingots were made by vacuummelting high-purity electrolytic iron (99.86%) and cobalt (99.87%) in an alumina crucible and casting into a steel mold 20 mm in diameter. Chemical analysis of the resulting alloys showed them to contain 90.35 wt.% of the desired Co content. The ingots obtained were cut, hot-forged and then machined to the required size (rectangular plates of 15 mm×10 mm × 3 mm).
Fig. 1. Phase diagram of Fe–Co [6].
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The specimens were heat treated in furnaces controlled within 9 1°C to obtain either the fully ordered or disordered states. It was expected that the ordered state would be obtained by the standard heat treatment— heating at 850°C for 2 h and then furnace cooling to 400°C at a rate of 100°C h − 1 —as well as by isothermal aging at 700°C. It also was expected that the disordered state would be obtained by heating to 800–1200°C with subsequent quenching in iced brine. During the process of heat treatment in air atmosphere very thin oxide films form in the surface layer of the specimen. The films peeled immediately during water quenching. X-ray examination showed the films are complex oxides. X-ray measurements at room and high temperatures were made using an X-ray diffractometer fitted with a diffracted-beam monochromator set for cobalt Ka radiation. In some cases it was necessary to carry out the X-ray measurements both at the surface and in the bulk of the specimens. For this purpose the surface layer was removed by sanding. It could be supposed that this mechanical surface treatment may produce disordering of the alloys studied. That is why for comparison the surface layer of some specimens was removed by electrolytic polishing. But the results in both cases were the same. The microhardness was measured under 500 g loading. The saturation magnetic induction was measured for different structure states of the alloys in an electric field equal to 15000 A m − 1.
3. Results and discussion We consider first the Fe–50Co alloy aged isothermally at 700°C in vacuum and air atmosphere. X-ray phase analysis was made at the surface and in the bulk (after removing not less than 0.1 mm of surface layer). Fig. 2 shows a portion near the (110) line of the X-ray patterns of specimens heat treated in air atmosphere. Interpretation of X-ray phase analysis in the bulk and at the surface showed that the long-range ordering process resulting in the B2-structure formation takes place only in a thin surface layer, not in the bulk. The percentage of B2 phase at the surface never exceeded 90. Removing the surface layers lowers the quantity of B2 phase. Fig. 3 shows the depth of B2-phase penetration into the bulk (i.e. the depth at which the X-ray lines of B2 phase can still be recorded) as a function of the 700°C aging duration. The rate of penetration during aging up to 72 h is logarithmic. After the standard heat treatment (850°C for 0.5 h, cooling down to 400°C at a rate of 100°C h − 1), carried out in air atmosphere, 55% of B2 phase is formed at the surface of the Fe–50Co alloy and the depth of B2-phase penetration is 0.03–0.04 mm; at the surface of ‘Permendur’ 80% of B2 phase is formed, its depth being the same. Thus it can be concluded that vanadium addition in-
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Fig. 2. Portions of the X-ray pattern near (110)a line, obtained in the bulk (a) and from the surface (b) of the Fe – 50Co alloy aged at 700°C in air atmosphere.
creases the quantity of B2 phase at the surface of the Fe–50Co alloy, but does not change the depth of B2 penetration into the bulk. If isothermal 700°C aging or the standard heat treatment are carried out in vacuum,
X-ray phase analysis shows that the B2 phase does not form either in the bulk or in the surface layer. (X-ray patterns in these two cases coincide with that shown in Fig. 2(a).) This indicates that air atmosphere is apparently acting as a catalyst in the A2B2 transformation. The B2-type long-range ordering of Fe–Co solid solutions, as is usually believed, results in intergranular brittle fracture, but addition of 1.5%V suppresses the brittle fracture [9]. Because the B2 phase is formed only Table 1 Microhardness of the microstructures (HV) Alloy
Place of measurement
Structure
Microhardness
Fe – 50Co
In the bulk At surface In the bulk At surface
A2 40%B2 A2 60%B2
320 240 260 210
‘Permendur’ Fig. 3. Growth rate of the B2 phase surface layer of the Fe – 50Co alloy during 700°C aging in air atmosphere.
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Fig. 4. Portion of the X-ray pattern near (110)a line, obtained from the surface of the Fe – 50Co alloy, water quenched from 1000°C for 4 h.
in a thin surface layer and only during air heat treatment, this phase cannot be responsible for the intergranular brittle fracture in the bulk. In an attempt to assess the tendency to brittleness of the alloys from their hardness, microhardness measurements were made at the surface and in the bulk of the Fe – 50Co and ‘Permendur’ alloys heat treated (850 – 400°C, at 100°C h − 1) in air atmosphere. The results are presented in Table 1. Two conclusions can clearly be made from Table 1: (1) The 1.5%V addition to the Fe – 50Co alloy considerably decreases the hardness level both in the bulk and at the surface. (2) Formation of the B2 phase in the thin surface layer decreases the surface hardness in comparison with the bulk hardness in the alloys both with and without vanadium addition. The surface microhardness of the specimens after the standard heat treatment in vacuum (the A2 B2 phase transformation does not occur) is the same as the microhardness in the bulk. Thus, B2 phase decreasing the hardness of the Fe– Co alloys as well as forming only at the free surface and in air atmosphere can not be the cause of intergranular brittle fracture of the Fe – Co alloys. Since the surface layer, where the B2 phase forms upon standard heat treatment in air atmosphere, is rather thin as compared to the thickness of sheets, bars and forgings, the influence of the layer on magnetic properties such as saturation magnetic induction and magnetostriction is of no importance. This is particu-
larly obvious because the standard heat treatment now accepted is carried out in vacuum resulting in no B2 phase formation at all. Therefore, the B2 phase does not define particular values of magnetic properties of Fe–Co alloys and does not result in their embrittlement. As mentioned above, at about 730°C, high maxima of specific heat capacity [3,4] and specific electrical resistivity [5] have been observed. (The studies [3–5] were carried out in air atmosphere.) Discovering B2 phase at the surface, the authors of these papers [3–5] considered the effects to be a result of A2 B2 phase transformation in all the bulk of Fe–Co alloys. However, since this transformation actually takes place only in a very thin surface layer (volume percentage of the converted material is almost always less than 1%), this transformation can not have such a profound effect on both specific heat capacity and specific electrical resistivity as they stated [3–5]. The question therefore arises: what is the cause of the changes in physical properties observed in those studies [3–5]? Precise measurements of the unit cell parameter in the bulk of the Fe–50Co alloy heat treated at different temperatures (from 500 to 1200°C at 100°C intervals) in air atmosphere were made. The time of exposure at every temperature was 1 h and then the specimens were Table 3 Kinetics of A2 l transformation Heat treatment
Table 2 Unit cell parameter of B2 phase Alloy
Fe–35Co
Fe–50Co
Permendur
Fe – 65Co
Parameter (nm)
0.2902
0.2905
0.2905
0.2909
700°C, water quenched 500°C A2 l transformation time at 500°C aging
Unit cell parameter (nm) Fe – 50Co
Fe –65Co
0.28585 0.28610 5th minute
0.28510 0.28535 10th minute
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Table 4 Induction of magnetic saturation of ‘Permendur’ as a function of the temperature NN
Temperature (°C)
State
Induction of magnetic saturation (Tl)
1 2 3 4
1000 700 700 500
Separation at surface B2 phase at the surface B2 phase was removed from the surface l-structure
0.76 0.47 0.50 2.20
quenched in water. After heat treatment at all temperatures from 600 to 1200°C, the unit cell parameter at room temperature in the bulk was the same and equal to 0.2859 nm, but after 500°C it was equal to 0.2861 nm. For comparison, these experiments were repeated in the vacuum chamber of the X-ray diffractometer, successively raising the temperature at 100°C intervals from 500°C to 1000°C, and then reducing the temperature down to 500°C at the same intervals. A 0.00025 nm jump of the unit cell parameter was observed between 500 and 600°C as in the case of heat treatment in air atmosphere. Hence, two conclusions can be made: (1) No phase transformations in the bulk occur in the temperature interval 600–1000°C, i.e. below the f.c.c. b.c.c. transition line (Fig. 1); this signifies that the long-range ordering of A2B2 type takes place only in the specific conditions—in the thin layer at the free surface and at air atmosphere. (2) Some phase transformation occurs in all the bulk at about 550°C (apparently, this is the so-called ‘550°C anomaly’). This transformation is discussed later. Because the specific heat capacity (cP) and the specific electrical resistivity (r) are physical properties characteristic for all the bulk of the alloy, their high maxima at about 730°C can not be a result of A2 B2 phase transformation, the volume share of which is too little. Because cP and r maxima, as well as B2 phase at the surface, appear at the same temperature, it can be supposed that there is some common cause for their appearance. We hope to try and find this cause. The specimens of all the alloys studied were heat treated at 800°C for 4 h and 1000°C for 4 h as well as at 1200°C for 1 h and then water quenched. Lines of the Fe–Co solid solution were absent in the X-ray patterns obtained from the surface after these heat treatments; instead, two systems of lines from pure Fe (unit cell parameter of 0.2848 nm) and pure Co (unit cell parameter of 0.3564 nm) were observed. The X-ray pattern of the Fe–50Co specimen heat treated at 1000°C for 4 h shows the line (110) of b.c.c. Fe and the line (111) of f.c.c. Co (Fig. 4). Separation of the Fe – Co solid solution into pure Fe regions and pure Co regions takes place in a thin surface layer (the depth is less than 0.15 mm). The hardness of the surface layer that underwent separation (HV 180) is essentially less than that of the
Fe–Co solid solution (HV 290) and B2 phase at the surface (HV 240). Analogous results were obtained in high temperature X-ray studies in vacuum: the separation in a thin surface layer of the Fe–50Co alloy into pure Fe and pure Co regions was observed at 900 and 1000°C. Separation of the Fe–Co solid solution into pure components above 730°C is possible if repulsive interaction between Fe and Co atoms takes place (Emix \0). The tendency for separation appears to exist in all the bulk of the alloy above 730°C, but for some reason it is manifest only at the surface. Long-range ordering of the Fe–Co solid solution by B2 type below 730°C is possible if the attractive interaction between Fe and Co atoms takes place (Emix \ 0). Although this tendency exists in all the bulk below 730°C, it shows up only at the surface and only during heating in the air atmosphere. Therefore, at 730°C the sign of the chemical interaction between Fe and Co atoms, i.e. the sign of mixing energy, reverses. The latter is possible if the electron structure of the alloy rearranges in some way. Analogous reconstruction of the electron structure was observed earlier in the Fe–Cr alloys [10]. It was found that the mixing energy reverses its sign at the upper and lower boundary of the temperature interval of s-phase existence. As in the case of Fe–Co alloys, the s-phase in Fe–Cr alloys was formed in a thin surface layer, although the thickness of the s-phase layer was somewhat more than in Fe–Co alloys. The separation process in Fe–Cr alloys took place in all the bulk above 830°C and below 600°C [10]. In our opinion, it is reasonable to call the alloys of Fe–Cr and Fe–Co systems as the alloys display reversible chemical orientation of components. Apparently, this property is characteristic not only for alloys of Fe–Cr and Fe–Co systems but for some alloys of other transition metals. Thus, the alloys of Fe–Co system are not exceptional in changing the sign of mixing energy with temperature, but it is not understood why the separation and longrange ordering processes in the Fe–Co system take place in thin surface layers only. True enough, in the latter case it can be explained by air atmosphere which can act as a catalyst of the A2 B2 phase transformation at the surface, but this explanation is not plausible for separations that are observed both in air atmosphere and vacuum but always in a thin surface layer only.
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Table 2 shows the results of the unit cell parameter measurements of the B2 phase formed at the surface of the alloys heat treated in air atmosphere. It can be seen that as the Co content in the alloy increases the unit cell parameter of the B2 phase also increases. A good correlation between the composition of the alloys and the B2 unit cell parameter is observed. This correlation may mean that the A2 B2 transforma-
Fig. 5. The lattice parameter of Fe–50Co solid solution measured at high temperatures. Time of holding at every temperature was about 0.5 h, except 820°C at which the X-ray measurement was carried out immediately in order to avoid measurement of the structure after separation.
Fig. 6. The Fe – Co phase diagram in terms of results obtained in the present paper. 1, critical temperature of change of chemical orientation; 2, critical temperature of change of magnetic configuration.
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tion occurs without (or with very short) a preliminary stage of up-hill diffusion. Usually that diffusion takes place in the alloys of non-stoichiometric composition and leads to cluster formation, the composition of which strives to achieve stoichiometric composition of the forming phase [11]. In non-stoichiometric Fe– 35Co and Fe–65Co alloys the B2 phase has a composition corresponding to the composition of the alloy and, therefore, clusters do not form in these alloys. At uniform long-range ordering of the nonstoichiometric alloys, the path of diffusion (especially, at the surface) is too short in comparison with the path during up-hill diffusion cluster formation and, therefore, the B2 phase at the surface (in air atmosphere) forms even in the process of water quenching. Hence some conclusions can be made: (1) The composition of B2 phase is subject to variation according to the composition of the alloy in which it forms. (2) In the alloys of non-stoichiometric composition the B2 phase forms uniformly over the whole surface but not at the local points. In all the preceding experiments the structures resulting from the separation or ordering were formed at the surfaces which before the corresponding heat treatment represented solid solutions. Naturally, it would be interesting to study the process of transformation of the structure after separation, for example, into the B2 phase structure. For this purpose the ‘Permendur’ alloy was heat treated first at 800°C for 2 h, and then at 650°C for different intervals of time. The structure after separation formed at 800°C is transformed gradually into solid solution during 2 h at 650°C, and the B2 phase begins to form at the surface only after the solid solution is formed in full. Now, the nature of a 0.00025 nm jump of the unit cell parameter will be discussed. The structural state of the solid solution with lattice parameter 0.00025 nm higher than its usual value will be designated as l-structure. Table 3 shows the conditions of the l-structure formation for the Fe–50Co and Fe–65Co alloys. The specimens were heated at 700°C, then water quenched and the unit cell parameters were measured. The subsequent heat treatment was carried out at 500°C for some time. The unit cell parameter increased in all the bulk of both alloys studied by the same value, namely 0.00025 nm, but the b.c.c. lattice is not changed or distorted. As mentioned above, a small maximum of the specific heat capacity-curves is observed at 550°C (‘the 550°C anomaly’) [3]. Earlier this extra peak in the cP curve at this temperature was shown to depend on the heating rate and was interpreted as being due to kinetic effects and not connected with any phase transitions at atomic or
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electron level [12]. But increasing the lattice parameter at this transformation lets us consider this transition as the phase transformation, especially as the magnetic properties of the alloys are grossly changed. For example, the saturation magnetic induction was measured for different structural states. The results are shown in Table 4, and allow us to reach the following conclusions: (1) high values of the saturation magnetic induction are characteristic for a structural state, named by us the l-structure, which is formed upon heat treatment at 550°C and below, and for which the parameter of the b.c.c. unit cell increases a little in comparison with the state after heat treatment at higher temperature; (2) other structural states, including those with B2 phase formed at the surface, are of no interest for use as a magnetically soft material. Therefore, A2 l transition is phase transformation in the whole bulk, accompaniing by a small peak of specific heat capacity, a small increase of lattice parameter and formation of a new magnetic configuration leading to high increase of the saturation magnetic induction. Thus, there are three temperature regions in which atomic interactions between Fe and Co atoms considerably differ and which are characterized by different slopes of the curve of the lattice parameter measured at different temperatures (Fig. 5): (1) the region above about 730°C (for both b.c.c. and f.c.c. lattices) where the tendency for separation exists and manifests itself structurally only at the surface; (2) the 550–730°C region where the tendency for ordering exists and manifests itself structurally as the B2 phase formation only at the surface and only in air atmosphere; (3) the region below about 550°C where the b.c.c. unit cell parameter increases a little in all the bulk (l-structure) and where a new magnetic configuration forms. X-ray phase analysis of Fe – 35Co and Fe – 65Co alloys showed that the curve of critical temperatures of chemical ordering (T B2 – A2), presented in Fig. 1, is in reality the curve of reversal of the mixing energy sign and just by way of the latter the curve can be drawn on the Fe–Co phase diagram (Fig. 6). The structures both above and below this curve can be different in dependence on proximity to surface of the specimen as well as on an atmosphere at heat treatment. Moreover, it is necessary to draw the line of the change of magnetic configuration on the phase diagram (Fig. 6) because this line determines the region of the l-structure whose existence is very important in technology as it has a high saturation magnetic induction.
4. Conclusion The B2 phase formation in Fe–Co alloys occurs only in a very thin surface layer and only during heat treatment below 730°C in air atmosphere. The composition of the B2 phase in non-stoichiometric alloys does not correspond to the Fe–Co stoichiometric composition. The B2 phase does not affect significantly the level of magnetic properties of the alloys studied and does not result in their embrittlement. No evidence of B2 phase formation was found in the bulk of the alloys heat treated either in vacuum or in air atmosphere, or in the surface layer after vacuum heat treatment. At temperatures above 730°C in the Fe–Co alloys heat treated in both vacuum and air atmosphere the separation of solid solution into its components takes place, and this separation occurs in a thin surface layer only. The maxima of specific heat capacity cP and electrical resistivity r observed at 730°C are apparently the result of transition from the tendency for ordering (below 730°C) to that for separation (above 730°C), i.e. the result of change of the mixing energy sign in Fe–Co solid solutions. Thus, the alloys of the Fe–Co system, together with those of the Fe–Cr system, belong to the alloys with reversible chemical orientation of components. The so-called ‘550°C anomaly’ is a phase transformation at which a new magnetic configuration forms and the b.c.c. lattice parameter increases.
Acknowledgements The authors thank Drs. S. Lenkov, Y. Tarukhan and A. Ruts for their assistance.
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