Samarium-cobalt magnet is an ideal material for the application in serve-motors, pump couplings, and sensors etc due to its extremely high magnetic properties, high thermal stability and excellent corrosion resistance. Since these applications require to operate at high temperature, or in abroad temperature range, or in a corrosive environment.
SmCo5 based magnet was investigated in 1960’s. RCo5 crystallizes in a hexagonal structure, Co atoms occupy at 2c and 3g crystal position. The crystal structural frame consists of a double layers of RCo2 and Co3. Hubbard et at found a special hard magnetic property in GdCo5, however, the magnetization is lower due to Gd spin alignment being antiparallel to that of Co, So, a modification study was done on the light rare earth RCo5 compound due to the light rare earth and Co spin alignment being parallel. Karl J. Strnat et al found a large anisotropy field in YCo5 in 1967. It resulted in an great interest growing on the study of R-Co 1:5 hard magnetic compound. However, SmCo5 was the only qualified hard magnet in RCo5 family, a milestone in energy product (BH)max = 20MGOe was obtained in 1969 by liquid phase sintering process. In order to decrease the cost of SmCo5, the test was done by aluminum and nickel substitution for Co, however, it induced the decrease of Curie temperature and energy product. The hydrogenation in SmCo5 compound was also tested to improve its magnetic properties, however, the results was not positive. According to Buschow’s reports, RCo5 type structure was not stable, and decomposed around 800℃. Thus, the later interest shifted to the Co rich Sm2Co17 compound.
Sm2Co17 has a high coercivity with a Curie temperature of 920℃. Its hard magnetic properties are much better than those of SmCo5. Sm2Co17 crystallizes in a hexagonal structure at high temperature and a rhombohedral structure, respectively. The high temperature phase can be stabilized by transition metals (such as Ti, Zr, Mn, Cr etc) or metal (such as Al, Ca) substitution for Co at room temperature. There is four kinds of inequivalent Co sites, i.e. 4f, 6g, 12j and 12k. The hexagonal R2Co17 structure is derived from RCo5 structure, where the Co atoms at 4f site in 2:17 structure could be regarded as being deriving from the Co dumbbell pair partly substitution for rare earth at Sm site in 1:5 structure. The relation between the lattice parameters of the hexagonal 2:17 and 1:5 phase is following by an equation of a2:17=sqrt(3×a1:5), c2:17=3c1:5. Since the crystalline anisotropy is weakened by the Co dumbbell pair partly substituting for rare earth at Sm site. And the intrinsic coercivity temperature coefficient is a little high. So, the later research for improving high iHc in stoichiometric 2:17 alloy were done, however, these tests were unsuccessful. An interest shifted toSm(Co3Fe2Cu)0.8-8 alloy in 1975. An energy product with 30MGOe was obtained in Sm ( CobalFe0.28Cu0.10Zr0.01)7.4 in 1977 by T. Ojima et al. It reveals an excellent magnet with a high energy products and high coercivity could be realized in 2:17 Sm-Co based magnet.
In 1983, Nd2Fe14B based magnet was introduced and rapidly developed due to its higher magnetization, maximum energy products, lower cost and better mechanical properties than Sm-Co magnets. Many applications benefited greatly from the introduction of Nd-Fe-B and today, industries including information technology (computers and telecommunications), automotive and medical (MRI equipment) all rely heavily on these magnets. But, most its applications involving exposure to temperature is less than 180℃, and its temperature coefficient is much higher than that of Sm-Co magnet. Another big problem during its application is its low corrosion resistance. So its application is limited due to the above mentioned problems. NdFeB magnet can not be used as a high temperature magnet. In recent years, with the rapid development on aviation industry, satellite communication, and military industry etc, it requires that the magnet works more than 450℃. More recently, a renew interest has attributed on the Sm-Co 1:7 compound. In 1998, a Sm-( Co2 Fe3 Cu2 Zr) 1:7 based magnet was developed with a room temperature coercivity of 2 T and more than IT at 500℃ as well as a small temperature coefficient (-0.11%/℃).
In comparison to Nd2Fe14B based magnet, Sm2Co17 based magnet exhibits an excellent temperature stability due to its high Curie temperature. Moreover, its high corrosion resistance allows use of the magnets without any coating, which is necessary for Nd2Fe14B magnet. Most applications involving exposure to temperature above 180℃ require the use of Sm-Co magnets. Generally, operating temperatures for a magnet should be about 200℃ below the Curie temperature. The Curie temperatures for Nd2Fe14B, 1:5 type and 2:17 type Sm-Co magnets are about 320℃, 700℃ and 920℃, respectively. Also, the application which requires a relatively constant magnetic field over a certain temperature range, must rely on Sm- Co magnet. Within the 25-150℃ range, the temperature coefficient of remanence Br is about -0.035%/℃ for uncompensated Sm-Co magnet, and as low as -0.001%/℃ for the temperature compensated Sm-Co magnets. The temperature compensated magnets are obtained by partial substitution with heavy rare earth metals, such as Gd, Ho, Dy, Tb, and Er, for some of the Sm. Applications involving environments where corrosion is likely, such as in water, high humidity and salty roads also benefit from the use of Sm-Co magnet.
1.Traveling Wave Tubes: used in space exploration, satellite communication, missiles, and combat aircraft
2.Inertial Devices: accelerometers and gyroscopes for guidance and stabilization of missiles, aircraft, ocean vessels, and satellites.
3.Motors and Generators.
4.Other Application: actuators, inductors, inverters, magnetic bearing, and regulators. Up to now Sm-Co based magnets are investigated more than 30 years, but, their basic magnetic properties such as magnetization processes, diffusion processes,…etc are still not completely understood. One main reason for these is that the processes take place at a nm-scale. The exact analysis failed to do even ten years ago due to the insufficient sensitivity of the measuring methods. Recently, the rapidly developing technology continues to create new demand for high temperature applications and has stimulated renewed interest in research and development of Sm-Co magnets.
Magnets of 2:17 type in commercial production now have a composition close to Sm (Co2 Fe2 Cu2 M)7.5, where the Fe atom increases Br, Cu permits the magnetic precipitation hardening, and often a small amount of another element such as Zr, Hf, Ti or the mixture of these to aid in the formation of the microstructure need for the precipitation hardening. The heat treatments applied to obtain high performance magnets are generally fairly complicated, the main process involving sintering and magnetic solution.
The transition metal content reducing from 8.5 to 7.5 in 2:17 Sm-Co magnet is due to heat treatment going to the lower temperature. This reduction of the homogeneity range is the season that typical microstructure are obtained after a low-temperature heat treatment. These microstructure consists of a more or less well-developed network of very small cell of the matrix phase, within the much large grains, which are separated and often completely surrounded by a thin boundary phase of 1:5 stoichiometry and CaCu5 type structure as shown in Fig 1. When high iHc magnets are heat treated to their optimum magnetic properties, the cells have linear dimension of about 100-200nm size and cell walls are typical 2-20nm thick. The cell interior has the rhombohedral (R) modification of the 2:17 structure and is heavily twinned, with the twin boundaries in the basal plane. There are also other, very thin layers visible under an electron microscope that are parallel to the basal plane and run across many cells and cell boundaries. They belong to a third phase, the so-called “platelet phase?or “z-phase?which contains most of the ZrSm (Co3Fe5Cu8Zr)z magnet product is a complex metallurgical system involving four compositional variables and at least an equal number of heat treating variables, and they are all interactive. A.E Ray developed a model describing the metallurgical behavior of these magnets. But the exact sequence of transformations by means of which the precursor separates into the three phases present in the microstructure of the aged alloy is still a matter of debate.
The discovery of the high-performance Nd-Fe-B magnet in 1983 has led to an extraordinary development of permanent magnet applications. The annual global production of these materials is presently over 10000 tones. The properties of these materials, however, rapidly degrade as temperature is increased and they cannot be used above 150℃. In the temperatures range of 150℃ - 300℃, the 2:17 Sm-Co magnet is preferred 2:17 Sm-Co magnet is good candidate for that potential applications in the fields of aeronauties, the space industry, electronics, and the automotive industry
For the high temperature application, the desired maximum operating temperature is above 450℃. Among all rare earth permanent magnets, the 2:17-type Sm-Co magnet has the highest Curie temperature and magnetization, therefore, are the most promising candidate of such high temperature applications. Serious efforts to improve high temperature performance of 2:17 type Sm-Co magnets began in 1995. A breakthrough was made for magnets to be used at high temperatures in 1997-1999, when a series of Sm(Co3Fe5Cu8Zr)z magnets with linear demagnetization curves was developed for up to 400℃ in 1997-1998, and then up to 500℃ in 1998-1999. Typical demagnetization curves of high temperature magnets are shown in fig.2. These magnets have substantially straight-line extrinsic demagnetization curves all the way up to 500℃.
The Sm (CobalFevCuyZrx)z magnets represent a complicated system with four compositional variables (x,y,v,z) and five heat treating variables ( Th, th, Tag, tag, dT/dt). Recently, the researchers have undertaken a comprehensive and systematic study on east alloys and sintered magnets to understand the effects of composition ( x, y, v, z) and processing on their magnetic hardening behavior, particularly on the temperature dependence of iHc. Systematic studies so far led to the development of new high temperature magnets with controlled and small temperature dependence of iHc (including an anomalous temperature dependence of coercivity in some magnets) having the record value of 10KOe at 500℃. Based on these studies, researchers are able to finely tune the microstructure and microchemistry of Sm (CobalFevCuyZrx)z magnets through adjustment in the composition and processing parameters, and design magnets for various applications.
1. Effect of Ratio z
The ratio z define the ratio of TTM/Sm ( TTM= total transition metal). According to the phase diagram, magnets with a lower ratio z (i.e. higher Sm content) would be expected to have more of the Sm(Co, Cu)5 cell boundary phase. It is found that when the ration z increases from 7.0 to 9.0, the average cell size increases from 88nm to 237nm. For a fixed Cu content, a larger cell size results in a larger amount of Cu in the 1:5 cell boundaries. This leads to a larger gradient in domain wall energy across the cell boundaries resulting in a stronger domain wall pinning, and gives rise to higher room temperature coercivity. For Sm (Co0.01Fe0.1Cu0.08Zr0.033)z magnets, when z=8.0, the coercivity is 28.7KOe at room temperature and 8.5kOe at 500℃
2. Effect of Zr
Although many previous studies have tried to interpreted the role of Zr in Sm(CobalFe0.1Cu0.08Zr8)8.5 magnets, the effect of Zr is not fully understood. The room temperature coercivity is below 2kOe for the sample without Zr. However, with increasing Zr content, the coercivity dramatically increases and reaches a value of around 40kOe for x in the range of 0.02 to 0.06. when x>0.06, the coercivity begins to decrease. At 500℃,although the coercivity is below 10kOe for all magnets, it still gradually increases with different x up to 0.06 and then decreases. The microstructures of Sm(CobalFe0.1Cu0.08Zr8)8.5 magnets with different x value have been investigated by TEM. It is seen that a cellular microstructure in not formed for the Zr-free magnet. Instead, a rod or needle-like 1:5 phase is found to be distributed in the 2:17 matrix. With increasing x from 0.01 to 0.08, the cell size decreases first slightly and then quickly from 120 to 35nm. On the other hand, the density of lamella phase increase first slightly and then slightly from 0.03 to 0.062 l/nm. Therefore, the increase of Zr leads to a finer cellular structure and higher density of lamella phase is formed leading to the decrease of the coercivity at both room and high temperature.
3. Effect of Cu
The effect of Cu content in the Sm(CobalFevCuyZrx)z microstructure and magnetic properties is sensitive, M3 decrease while iHc increases with increasing Cu content. It is noted that iHc of the magnets with z=8.5 reaches a maximum of about 40kOe when Cu content at 0.088. However, for the magnets with z=7.5, iHc continues to increase with increasing Cu content x up to 0.169. the microstructure of Sm(CobalFe0.1Cu2Zr0.04)8.5 magnets shows the cell size to decrease with increasing Cu content with a stronger dependence in the case of high ratio z. the average cell size decrease from 120 to 70 nm with increasing Cu content from 0.048 to 0.169. Nanoprobe chemical analysis shows that the Cu concentration at the cell boundaries increases with increasing Cu content. Since the magnet with low ratio z has a smaller cell size, the proportion of the cell boundaries is larger. More Cu is needed to obtain a large domain wall energy gradient across the cell boundaries. Even when the Cu content is up to 0.168, the coercivity has not yet reached its maximum. The temperature dependence of Sm(Co0.01Fe0.1Cu2Zr0.04)2 magnets with different z and Cu content y shows that the magnets with higher Cu content and low z exhibit higher iHc at high temperatures up to 500℃. The iHc of over 10kOe at 500℃ is obtained for these magnets.
The progress made in magnetic materials has allowed the dramatic miniaturization of device in which the magnet was previously a major part of the volume and weight of the devices. High performance permanent magnets are crucial components in a majority of high technology system and subsystem, which require a large and stable magnetic field in a wide variety of environmental conditions. The applications include microwave tubes, accelerometers, reaction and momentum wheels to control and stabilizer satellites, magnetic amplifiers and bearings. According to these commands, the magnet materials are required to have a uniform and stable magnetic property in the temperature range from 50℃ to 200℃.2:17-type Sm-Co magnet is an ideal material for these applications due to their high Curie temperatures and magnetocrystalline anisotropies with a low temperature coefficients of remanence and a high coercivity.
Recently, the strip casting method has been employed to prepare the high performance Nd2Fe14B magnets, in which the microstructure of cast ingots change significantly due to the high cooling rate comparing to the conventional mold casting technique. The strip casting method was utilized to prepare the Sm(Co0.66Fe0.27Cu0.05Zr0.02)7.5 mother alloy by K. Sakaki et al. Microstructure analysis shows that both the book mold and the strip cast ingots consist of the Th2Zn17-type, TbCu7-type, Zr-rich, and Sm-rich phase, but the average grain size of the strip casting ingots is around 10µm with finely dispersed Zr-rich and Sm-rich phases as shown in fig3. Sintered magnet made in strip cast ingot shows a lower remanence and poor grain alignment because of poly-crystalline particles present after jet milling. Proper heat treatments, however, it was found to be effective in obtaining homogenous microstructure with a large grain size of >50µm, and results in the enhancement of Br by 30mT and (BH)max by 17kJ/m^3, comparing to magnets made from mold cast ingots. The same treatment on the conventional ingots, which showed microstructure too coarse to be homogenized, did not improve the magnetic property of sintered magnet. Strip casting and proper heat treatment are effective for obtain high performance 2:17 type Sm-Co magnet.
Another approach to increasing (BH)max in 2:17 magnet is to develop pure Sm2(Co2Fe)17 (without Cu and Zr) magnet with high iHc. Traditionally, the 2:17 type Sm-Co magnet is made using conventional powder metallurgical process, and the commercial product composition is close to Sm(Co3Fe2Cu2Zr)7.5, where iron doping is beneficial for increasing Br, copper doping for permitting the magnetic precipitation hardening, and a small amount of zirconium doping for forming the microstructure with the precipitation hardening. However, Cu and Zr are non-magnetic elements, magnetization is reduced. The saturation magnetization Ms of pure Sm2(Co1Fe)17 is fairly high Ms of Sm2(Co0.8Fe0.2)17 is 1.4T. If iHc could have reached at 14kOe, a maximum energy product up to 42MGOe was realized in pure 2:17 Sm-Co magnet.
One of the most advantageous properties of Sm-Co based magnet is its high Curie temperature and the concomitant low temperature dependence of remanence as well as the modest temperature dependence of the coercivity. It is possible to content the temperature coefficient of Br in Sm-Co magnets by substituting heavy rare earth (HRE) such as Tb, Dy etc for Sm.
The origin of the reversible temperature coefficients lies in the temperature dependence of the molecular moment. The rare earth sublattice moment dependents on temperature more strongly than that of cobalt. Since the rare earth and Co spin alignment being parallel, the overall magnetization exhibits a negative temperature dependence characterization. The temperature dependence of magnetic moment will be improved by heavy rare earth doping in Sm-Co magnet due to the heavy rare earth spin alignment being anti parallel. In the temperature range of interest the HRE-Co compounds have positive slope for the magnetic flux dependence on temperature, whereas the I.R.E-Co compounds have positive slope. A proper combination of light and heavy rare earth should produce nearly temperature independent magnetic flux over the temperature range of interest.
In order to develop high energy product in temperature compensated magnets, Sm is partially substituted with Gd, Dy, Tb, Ho, and Er. Using a modified powder metallurgy process, these compensated Sm-Co magnet with heavy rare earth can be produced with a high coercivity. Fig 4 and fig.5 show the commercially available temperature compensated 1:5 and 2:17 Sm-Co magnets, which consist mostly of Sm, Gd, Er, Co and other transition metals, respectively. The temperature coefficients aof these compensated magnets are in the range of -0.001-0.002%/℃ at -50℃ to +150℃. The remanences of these magnets at room temperature are about 0.611T and 0.825T for 1:5 and 2:17 type Sm-Co magnets respectively. Generally, a small value of temperature coefficient is associated with lower Br.
Strong brittleness is a fatal shortcoming in Sm-Co magnet whose fracture toughness is even lower than 2Mpa/m^1/2. The strong brittleness and poor machinability of sintered Sm-Co magnets are unfavorable for instrument developing toward to small size and light weight, and also limits its application in shock or vibration environment.
It is a long time that the studies on Sm-Co magnets have been mainly focused on their magnetic properties, but the systematic studies on the mechanical characteristics and fracture mechanism of Sm-Co-based alloys are very limited. Recently, we have investigated the fracture mechanism of sintered 2:17 type Sm-Co magnets, and provided some theoretical bases for improving the strength and toughness of these magnets.
1. Mechanism of cleavage fracture of Sm2Co17-based alloys
The microfractography and EDAX were immediately done in a S-250MK3 type scanning electron microscope (SEM) after the 3-point bend tests. Fig.6 is a typical microfractograph of experimental alloy. In high magnifications (from 1000 to 3000 magnification), the morphological characteristics of cleavage fracture such as river patterns, cleavage steps etc, were found in fracture as shown in Fig.6a, Fig.6b reveals that cleavage steps (also called “secondary cracks? is formed due to secondary cleavage. Rivers continuously flow through grain boundary with a small angle of slop as shown in Fig.6c. Rivers increase suddenly as cleavage cracks propagate through the twist grain boundary as shown in Fig.6d. According to the above mentioned facts, it is concluded that micromechanism fracture of sintered Sm2Co17 type permanent magnetic materials is its cleavage fracture.
2. Effect of Sm-rich impurities on the cleavage fracture of Sm2Co17-based alloys
Figure 7 shows there are some particle-shaped Sm-rich impurities in fracture. Sm-rich impurity is a brittle phase which is derived from the broken piece of some large impurities. The breakage of impurities doesn’t come form the stress concentration since no microcracks were found around ductile dimples in impurites. Many Sm-rich impurities with small size were stripped off during fracture and there are some “tail?appearance in the direction of crack propagation (see Fig.7b). Energy us expensed in the formation process of these “tail? which reduces the stress concentration and hinders the cleavage fracture of alloy. Fig. 7c exhibits that the cleavage cracks stop propagating before holes of bigger Sm-rich impurities. According to the microstructure analysis in Fig.7d, it suggests that Sm-rich impurities also change the direction of crack propagation, and thereby possibly decrease the tendency of cleavage fracture of Sm2 Co17-based alloy.
Cleavage fracture belongs to one kind of transgranular fracture under normal stress. Cleavage fracture occurs when the stress increases more than the cohesion force of metals, and gives rise to bond rupture between atoms. It implies that the two conditions are necessary for starting cleavage fracture: one is a certain mechanism of stress concentration, and the other one is that the non-relax high stress created at a particular section. A summary is made as below for interpret the cleavage fracture in the sintered Sm2Co17-based alloys.
Firstly, the rhombohedral Sm2Co17-based alloy is the same layer-packed structure as that of hexagonal closed-packed(hcp) structure. But its symmetry is lower than that of hep structure. Thus, it implies that the number of the slip deformation modes of Sm2Co17-based alloys is fewer than that in hep structure, and the deformation in Sm2Co17-based alloys is very difficult. However, more or less plastic deformation always exists in the region of the crack front in spite of the alloys?brittleness. Plastic deformation occurs in the Sm-Co-based alloys under the limit of load value. Later dislocation reaction is easy to form sessile dislocation for lack of slip deformation modes in alloys. Following moving dislocation stops up before sessile dislocation, which results into a very high stress that can’t be relaxed. Thus, the cleavage cracks appear. Once cleavage cracks appear in brittle materials, it will propagate to cause cleavage fracture due to the effect of tip and notch.
Secondly, since the sintered Sm2Co17 type permanent magnetic materials are orientated by an applied field before sintering Neighbor grains almost have the same c axes orientation. So the wrong-packed degree of grains is very low in sintered Sm2Co17-based alloys, and the grain boundary is not in the region with concentrated dislocations as that in ordinary metals. The microstructure of sintered Sm2Co17 type magnets just like the texture in the metals with a large amount of cold machining deformation. According to reports, grain boundary, especially grain boundary with big angle, is a great obstacle to the propagation of cleavage cracks. While obvious texture appears in metals, cracks propagate through grains with the practical same orientation over a long distance, which makes cleavage fracture proceed easily and thereby the toughness of the alloys be decreased. The lack of slip deformation modes and the problems accompanied by the preparing process of powder metallurgy, has caused the low strength and poor roughness of sintered Sm2Co17 type magnets. However the strong anisotropy of the rhombohedral lattice and the sintering process are essential for the high magnetic properties in sintered Sm2Co17 type magnets. Thus, the high strength and good toughness seem to be in contradiction with the high magnetic properties of sintered Sm2Co17-type magnets. Literatures revealed that the particles are the obstacle of cleavage fracture because they make stress concentration relax by forming holes. We have improved the strength and toughness of sintered Sm2Co17 type magnets by introducing some spherical particles or fibers with high strength and high toughness (firmly combined with matrix) into the matrix which will not obviously decrease the magnetic properties of the magnets. This method has been widely used in strengthening and toughening brittle materials such as ceramics etc. The fracture of sintered Sm2Co17 type permanent magnetic materials belongs to a brittle cleavage fracture. The strong brittleness of sintered Sm2Co17-based alloys is mainly attributed to the lack of the slip deformation modes in the alloys.
The Sm-rich impurity in sintered Sm2Co17-based alloys is beneficial for decreasing the cleavage brittleness of the alloys. It suggests that it is possible to improve the strength and toughness of sintered Sm2Co17 type magnets by introducing some spherical particles or fibers with high strength and good toughness(firmly combined with matrix) into the matrix of the materials.
In a conclusion, we have presented the research back ground and the recent development of Sm-Co magnets systematically, which includes crystal structure, microstructure, hard magnetic properties, mechanically property and their correlation. It reveal that Sm-Co high temperature magnet with a low temperature coefficient and high energy product is very active research field, and is possible to become to a new generation magnet.
Prof. Li Wei graduated from Physics Department, Shandong University in 1982. Then he joined Central Iron & Steel Research Institute. His main research field centers in the microstructure, magnetic properties, temperature characteristic, coercivity mechanism of Nd-Fe-B, Sm-Co and manocrystalline rare earth permanent magnetic materials and their manufacturing equipments and process. He participated and took charge of more than 20 major scientific and research programme, among them: Programme of International Cooperation, National Key Technologies R & D Programme, National High Tech Research and Development (863) programme and Programme of National Natural Science Foundation.