materials

Article Structure of Alloys for (Sm,Zr)(Co,Cu,Fe)Z Permanent Magnets: First Level of Heterogeneity

Andrey G. Dormidontov *, Natalia B. Kolchugina, Nikolay A. Dormidontov and Yury V. Milov

LLC “MEM”, Moscow 123458, Russia; [email protected] (N.B.K.); [email protected] (N.A.D.); [email protected] (Y.V.M.) * Correspondence: [email protected]



Received: 2 August 2020; Accepted: 28 August 2020; Published: 3 September 2020


Abstract: An original vision for the structural formation of (Sm,Zr)(Co,Cu,Fe)Z alloys, the compositions of which show promise for manufacturing high-coercivity permanent magnets, is reported. Foundations arising from the quantitative analysis of alloy microstructures as the first, coarse, level of heterogeneity are considered. The structure of the alloys, in optical resolutions, is shown to be characterized by three structural phase components, which are denoted as A, B, and C and based on the 1:5, 2:17, and 2:7 phases, respectively. As the chemical composition of alloys changes monotonically, the quantitative relationships of the components A, B, and C vary over wide ranges. In this case, the hysteretic properties of the (Sm,Zr)(Co,Cu,Fe)Z alloys in the high-coercivity state are strictly controlled by the volume fractions of the A and B structural components. Based on quantitative relationships of the A, B, and C structural components for the (R,Zr)(Co,Cu,Fe)Z alloys with R = Gd or Sm, sketches of quasi-ternary sections of the (Co,Cu,Fe)-R-Zr phase diagrams at temperatures of 1160–1190 ◦C and isopleths for the 2:17–2:7 phase composition range of the (Co,Cu,Fe)–Sm–Zr system were constructed.

Keywords: Sm–Co permanent magnet alloys; highly coercive structures; composition-structure- properties diagrams; (Co,Cu,Fe)–R–Zr phase diagrams sketches

1. Introduction

Permanent magnets based on the (Sm,Zr)(Co,Cu,Fe)Z alloys, which were developed and commercialized a while ago [1,2], are complex metallurgical systems and their hysteretic properties and structural state are closely linked to the chemical composition of the material and heat treatment conditions. Despite numerous investigations into the structural formation and properties of the alloys, the studies are far from over. The phase transformations, which occur during stepped tempering, and the transformations that cause the instant “recovery of properties” upon repeated heating of samples to the isothermal tempering temperature being among them [3,4], have not yet been described. The phase composition of boundaries between cells at different states of heat treatments [3–6] meets with strongly conflicting views. There are sufficiently exotic variants of the phase structure at the boundary–cell interface [7]. For example, Popov et al. [7] assume that the boundary phase is separated so that Cu is localized in the 1:5 phase near the (1:5)/(2:17) interface rather than in the center of the 1:5 boundary phase. However, the structural peculiarity of the boundary is the alternation of phase layers strictly across the “c” axis, which is common for the whole anisotropic massive of samples [3,8]. Within such a structure, the formation of additional phase separation along the most low-size coordinate of boundary structural element is not likely energetically reasonable. Moreover, this assumption certainly contradicts with the fact that the Cu concentration in the center of the boundary phase in ternary junctions of the cells is maximal [8].

Materials 2020, 13, 3893; doi:10.3390/ma13173893 www.mdpi.com/journal/materials Materials 2020, 13, 3893 2 of 18

It should be recognized that currently there is no objective holistic view of the formation of high-coercivity structure of the (Sm,Zr)(Co,Cu,Fe)Z alloys. This hinders the progress in the development and improvement of both new compositions of alloys for permanent magnets and manufacturing processes for them. The aim of the present study was to detail the interrelations of the chemical and phase compositions of the (Sm,Zr)(Co,Cu,Fe)Z alloys in the composition ranges, which are of importance for the manufacturing of permanent magnets because of the functionality of their phase structure. The magnetic hardness of the (Sm,Zr)(Co,Cu,Fe)Z alloy for permanent magnets is ensured by its precipitation hardening in the course of complex heat treatment. The detailing is performed based on the generalizing of both original results, which are not given in widely available periodicals and the known literature data. The study is also aimed at concretizing the phase transformations of the Sm–Zr–Co–Cu–Fe system in accordance with our understanding of the occurred processes and phenomena.

2. Methodological Features

2.1. On the Formula of Alloys for the (Sm,Zr)(Co,Cu,Fe)Z Permanent Magnets Among the fundamental studies that have determined the methodology of investigations of (Sm,Zr)(Co,Cu,Fe)Z alloys, there are investigations related to Ray’s metallurgical model [9,10] in which the process of the formation of the high-coercivity structure was improved. The behavior of alloys during heat treatments can be determined by the following three main model postulates:

1. The starting point is the matrix, which is the disordered single-phase 2:17R precursor; the decomposition of the phase during isothermal tempering leads to the formation of the cellular-lamellar structure of phases. The formed phases are modified at the expense of interphase interdiffusion during stepped aging.

2. Zirconium with a vacancy occupies Co2 dumbbell sites when entering the Sm2(Co,Fe,Cu,Zr)17 matrix. 3. The composition of the alloy for manufacturing the permanent magnets should be maximally close to the 2:17 stoichiometry taking into account Sm and Zr losses for the oxidation and reactions with other nonmetallic impurities.

In this context, the formulas Sm2(Co,Fe,Cu,Zr)17 and Sm(Co,Cu,Fe,Zr)Z have been logically and widely used by investigators not only for the generic designation of alloys and magnets based on them, but also for plotted dependences “composition–physical properties”. We assumed that the alloy formula of Sm1-XZrX(Co,Cu,Fe)Z suggested by researchers of the Department of Magnetism, Tver State University supervised by Prof. Dmitry D. Mishin, Ph.D (M.B. Lyakhova et al. [11,12]) is physically more true. In the case of such a formula, at least, the dependences composition-properties can be concretized personally for 4f, 4d, and 3d elements of the alloy. There are additional arguments in favor of the formula. Currently, it is obvious that during heat treatment for the high-coercivity state, at the end of the solid solution heat treatment (SSHT), the starting matrix, in the classical sense of this term, is formed based on the disordered 1:7H phase [3–6] (from herein, the designation of AnBm intermetallics is given in the form of n:mH or n:mR, where H and R indicate the hexagonal and rhombohedral crystal lattices, respectively). The 1:7H structure is the hexagonal TbCu7-type phase (space group P6/mmm), rather than 2:17R, which is typical of 1:5H and 2:17H [13] or P63/mmc [14]. According to Lefevre et al. [14], the hexagonal structure is not a disordered form of the rhombohedral Sm2Co17 phase. It is obtained by deformation of the SmCo5 structure through the disordered SmCo7 structure. In such a structure, Zr atoms occupy Sm sites. The substitution of Zr atoms in the close hexagonal Th2Ni17-type structure refined by the Rietveld method corresponds approximately to 0.5/0.5, 0.75/0.25, and 0.9/0.1 Sm/Zr or Co2/Zr population on Materials 2020, 13, 3893 3 of 18 sites 2b, 2d, and 4f/2c, respectively. Thus, the preferred sites for accepted Zr atoms are the Sm atom sites [14]. The analogous situation is characteristic for the Fe–Gd–Zr system. The Rietveld refinement of the Fe17(Gd,Zr)2 structure indicated that Zr atoms reside only on the 2b site [15,16]. Thus, the matrix being the initial solid solution, with good reason, is written in the form of Sm1-XZrX(Co,Cu,Fe)Z. The substitutions Sm–Zr in crystal lattices of phases of the Smn-1Co5n-1 homological series has not been in doubt for a long time. As for the 2:17R phase, Zr should definitely be included in parentheses together with the Co or 3d-metal mixture, Sm2(Co,Zr)17. However, this phase, despite its high final volume in the alloy and high role in the formation of magnetic properties, is the secondary product of phase transformations. Thus, the arguments for the original formula of the Sm1-XZrX(Co,Cu,Fe)Z alloys are: - the separation of the effect of main elements qualitatively differing in the configurations of electron shells (4f–Sm, 4d–Zr, and 3d–Co,Cu,Fe) on the structure and properties of the studied alloys; and - during the formation of the alloy structure in the course of heat treatment, the matrix (the initial structural state of test object) is the disordered 1:7H solid solution and almost all intermediate and final phases formed in the course of phase transformations, except 2:17R, adequately correspond to the formula Sm1-XZrX(Co,Cu,Fe)Z.

2.2. On the Heterogeneity of Alloys for the (Sm,Zr)(Co,Cu,Fe)z Permanent Magnets

There are many results of experimental investigations of (Sm,Zr)(Co,Cu,Fe)z materials for permanent magnets in the literature, which were obtained by modern experimental equipment that is perfect from the point of view of the methodological approach [3,5–8]. Results obtained by methods including high-resolution transmission electron microscopy (HRTEM), nanoprobe energy dispersive x-ray spectroscopy (EDXS), nano-beam diffraction (NBD), etc. have allowed one to estimate the fine structure of (Sm,Zr)(Co,Cu,Fe)Z sintered magnet samples using results of the local analysis of some phases [3,5,6,8]. However, sometimes, the relation of results obtained for the structure of magnets and the initial alloy structure is overlooked. It is obvious that the real sintered rare-earth permanent magnets are produced from thin powders, otherwise, the energy potential cannot be realized completely. It should be noted that the homogenization of the alloy in the course of magnet production starts directly after milling rather than in reaching the working temperature of SSHT. The manufacturing operations (milling, sintering, and SSHT) homogenizing the alloy shift the experimental pattern of phase transformations from realism to impressionism, which makes their interpretation difficult. The methodology of this study was built taking into account the evidence of several levels of heterogeneity of the (Sm,Zr)(Co,Cu,Fe)Z alloys and magnets based on them:

1. The first obvious level of heterogeneity of the (Sm,Zr)(Co,Cu,Fe)Z alloys, which was noted in numerous studies, can be observed with optical resolutions [11,12,17–21]. It is characterized by three clear components comprising the structure, which, with optical magnifications, have signs of all phases, which, with the higher magnifications, are obviously heterogeneous. We denoted these “optical phases” as structural phase components (SPC).

2. The second level of heterogeneity typical of both (Sm,Zr)(Co,Cu,Fe)Z alloys and magnets based on them is the scale of the so-called cellular structure, which has been repeatedly studied, and consist of cells and cell boundaries that are penetrated by thin plates along the basal planes. 3. The third level of heterogeneity is the structure of boundaries between cells, which obviously undergo phase transformations at all stages of thermal aging of the (Sm,Zr)(Co,Cu,Fe)Z alloys and magnets based on them. This study was devoted to regularities mainly typical of the first, the most rough, level of the heterogeneity of materials based on the (Sm,Zr)(Co,Cu,Fe)Z alloys. Materials 2020, 13, 3893 4 of 18

3. Materials and Methods

The (Sm,Zr)(Co,Cu,Fe)Z alloys were prepared by high-frequency induction melting in an argon atmosphere at an excess pressure of 5–10% using Al2O3 crucibles (NTC Bakor, Ltd, Moscow, Russia) and individual components such as Sm, Zr, Co, Fe, and Cu of >99.9, >99.97, >99.98, >99.7, and >99.97 wt.% purity, respectively. After melting, the alloys were not cast in a mold but slowly cooled in the crucible, which was carefully thermally isolated from a water-cooled inductor. As a result, an ingot characterized by coarse grains 4–6 mm in average size was obtained. With respect to the magnetic state, each grain had a single easy magnetization axis (EMA). These samples could not be classified among traditional single crystals since they are not single-phase. However, all phases of one grain were strictly collinear; in other words, the EMAs of each phase within a grain were parallel to each other. For simplicity, from here on, we call these samples pseudo-single crystals. The chemical composition of the Sm1-XZrX(Co1-a-bCuaFeb)Z samples of the experimental series is given in Table1.

Table 1. Chemical composition of the experimental alloys Sm1-XZrX(Co1-a-bCuaFeb)Z.

Series no. x a b z 1 0.13 0.088 0.210 6.0–6.8 2 0.15 0.088 0.210 6.0–6.8 3 0.17 0.088 0.210 6.0–6.8 4 0.19 0.088 0.210 6.0–6.8 5 0.15 0.075 0.260 6.0–6.8 6 0.13 0.070 0.240 6.1–6.5

The chemical analysis of alloys was performed by optical emission spectroscopy with inductively coupled plasma (ULTIMA 2 Jobin-yvon ICP-OES, Tokyo, Japan). The heat treatment conditions used were similar to those described in the literature [2–8]. Heat treatment for the solid solution (SSHT) at 1150–1180 ◦C for 5 h and subsequent water cooling were performed. The isothermal tempering at 800 ◦C was carried out for 20 h; after that, samples were either water-quenched or subjected to step tempering to 400 ◦C at an average cooling rate of 100◦/h followed by furnace cooling. All heat treatments were performed after preliminary degassing during heating to 600 ◦C in a vacuum and subsequent high-purity argon puffing to a pressure that slightly exceeded (to 5–10%) the atmospheric pressure. Pseudo-single crystal samples of each of the compositions in as-cast and heat-treated (under different conditions) states were ground to form balls 2.5–3.5 mm in diameter by grinding with gaseous nitrogen supplied under a pressure between two abrasive caps. The magnetic measurements of major and minor hysteresis loops of pseudo-single crystal spherical samples were performed at room temperature using a vibrating sample magnetometer (VSM LDJ Electronics Inc., Model 9600, Troy, NY, USA) (HMAX = 30 kOe). To measure the major hysteresis loop, the samples were preliminarily magnetized in a magnetic field (Ningbo Canmag Electronics Co., Model KCJ-3560G, Ningbo, China) of no less than 100 kOe. The VSM was graduated using a Ni standard with σ = 54.4 G cm3/g. A VSM-option of PPMS-9T Quantum Design installation was used to study the SNi × magnetic characteristics of samples, the coercive force of which at room temperature is above 30 kOe. The samples completely evaluated with respect to magnetic parameters were demagnetized using alternating magnetic field of VSM with decreasing amplitude and were fixed in mandrels with a fast curing epoxy compound. The fixation of samples was performed using a magnetic field and a specific facility in order to align the basal or prismatic planes of the sample in parallel to the surface of the prepared section. The sections were prepared in accordance with standard techniques using diamond pastes with a Materials 2020, 13, 3893 5 of 18 gradual reduction in grain size. The mandrels for the fixations of samples allowed us to mount them in a holder of VSM and perform additional manipulations in the applied magnetic field. The polished and etched sections were studied using optical magnifications in order to visualize the domain structure by Kerr-effect method and the microstructure, respectively. The quantitative relationships of structural components were determined in accordance with standard stereometric metallography techniques. MaterialsThe 2020 fine, 13, structure x FOR PEER of REVIEW samples was studied by scanning electron microscopy (SEM) (using5 of 19 a scanning electron microscope (FEI Company, Fremont, CA, USA with EDS, EDAX Inc. Mahwah, NJ,USA USA).). The Thecompositions compositions of structural of structural elements elements were studied were studied by local by x-ray local energy x-ray di energyspersive dispersive analysis analysisusing pure using elements pure elements as the standards.as the standards. The EDX The analysis EDX analysis of each of eachof samples of samples start starteded from from the thedetermination determination of the of integral the integral chemical chemical composition composition for the for maximally the maximally possible possible section section area in area order in orderto control to control the correspondence the correspondence of the ofcompositio the compositionn of the sample of the sample to the chemical to the chemical analysis analysis data for data the forassociated the associated series. series. Analogously, Analogously, individual individual structural structural components components were were studied studied using using areas knowingly apart from thethe interphaseinterphase boundaries. boundaries. Element scanning across the boundaries of the structural componentscomponents waswas alsoalso used.used. Anisotropic sinteredsintered powderpowder samples were prepared fromfrom thesethese alloys using a standard powder metallurgy technique, which includes crashing in a nitrogen atmosphere, and milling in a vibratingvibrating ball mill. Compaction of powders in a transversetransverse magnetic field,field, cold isostatic pressing, sintering in a hydrogen atmosphere, and heat treatment in an argonargon atmosphereatmosphere were performedperformed in accordanceaccordance withwith regimesregimes describeddescribed above.above. The magnetic properties of the sinteredsintered samplessamples (Ø15(Ø 15 × 7 mm) were × measured in fieldsfields of 30 kOe using a completely closed magnetic circuit and an automatic recording fluxflux meter (B-H(B-H tracertracer LDJLDJ ElectronicsElectronics Inc.,Inc., ModelModel 5500H,5500H, Troy,Troy, NY, NY, USA). USA).

4. Results

4.1. Dependence of Magnetic Properties of Samples on the Chemical Composition of the 4.1. Dependence of Magnetic Properties of Samples on the Chemical Composition of the (Sm,Zr)(Co,Cu,Fe)z (Sm,Zr)(Co,Cu,Fe) Alloys Alloys z Figure1 shows the major and minor hysteresis loops of the Sm 0.87Zr0.13(Co0.690Cu0.070Fe0.240)6.5 Figure 1 shows the major and minor hysteresis loops of the Sm0.87Zr0.13(Co0.690Cu0.070Fe0.240)6.5 sample, which are typical of all samples under study. sample, which are typical of all samples under study.

Figure 1. Major and minor hysteresis loops of pseudo-single crystal Sm0.87Zr0.13(Co0.60Cu0.070Fe0.240)6.5 Figure 1. Major and minor hysteresis loops of pseudo-single crystal Sm0.87Zr0.13(Co0.60Cu0.070Fe0.240)6.5 sample in the high-coercivity state; (bottom) and (top) curves correspond to the magnetization curves of sample in the high-coercivity state; (bottom) and (top) curves correspond to the magnetization curves the sample demagnetized by reverse field and alternating filed, respectively (VSM, without allowance of the sample demagnetized by reverse field and alternating filed, respectively (VSM, without for the demagnetizing factor, N = 1/3). allowance for the demagnetizing factor, N = 1/3). The given hysteresis loop is sufficiently wide for detailing the minor hysteresis loops and is also almostThe completely given hysteresis within loop the operating is sufficiently fields wide of VSM. for detailing the minor hysteresis loops and is also almostThe completely bottom curves within correspond the operating to the fields magnetization of VSM. curves of sample demagnetized by reversing fieldThe and the bottom series curves of minor correspond hysteresis loops to the measured magnetization with progressively curves of increasedsample demagneti magnetizedzed field. by Thereversing top curves field indicate and the magnetization series of minor curves hysteresis of the same loops sample measured demagnetized with progressively with the alternating increased magnetized field. The top curves indicate magnetization curves of the same sample demagnetized with the alternating field with decreasing amplitude and the series of minor hysteresis loops measured with progressively increased magnetized field. This sample and samples of all alloys under study indicate the classic magnetic hysteresis related to the domain wall pinning mechanism. It can be seen that from the viewpoint of maximum energy product, the hysteresis loop is ultimate. The decrease in the magnetization, with allowance for the demagnetizing factor, starts in negative fields of ~ 8–9 kOe; the specific saturation magnetization of the sample was σS = 116 G × cm3/g, which corresponds to values 4πJS = 12.5 kG and (BH)MAX = 39 MGOe.

Materials 2020, 13, 3893 6 of 18

field with decreasing amplitude and the series of minor hysteresis loops measured with progressively increased magnetized field. This sample and samples of all alloys under study indicate the classic magnetic hysteresis related to the domain wall pinning mechanism. It can be seen that from the viewpoint of maximum energy product, the hysteresis loop is ultimate. The decrease in the magnetization, with allowance for the demagnetizing factor, starts in negative 3 fields of ~8–9 kOe; the specific saturation magnetization of the sample was σS = 116 G cm /g, Materials 2020, 13, x FOR PEER REVIEW × 6 of 19 which corresponds to values 4πJS = 12.5 kG and (BH)MAX = 39 MGOe. The hysteresis loops of the other samples are qualitatively similar to that given in Figure1. The hysteresis loops of the other samples are qualitatively similar to that given in Figure 1. The The differences consist in scales of loops. Slight differences were observed along the ordinate axis, differences consist in scales of loops. Slight differences were observed along the ordinate axis, which which were in accordance with the relative concentrations of Co and Fe, and significant differences were in accordance with the relative concentrations of Co and Fe, and significant differences were were observed along the abscissa axis, which were due to the structural compositions of the samples. observed along the abscissa axis, which were due to the structural compositions of the samples. The magnetization reversal portions of the major hysteresis loops of samples of series no. 2 The magnetization reversal portions of the major hysteresis loops of samples of series no. 2 (Table1) with the fixed x, Sm 0.85Zr0.15(Co0.702Cu0.088Fe0.210)Z, and samples of series nos. 1–4 with the (Table 1) with the fixed x, Sm0.85Zr0.15(Co0.702Cu0.088Fe0.210)Z, and samples of series nos. 1–4 with the fixed fixed z, Sm1-XZrX(Co0.702Cu0.088Fe0.210)6.4, are given in Figure2. z, Sm1-XZrX(Co0.702Cu0.088Fe0.210)6.4, are given in Figure 2.

Figure 2. Magnetization reversal portions of major hysteresis loops of pseudo-single crystal samples in

high-coercivityFigure 2. Magneti state:zation (a) Smreversal0.85Zr portions0.15(Co0.702 ofCu major0.088 Fehysteresis0.210)z with loops z = of6.0 pseudo (1); 6.2-single (2); 6.3 crystal (3); 6.4 samples (4); 6.5 (5)in high and- 6.8coercivity (6), and state: (b) Sm (a)1-X SmZr0.85XZr(Co0.150.702(CoCu0.7020.088Cu0.088Fe0.210Fe0.210)6.4)z with zх == 6.00.13 (1); (1); 6.2 0.15 (2); (2);6.3 0.17(3); 6.4 (3) (4); and 6.5 0.19 (5) (VSM,and 6.8 without (6), and allowance (b) Sm1-XZr forX(Co the0.702 demagnetizingCu0.088Fe0.210)6.4 factor, with Nх = 10.13/3). (1); 0.15 (2); 0.17 (3) and 0.19 (VSM, without allowance for the demagnetizing factor, N = 1/3). It can be seen clearly that as z increased for the Sm0.85Zr0.15(Co0.702Cu0.088Fe0.210)z alloys (Figure2a), the coerciveIt can be force seen increased clearly that monotonously as z increased over for the the Sm whole0.85Zr composition0.15(Co0.702Cu0.088 range.Fe0.210 However,)z alloys (Figure beginning 2a), fromthe coercive certain force z, at lowincreased values mono of appliedtonously magnetic over the field, whole the composition magnetization range. reversal However, curve beginning indicated afrom “shoulder” certain z, (i.e., at low the values squareness of applied of the magnetic loop decreases). field, the Such magnetization behavior is reversal completely curve typical indicate ofd all a samples“shoulder” under (i.e., study. the squareness of the loop decreases). Such behavior is completely typical of all samplesThe under difference study consists. only in the value of the “shoulder”. The lower the relative zirconium concentration (x), the clearer the “shoulder”. Moreover, as the relative zirconium concentration (x)

Materials 2020, 13, x FOR PEER REVIEW 7 of 19 Materials 2020, 13, 3893 7 of 18 The difference consists only in the value of the “shoulder”. The lower the relative zirconium concentration (x), the clearer the “shoulder”. Moreover, as the relative zirconium concentration (x) increases,increases, the the z z value value corresponding corresponding to to the the worsening worsening loop loop squareness squareness shifts shifts to the to lowthe low values. values. This This can becan clearly be clearly observed observed in comparing in comparing the dependences the dependences in Figure in Figure2b. 2b. TheThe dependencesdependences ofof thethe magneticmagnetic propertiesproperties ofof thethe sinteredsintered powderpowder samplessamples onon thethe chemicalchemical compositioncomposition ofof thethe SmSm1-X1-XZrX(Co,Cu,Fe)(Co,Cu,Fe)z alloysalloys were were qualitat qualitativelyively similar to those of the pseudo-singlepseudo-single crystalcrystal samples.samples. As anan example,example, FigureFigure3 3 shows shows the the characteristic characteristic magnetization magnetization reversal reversal curves curves of of sinteredsintered powderpowder samplessamples inin thethe high-coercivityhigh-coercivity statestate (series(series no.no. 2).2).

Figure 3. Magnetization reversal portions of hysteresis loops of sintered powder samples in the

high-coercivityFigure 3. Magnetization state for thereversal Sm0.85 portionsZr0.15(Co of0.702 hysteresisCu0.088Fe loops0.210 )ofz alloyssintered with powder z = 6.0 samples (1); 6.4 (2);in the 6.7 high (3). - coercivity state for the Sm0.85Zr0.15(Co0.702Cu0.088Fe0.210)z alloys with z = 6.0 (1); 6.4 (2); 6.7 (3). The main difference of the sintered powder samples consists of the fact that the characteristic “shoulder”The main in the difference magnetization of the reversal sintered curves powder of powder samples samples consists appeared of the fact for thethat compositions the characteristic with z“shoulder” lower by 0.15–0.2 in the magnetization than that for thereversal pseudo-single curves of crystalpowder samples samples of appear analogoused for compositions the compositions from allwith series z lower of alloys by 0.15 under–0.2 study.than that It is for likely the thatpseudo the- decreasesingle crystal in the samples threshold of ofanalogous the appearance compositions of the “shoulder”from all series is related of alloys to samarium under study. and It zirconium is likely that losses the due decrease to the oxidationin the threshold and to theof the reactions appearance with nonmetallicof the “shoulder” impurities is related and samarium to samarium losses and due zirconiumto evaporation losses during due theto the course oxidation of manufacturing and to the thereactions sintered with samples. nonmetallic impurities and samarium losses due to evaporation during the course of manufacturing the sintered samples. 4.2. Dependence of the Structure of Samples on the Chemical Composition of the (Sm,Zr)(Co,Cu,Fe)z Alloys 4.2. DependenceFigure4 shows of the the Structure microstructure of Samples on on the the prismatic Chemical planeComposition of the pseudo-singleof the (Sm,Zr)(Co,Cu,Fe) crystal samplez Alloys of Sm0.85FigureZr0.15(Co 4 shows0.702Cu the0.088 microstructureFe0.210)6.4 alloy on in the the prismatic (a) as-cast plane state, of (b) the after pseudo solid- solutionsingle crystal heat treatment,sample of andSm0.85 (c)Zr after0.15(Co complete0.702Cu0.088 cycleFe0.210 of)6.4 heat alloy treatment in the for(a) theas-cast high-coercivity state, (b) after state, solid which solution are typical heat oftreatment, all series ofand samples. (c) after complete cycle of heat treatment for the high-coercivity state, which are typical of all seriesThe of samples. microstructure of all samples under study consisted of three structural phase components (SPCs):The dark-grey microstructure SPC (fromof all samples here on, under A); bright-grey study consist dendritic-likeed of three structural SPC (B), phase and almost components white anisotropic(SPCs): dark SPC-grey elongated SPC (from along here the basalon, A) plane; bright of sample-grey dendritic (C), which,-like for SPC all samples, (B), and was almost localized white withinanisotropic SPC A.SPC elongated along the basal plane of sample (C), which, for all samples, was localized withinThe SPC heat A. treatment did not lead to any substantive changes in the quantitative relations of structuralThe heatcomponents treatment in all did studied not lead samples to any with substantive the same chemical changes composition. in the quantitative After solid relations solution of heatstructural treatment components and quenching, in all studied the characteristic samples with composition the same contrast chemical in composition. the section disappeared; After solid however,solution heatthe cast treatment prehistory and of quenching, the cast state the was characteristic clearly observed. composition After a complete contrast heat in the treatment section cycle,disappear the contrasted; however, recovered the cast but diprehistoryffered in theof the halftone cast state pattern. was clearly observed. After a complete heat Atreatment portion ofcycle, the microstructurethe contrast recover in Figureed but4c wasdiffer contrasteded in the halftone in order pattern. to perform the quantitative analysisA portio of then image.of the microstructure in Figure 4c was contrasted in order to perform the quantitative analysisThe of same the image. colors were used in Figure5 to show changes in the relationships of structural componentsThe same with colors progressive were used change in in Figure the chemical 5 to show composition changes of in the the alloys relationships of series nos. of 1, structural 2, and 4 (Tablecomponents1). It is with clearly progressive observed chang that thee in microstructures the chemical com ofposition all studied of the samples alloys in of the series high-coercivity nos. 1, 2, and state4 (Table were 1). qualitativelyIt is clearly observed identical that and the were microstructures the superposition of all studied of three samples structural in the components high-coercivity A, B, andstate C, were and thequalitatively quantitative identical relationships and were between the superposition which varied monotonously of three structural with changingcomponents chemical A, B, composition within each series of alloys.

Materials 2020, 13, x FOR PEER REVIEW 8 of 19 anMaterialsd C, 20and20, 13the, x FOR quantitative PEER REVIEW relationships between which varied monotonously with changing8 of 19 chemical composition within each series of alloys. anMaterialsd C, 2020and, 13 the, 3893 quantitative relationships between which varied monotonously with changing8 of 18 chemical composition within each series of alloys.

Figure 4. Microstructure on the prismatic plane of the pseudo-single crystal samples of the SmFigure0.85Zr0.15 4. (CoMicrostructure0.702Cu0.088Fe0.210 on)6.4 alloy the prismaticin the (a) planeas-cast of state, the ( pseudo-singleb) after solid-solution crystal samplesheat treatment, of the

andFigureSm0.85 (c)Zr after 4.0.15 Microstructure(Co complete0.702Cu 0.088cycle Feon of0.210 heat the)6.4 prismatictreatmentalloy in the (optical plane (a) as-cast of microscope, the state, pseudo (b) afteretching-single solid-solution with crystal 5% alcohol samples heat treatment, solution of the ofSmand HNO0.85 (cZr) after30.15). (CoThe complete0.702 easyCu 0.088magnetization cycleFe0.210 of)6.4heat alloy axis treatment in isthe in (thea) (optical as image-cast microscope, state,plane (andb) after strictly etching solid horizontal. with-solution 5% alcohol heatDefinitions treatment, solution for

structuralandof HNO (c) after3). components The complete easy magnetization cyclefor A, of B andheat C axistreatment in ( isc) inare the (opticalgiven image in microscope, the plane text. and strictly etching horizontal. with 5% alcohol Definitions solution for ofstructural HNO3). componentsThe easy magnetization for A, B and axis C in is (c )in are the given image in theplane text. and strictly horizontal. Definitions for structural components for A, B and C in (c) are given in the text.

FigureFigure 5. 5. ChangesChanges of of relationships relationships of of structural structural phase phase components components A, A, B, B, and and C C(are (are shown shown in inpanel panel c c and in Figure4) in pseudo-single crystal samples of Sm Zr (Co Cu Fe )z alloys in the and in Figure 4) in pseudo-single crystal samples of Sm1-X1-XZrX(CoX 0.7020.702Cu0.088Fe0.0880.210)z 0.210alloys in the high- coercivityFigurehigh-coercivity 5. Changesstate statewith of withxrelationships = 0.13 x = 0.13(a1— (ofa3);1—3); structural 0.15 0.15(b1— (phaseb3)1—3) and components and 0.19 0.19 (c1 (—c1—3), 3),A, andB, and z = zC =6.2 (are6.2 (1); shown (1); 6.4 6.4 (2); in (2); panel6.7 6.7 (3) (3) c (prismatic plane, the easy magnetization axis is in the image plane and strictly vertical). (prismaticand in Figure plane, 4) inthe pseudo easy magnetization-single crystal axis samples is in theof Smimage1-XZr planeX(Co0.702 andCu strictly0.088Fe0.210 vertical).)z alloys in the high- coercivityThe energy-dispersive state with x = X-ray 0.13 ( (EDX)a1—3); microanalysis0.15 (b1—3) and performed 0.19 (c1— for3), theand samples z = 6.2 of(1); all 6.4 series (2); 6.7 of alloys(3) in the high-coercivity(prismatic plane, state the showedeasy magnetization the similarity axis of is all in chemicalthe image compositionsplane and strictly of the vertical). structural components.

It is possible to state that the intermediate compositions corresponded to the common formulas: in SPC A, (Sm,Zr)(Co,Cu,Fe)6.3; in SPC B, (Sm,Zr)(Co,Cu,Fe)7.1; and in SPC C, (Sm,Zr)(Co,Cu,Fe)3.7 Materials 2020, 13, x FOR PEER REVIEW 9 of 19

The energy-dispersive X-ray (EDX) microanalysis performed for the samples of all series of Materialsalloys 2020in the, 13 high, 3893-coercivity state showed the similarity of all chemical compositions of the structural9 of 18 components. It is possible to state that the intermediate compositions corresponded to the common formulas: in SPC A, (Sm,Zr)(Co,Cu,Fe)6.3; in SPC B, (Sm,Zr)(Co,Cu,Fe)7.1; and in SPC C, (in alternative variants, Sm(Co,Cu,Fe,Zr) , Sm(Co,Cu,Fe,Zr) , and Sm(Co,Cu,Fe,Zr) , respectively). (Sm,Zr)(Co,Cu,Fe)3.7 (in alternative 7.5 variants, Sm(Co,Cu,Fe,Zr)8.4 7.5, Sm(Co,Cu,Fe,Zr)12 8.4, and Thus, the structural components were formed based on the 1:5 (A), 2:17 (B), and 2:7 (C) phases. Sm(Co,Cu,Fe,Zr)12, respectively). Thus, the structural components were formed based on the 1:5 (A), This2:17 corresponds (B), and 2:7 (C) to resultsphases. reported This corresponds by Kianvash to results et al. [ 17reported]. by Kianvash et al. [17]. 4.3. Interrelations between the Chemical Composition, Structure, and Hysteresis Parameters of 4.3. Interrelations between the Chemical Composition, Structure, and Hysteresis Parameters of (Sm,Zr)(Co,Cu,Fe)z Alloys in the High-Coercivity State (Sm,Zr)(Co,Cu,Fe)z Alloys in the High-Coercivity State Figure6 shows the dependences of the coercive force and quantitative relationships of the Figure 6 shows the dependences of the coercive force and quantitative relationships of the structural components on the chemical composition of samples in the high-coercivity state for the structural components on the chemical composition of samples in the high-coercivity state for the Sm1-XZrX(Co0.702Cu0.088Fe0.210)z alloys with the same relation of 3d elements (series 1–4). Sm1-XZrX(Co0.702Cu0.088Fe0.210)z alloys with the same relation of 3d elements (series 1–4).

Figure 6. Dependences of the volume percentage of structural components A, B, and C ((Vi/ΣVi) 100) Figure 6. Dependences of the volume percentage of structural components A, B, and C ((Vi/ΣVi)x100)× and coercive force (HCJ) of the Sm1-XZrX(Co0.702Cu0.088Fe0.210)z alloys with x = 0.13 (a), 0.15 (b), 0.17 (c), andand 0.19 coercive (d) on force the chemical(HCJ) of the composition Sm1-XZrX(Co z.0.702 TheCu0.088 verticalFe0.210) dashedz alloys linewith shows x = 0.13 the (a compositions), 0.15 (b), 0.17 with (c), equaland 0.19 volume (d) on percentage the chemical of structural composition components z. The vertical A and B.dashed line shows the compositions with equal volume percentage of structural components A and B. The analysis of the dependences given in Figure6 allowed us to infer the following: AsThe the analysis relative of Zr the content dependences (x) increases, given in the Figure volume 6 allowed percentage us to infer of the the structural following component: C increases.As the In relative this case, Zr forcontent each (x) of theincreases, series the of alloys,volume as percentage the content of ofthe 3d structural elements component (z) increased, C theincreases. volume In percentage this case, for of structural each of the component series of alloys, C decreased. as the content of 3d elements (z) increased, the volumeThe volumepercentage percentages of structural of structural component components C decrease Ad and. B corresponding to more than 90% of the total alloyThe volume volume percentages changed monotonously of structural (forcomponents all experimental A and B series)corresponding from the to dominating more than volume90% of the total alloy volume changed monotonously (for all experimental series) from the dominating percentage of structural component A to the dominating volume percentage of structural component B. volume percentage of structural component A to the dominating volume percentage of structural As the relative content of 3d elements (z) and the volume percentage of structural component component B. B increased in compositions corresponding to the equality of the volumes of structural components A and B (V = V ), an intense increase in the coercive force was observed; after that (at V < V ), A B A B the increase in HCJ decelerated and the dependence reached the horizontal “plateau”. Materials 2020, 13, x FOR PEER REVIEW 10 of 19

As the relative content of 3d elements (z) and the volume percentage of structural component B Materialsincrease2020d in, 13 compositions, 3893 corresponding to the equality of the volumes of structural components10 of A 18 and B (VA = VB), an intense increase in the coercive force was observed; after that (at VA < VB), the increaseThe in comparison HCJ decelerate of thed and data the given dependence in Figure reached2a,b and the horizontal Figure5b shows“plateau”. that for the series of The comparison of the data given in Figures 2a,b and 5b shows that for the series of Sm1- Sm1-XZrX(Co0.702Cu0.088Fe0.210)Z alloys, the transition through the chemical composition corresponding XZrX(Co0.702Cu0.088Fe0.210)Z alloys, the transition through the chemical composition corresponding to the to the equality of volume contents of structural components A and B (VA = VB) was accompanied by equality of volume contents of structural components A and B (VA = VB) was accompanied by the the appearance of a “shoulder” in the magnetization reversal curves along with the end to increase in appearance of a “shoulder” in the magnetization reversal curves along with the end to increase in HCJ. Thus, the reaching of the alloy chemical compositions corresponding to the condition VB > VA HCJ. Thus, the reaching of the alloy chemical compositions corresponding to the condition VB > VA was accompanied by the decrease in the squareness of hysteresis loops. This was satisfied for all series was accompanied by the decrease in the squareness of hysteresis loops. This was satisfied for all of Sm1-XZrX(Co,Cu,Fe)Z alloys. series of Sm1-XZrX(Co,Cu,Fe)Z alloys. Figure7 also demonstrates the validity of the noted regularities for series no. 5, characterized by the Figure 7 also demonstrates the validity of the noted regularities for series no. 5, characterized by other relationship of 3d elements in the Sm0.85Zr0.15(Co0.665Cu0.075Fe0.260)Z alloy. Thus, the following the other relationship of 3d elements in the Sm0.85Zr0.15(Co0.665Cu0.075Fe0.260)Z alloy. Thus, the following conclusion can be inferred. conclusion can be inferred.

Figure 7. (a) Magnetization reversal portions of major hysteresis loops and (b) dependences of the Figure 7. (a) Magnetization reversal portions of major hysteresis loops and (b) dependences of the volume percentage of structural phase components A, B, and C ((Vi/ΣVi)x100) and coercive force (HCJ) volume percentage of structural phase components A, B, and C ((Vi/ΣVi)x100) and coercive force (HCJ) of the Sm0.85Zr0.15(Co0.665Cu0.075Fe0.260)z alloys with z = 6.0(1); 6.2(2); 6.4(3) and 6.5(4) on the chemical of the Sm0.85Zr0.15(Co0.665Cu0.075Fe0.260)z alloys with z = 6.0(1); 6.2(2); 6.4(3) and 6.5(4) on the chemical composition z (VSM, without allowance for the demagnetizing factor, N = 1/3). composition z (VSM, without allowance for the demagnetizing factor, N = 1/3).

The major hysteresis loops for the Sm1-XZrX(Co,Cu,Fe)Z alloys in the high-coercivity state, which The major hysteresis loops for the Sm1-XZrX(Co,Cu,Fe)Z alloys in the high-coercivity state, which are characterized by the maximum (for a certain relation of alloy components) coercive force (HCJ) are characterized by the maximum (for a certain relation of alloy components) coercive force (HCJ) along with the high squareness of the loop, are reached in the case of equal volumes of structural along with the high squareness of the loop, are reached in the case of equal volumes of structural components A and B comprising the main volume of the alloy and largely determining the hysteretic components A and B comprising the main volume of the alloy and largely determining the hysteretic properties of the alloy. properties of the alloy. This conclusion also fully corresponds to samples of sintered magnets produced from the powders This conclusion also fully corresponds to samples of sintered magnets produced from the of the alloys in the experimental series (with allowance for notes given above in the caption for powders of the alloys in the experimental series (with allowance for notes given above in the caption Figure3). for Figure 3). 5. Discussion 5. Discussion 5.1. Alloy-Formation in the Co–Sm–Zr and (Co,Cu,Fe)–Sm–Zr Systems 5.1. Alloy-Formation in the Co–Sm–Zr and (Co,Cu,Fe)–Sm–Zr Systems Note that it is difficult to compare the data on the phase diagrams constructed by different investigators,Note that which it is difficult use diff erent to compare coordinate the data systems. on the Due phase to this, diagrams we compiled constructed the available by different data andinvestigators, constructed which ternary use phasedifferent diagrams coordinate in a systems. form more Due convenient to this, we for compile the presentd the available study, namely, data cobaltand construct was alwaysed ternary in the leftphase corner diagrams of the in binary, a form quasi-binary more convenient sections, for andthe ternarypresent phasestudy, diagrams;namely, rare-earthcobalt was metal always was in alwaysthe left incorner the top of the corner binary, of the quasi ternary-binary systems. sections The, and alloying ternary component phase diagrams; was in therare right-earth corner. metal was always in the top corner of the ternary systems. The alloying component was in the right corner. The structural formation of the Sm–Zr–Co and Sm–Zr–Co–Cu–Fe alloys has been considered in a great number of studies [14,17–31]. The analysis of these results allowed us to formulate the following regulations.

The introduction of zirconium into the alloy leads to the formation of a wide TbCu7-based solid solution region in the composition range of existence in the 2:17 and 1:5 phases. The 1:7 region was Materials 2020, 13, x FOR PEER REVIEW 11 of 19

The structural formation of the Sm–Zr–Co and Sm–Zr–Co–Cu–Fe alloys has been considered in a great number of studies [14,17–31]. The analysis of these results allowed us to formulate the following regulations. The introduction of zirconium into the alloy leads to the formation of a wide TbCu7-based solid solution region in the composition range of existence in the 2:17 and 1:5 phases. The 1:7 region was Materials 2020, 13, 3893 11 of 18 typical of high temperatures corresponding to the SSHT temperatures of the (Sm,Zr)(Co,Cu,Fe)Z alloys and is directly adjacent to the homogeneity range of the 2:17 phase in the 2:17H modification typicalwith the of highTh2Ni temperatures17-type structure corresponding (Figure 8 [25]). to the SSHT temperatures of the (Sm,Zr)(Co,Cu,Fe)Z alloys and isThe directly decomposition adjacent to of the the homogeneity solid solution range led to of the the precipitation 2:17 phase in of the the 2:17H 2:17 modificationphase and continuous with the Thformation2Ni17-type of structurephases of (Figurethe homologous8[25]). row Smn-1Co5n-1.

Figure 8. Isothermal section of Co corner of the ternary Co–Sm–Zr phase diagram at 1200 ◦C (compilationFigure 8. Isothermal of data of Nishio section et of al. Co [25 corner]). of the ternary Co–Sm–Zr phase diagram at 1200 °С (compilation of data of Nishio et al. [25]). The decomposition of the solid solution led to the precipitation of the 2:17 phase and continuous formationThe analysis of phases of of numerous the homologous microstructures row Smn-1 ofCo Sm5n-11-.XZrX(Co,Cu,Fe)Z samples under study in the as-castThe and analysis heat of treated numerous states microstructures shows that, for of Sm all 1-XofZr theX(Co,Cu,Fe) studied compositionsZ samples under (Table study 1) in upon the as-castsolidification and heat and treated subsequent states shows solid that, solution for all decomposit of the studiedion, compositions the discontinuity (Table 1was) upon characteristic solidification for andtwo subsequent primary independently solid solution solidified decomposition, 2:17 andthe 2:7 discontinuityphases (Figure was 4). The characteristic phase components for two primary B and C independentlywere formed based solidified on the 2:17 phases. and 2:7 phases (Figure4). The phase components B and C were formed basedRegarding on the phases. structural phase component A, its structure is likely to correspond to the statement [26,27]Regarding on continuous structural phase phase formation. component A, its structure is likely to correspond to the statement [26,27] on continuousFeatures of phase the structur formation.al formation at the 2:17 phase side, which are given in Morita et al. [22– 24],Materials Featureswere collected2020 of, 13 the, x FOR structuralby PEERus as REVIEW a formationlogical sequence at the 2:17 in Figure phase side,9. which are given in Morita et al.12 of [22 19– 24], were collectedNote that by w ushen as plotting a logical the sequence compiled in Figure data 9of. Morita et al. [22–24], we allowed ourselves to correct some inherent original obvious annoying inaccuracies. However, in our opinion, to completely present the alloy formation of the R1-XZrX(Co,Cu,Fe)Z compositions, the structural formation at the side of the 2:7 phase is also of importance. To refine the processes that occurred, we considered the previously published results.

Figure 9. Phase formation in the (Sm,Zr)(Co,Cu,Fe)Z alloys was demonstrated in partial isopleths of quasi-ternaryFigure 9. Phase phase formation diagrams in ( athe–f )(Sm,Zr)(Co,Cu,Fe) in the homogeneityZ alloys range was of demonstrated the 2:17 phase in (compilationpartial isopleths based of on dataquasi of Morita-ternary et phase al. [22 diagr–24]ams with (a some–f) in corrections).the homogeneity range of the 2:17 phase (compilation based on data of Morita et al. [22–24] with some corrections).

5.2. Alloy-Formation in the (Co,Cu,Fe)–Gd–Zr Systems For the first time, the existence of the strict correspondence between the alloy composition, alloy microstructure (in optical resolutions), and hysteretic properties, which is given in Section 4.2, was found by Lyakhova et al. [11,12] for the Gd1-XZrX(Co,Cu,Fe)Z compositions. It should be noted that the Gd-containing alloys were found to be preferred as model alloys for the investigation of composition–property correlations. The ferrimagnetic coupling of rare-earth and transition metal sublattices of the main phases decreased the magnetic moment of the samples and led to lower anisotropy field and, therefore, coercive force values. This is convenient for magnetic measurements in the limited range of quasi-static external magnetic fields. Moreover, the oxidation of Gd-containing alloys is lower than that of Sm-containing alloys, and the vapor pressure of Gd at 1000 °С is lower than that of Sm by seven orders of magnitude. These circumstances facilitate the preparation of samples. The microstructure of the Gd1-XZrX(Co,Cu,Fe)Z alloys can be easily etched and are more in contrast when compared to that of the Sm alloys because of the more simple set and configuration of phases observed for the actual composition range. In particular, the 5:19 phase was absent in the Gd–Co system. In the rare-earth series, when the atomic number increased, the atomic radius decreased and the 2:17H structure was stabilized, in particular, when Zr atoms were substituted for R atoms [14]. It should be noted that Khan [32] had already noted a certain similarity of the Sm–Co and Gd– Co systems, namely, the existence of the 1:7 phase, as a polymorphic modification of the 2:17 phase existing at high temperatures without introducing an additional alloying element, only in binary cobalt systems with samarium and gadolinium. Lyakhova et al. [11,12] showed the qualitative identity of the dependences of the volume of structural components and magnetic properties on the chemical composition over a wide composition range of Gd1-XZrX(Co,Cu,Fe)Z alloys (Figure 10). Figure 10a shows the positions of the experimental series of Gd1-XZrX(Co,Cu,Fe)Z alloys in the (Co,Cu,Fe) corner of the quasi-ternary (Co,Cu,Fe)–Gd-Zr system (Figure 10b–h). The dependences properties vs. composition are shown by corresponding letters. As a whole, the experimental dependences for the Gd1-XZrX(Co,Cu,Fe)Z alloys were similar to those for the Sm1-XZrX(Co,Cu,Fe)Z alloys (Figures 5–7). The difference between the data given for the (Gd,Zr)(Co,Cu,Fe)Z alloys and those given for the (Sm,Zr)(Co,Cu,Fe)Z alloys consists of the fact that because of the peculiarities noted for the Gd alloys,

Materials 2020, 13, 3893 12 of 18

Note that when plotting the compiled data of Morita et al. [22–24], we allowed ourselves to correct some inherent original obvious annoying inaccuracies. However, in our opinion, to completely present the alloy formation of the R1-XZrX(Co,Cu,Fe)Z compositions, the structural formation at the side of the 2:7 phase is also of importance. To refine the processes that occurred, we considered the previously published results.

5.2. Alloy-Formation in the (Co,Cu,Fe)–Gd–Zr Systems For the first time, the existence of the strict correspondence between the alloy composition, alloy microstructure (in optical resolutions), and hysteretic properties, which is given in Section 4.2, was found by Lyakhova et al. [11,12] for the Gd1-XZrX(Co,Cu,Fe)Z compositions. It should be noted that the Gd-containing alloys were found to be preferred as model alloys for the investigation of composition–property correlations. The ferrimagnetic coupling of rare-earth and transition metal sublattices of the main phases decreased the magnetic moment of the samples and led to lower anisotropy field and, therefore, coercive force values. This is convenient for magnetic measurements in the limited range of quasi-static external magnetic fields. Moreover, the oxidation of Gd-containing alloys is lower than that of Sm-containing alloys, and the vapor pressure of Gd at 1000 ◦C is lower than that of Sm by seven orders of magnitude. These circumstances facilitate the preparation of samples. The microstructure of the Gd1-XZrX(Co,Cu,Fe)Z alloys can be easily etched and are more in contrast when compared to that of the Sm alloys because of the more simple set and configuration of phases observed for the actual composition range. In particular, the 5:19 phase was absent in the Gd–Co system. In the rare-earth series, when the atomic number increased, the atomic radius decreased and the 2:17H structure was stabilized, in particular, when Zr atoms were substituted for R atoms [14]. It should be noted that Khan [32] had already noted a certain similarity of the Sm–Co and Gd–Co systems, namely, the existence of the 1:7 phase, as a polymorphic modification of the 2:17 phase existing at high temperatures without introducing an additional alloying element, only in binary cobalt systems with samarium and gadolinium. Lyakhova et al. [11,12] showed the qualitative identity of the dependences of the volume of structural components and magnetic properties on the chemical composition over a wide composition range of Gd1-XZrX(Co,Cu,Fe)Z alloys (Figure 10). Figure 10a shows the positions of the experimental series of Gd1-XZrX(Co,Cu,Fe)Z alloys in the (Co,Cu,Fe) corner of the quasi-ternary (Co,Cu,Fe)–Gd-Zr system (Figure 10b–h). The dependences properties vs. composition are shown by corresponding letters. As a whole, the experimental dependences for the Gd1-XZrX(Co,Cu,Fe)Z alloys were similar to those for the Sm1-XZrX(Co,Cu,Fe)Z alloys (Figures5–7). The difference between the data given for the (Gd,Zr)(Co,Cu,Fe)Z alloys and those given for the (Sm,Zr)(Co,Cu,Fe)Z alloys consists of the fact that because of the peculiarities noted for the Gd alloys, the dependence of the coercive force on the composition for all series exhibited a clear extremal behavior. The maximum of the dependence of coercive force strictly corresponds to the equality of the volumes of structural components A and B. All the conclusions formulated by us based on the results obtained for the Sm-containing series of alloys were satisfied for the Gd-containing alloys. However, the extremal behavior of the dependences of the coercive force on the chemical and phase compositions for the Gd alloys allowed us to made an additional conclusion formulated for the (Sm,Zr)(Co,Cu,Fe)Z alloys. For the experimental series of (Gd,Zr)(Co,Cu,Fe)Z alloys, a trend was clearly observed to broaden the coercivity peak (decrease in sharpness) with increasing Zr content and trend to shifting the peak to the low relative fraction of 3d-elements (z). Materials 2020, 13, x FOR PEER REVIEW 13 of 19

the dependence of the coercive force on the composition for all series exhibited a clear extremal behavior. The maximum of the dependence of coercive force strictly corresponds to the equality of the volumes of structural components A and B. All the conclusions formulated by us based on the results obtained for the Sm-containing series of alloys were satisfied for the Gd-containing alloys. However, the extremal behavior of the dependences of the coercive force on the chemical and phase compositions for the Gd alloys allowed us to made an additional conclusion formulated for the (Sm,Zr)(Co,Cu,Fe)Z alloys. For the experimental series of (Gd,Zr)(Co,Cu,Fe)Z alloys, a trend was clearly observed to broaden Materials 2020, 13, 3893 13 of 18 the coercivity peak (decrease in sharpness) with increasing Zr content and trend to shifting the peak to the low relative fraction of 3d-elements (z).

Figure 10. (a) Experimental series of Gd1-XZrX(Co0.70Cu0.09Fe0.21)z alloys in the section of the cobalt cornerFigure of the 10. quasi-ternary(a) Experimental (Co 0.70 seriesCu 0.09 of GdFe0.211-XZr)–Gd–ZrX(Co0.70Cu phase0.09Fe diagram0.21)z alloys and in dependences the section of of the volumecobalt percentagecorner of the of structuralquasi-ternary components (Co0.70Cu0.09 A,Fe B,0.21 and)–Gd C–Zr ((Vi phase/ΣVi) diagram100) and dependences coercive force of (Hthe volume) on the × CJ chemicalpercentage composition of structural z for components the alloys with A, xB,= and0.11 C ( b ((Vi/ΣVi)), 0.15 (c ),× 0.19100) ( d and), and coercive 0.30 (e force) and (H onCJ x) for on fixed the z =chemical5.2 (f), 5.6 composition (g), and 6.0 z for (h). the Based alloys on with the compilationx = 0.11 (b), 0.15 of results (c), 0.19 available (d), and 0.30 in Lyakhova (e) and on et x al.for [ 11fixed,12 ]. Materials 2020, 13, x FOR PEER REVIEW 14 of 19 z = 5.2 (f), 5.6 (g), and 6.0 (h). Based on the compilation of results available in Lyakhova et al. [11,12].

The (Sm,Zr)(Co,Cu,Fe)Z samples in the high-coercivity state (see Figures1,2, and7) and pseudo- The (Sm,Zr)(Co,Cu,Fe)Z samples in the high-coercivity state (see Figures 1, 2, and 7) and pseudo- single crystals (Gd,Zr)(Co,Cu,Fe) Z in the high-coercivity state demonstrate the ultimate hysteresis single crystals (Gd,Zr)(Co,Cu,Fe)Z in the high-coercivity state demonstrate the ultimate hysteresis loops (Figure 11). loops (Figure 11).

Figure 11. Major hysteresis loops of pseudo-single crystal samples of Gd0.85Zr0.15(Co0.70Cu0.09Fe0.21)Z alloysFigure with 11. Major z = 5.2(1), hysteresis 5.6(2), loops 6.0(3), of pseudo 6.2(4),-single 6.4(5), crystal and 6.8(6); samples VSM, of Gd without0.85Zr0.15 allowance(Co0.70Cu0.09 forFe0.21 the)Z demagnetizingalloys with z =factor, 5.2(1), N = 5.6(2),1/3); compilation 6.0(3), 6.2(4), of data6.4(5), of andLyakhova 6.8(6); et VSM, al. [11 ]. without allowance for the demagnetizing factor, N = 1/3); compilation of data of Lyakhova et al. [11].

The comparison of Figures 10c and 11 clearly shows that as the chemical composition changes monotonically in the Gd0.85Zr0.15(Co0.70Cu0.09Fe0.21)z alloy series, the excess of the volume content of component B over component A (VB > VA) is accompanied by the decrease in the coercivity and, like in the case of (Sm,Zr)(Co,Cu,Fe)Z alloys, by the progressive decrease in the squareness of the hysteresis loop. We calculated the phase relations for the (Co,Cu,Fe)–Gd–Zr system based on results given in Figure 10 taking into account the behavior of the structure-sensitive characteristic such as the intrinsic coercive force of samples (HCJ), localization of Zr in crystal lattices of Smn-1Co5n-1 phases, and the above constructed Co–Sm–Zr and (Co,Cu,Fe)–Sm–Zr phase diagrams. The results are given in Figure 12.

Figure 12. Sketch of the 3-dM angle of the quasi-ternary (Co,Cu,Fe)–Gd–Zr phase diagram for a temperature range of 1160–1190 °С.

This sketch of the diagram corresponds to temperatures slightly below the upper interval of SSHT temperature, at which, according to results of the analysis of microstructures of the

Materials 2020, 13, x FOR PEER REVIEW 14 of 19

The (Sm,Zr)(Co,Cu,Fe)Z samples in the high-coercivity state (see Figures 1, 2, and 7) and pseudo- single crystals (Gd,Zr)(Co,Cu,Fe)Z in the high-coercivity state demonstrate the ultimate hysteresis loops (Figure 11).

Figure 11. Major hysteresis loops of pseudo-single crystal samples of Gd0.85Zr0.15(Co0.70Cu0.09Fe0.21)Z alloys with z = 5.2(1), 5.6(2), 6.0(3), 6.2(4), 6.4(5), and 6.8(6); VSM, without allowance for the Materials 2020, 13, 3893 14 of 18 demagnetizing factor, N = 1/3); compilation of data of Lyakhova et al. [11].

TheThe comparison comparison of of Figures Figures 10c 10 cand and 11 11 clearly clearly shows shows that that as as the the chemical chemical composition composition changes changes monotonically in the Gd0.85Zr0.15(Co0.70Cu0.09Fe0.21)z alloy series, the excess of the volume content of monotonically in the Gd0.85Zr0.15(Co0.70Cu0.09Fe0.21)z alloy series, the excess of the volume content component B over component A (VB > VA) is accompanied by the decrease in the coercivity and, like of component B over component A (VB > VA) is accompanied by the decrease in the coercivity and, in the case of (Sm,Zr)(Co,Cu,Fe)Z alloys, by the progressive decrease in the squareness of the like in the case of (Sm,Zr)(Co,Cu,Fe)Z alloys, by the progressive decrease in the squareness of the hysteresishysteresis loop. loop. WeWe calculated calculated the the phase phase relations relations for for the the (Co,Cu,Fe) (Co,Cu,Fe)–Gd–Zr–Gd–Zr system system based based on on results results given given in in FigureFigure 10 10 taking taking into into account account the the behavior behavior of of the the structure structure-sensitive-sensitive characteristic characteristic such such as as the the intrinsic intrinsic coercive force of samples (HCJ), localization of Zr in crystal lattices of Smn-1Co5n-1 phases, and the above coercive force of samples (HCJ), localization of Zr in crystal lattices of Smn-1Co5n-1 phases, and the constructedabove constructed Co–Sm– Co–Sm–ZrZr and (Co,Cu,Fe) and (Co,Cu,Fe)–Sm–Zr–Sm–Zr phase diagrams. phase diagrams. TheThe results results are are given given in in Figure Figure 12. 12 .

Figure 12. Sketch of the 3-dM angle of the quasi-ternary (Co,Cu,Fe)–Gd–Zr phase diagram for a

Figuretemperature 12. Sketch range of of the 1160–1190 3-dM angle◦C. of the quasi-ternary (Co,Cu,Fe)–Gd–Zr phase diagram for a temperature range of 1160–1190 °С. This sketch of the diagram corresponds to temperatures slightly below the upper interval of SSHT temperature,This sketch at of which, the diagram according corresponds to results of to the temperatures analysis of microstructures slightly below ofthe the upper (R,Zr)(Co,Cu,Fe) interval of z SSHTalloys, temperature, obvious melting at which,on the edge according of the structural to results component of the analysis C (2:7) was of observed. microstructures Similar of melting the was observed by Kianvash et al. [17].

5.3. Alloy-Formation in the (Co,Cu,Fe)–Sm–Zr Systems

Taking into account the data given in Figure6, the phase relations in the Gd 1-XZrX(Co,Cu,Fe)Z alloys (Figure 12) easily transform into the corresponding diagram for the Sm1-XZrX(Co,Cu,Fe)Z alloys. Calculations show that the transition from the constructions performed for the Gd-containing compositions to the constructions for the Sm-containing compositions was mainly reached by elongation of the diagram in Figure 12 along the equi-composition axes R–Zr. In this case, it should be taken into account that the diagram for Sm alloys has an additional “arrow” of the 5:19 phase and areas of interaction of the phase with neighboring phases. The obtained sketch of the diagram, which indicates the phase relations in the Sm1-XZrX(Co,Cu,Fe)Z alloys (used for manufacturing of permanent magnets) in the temperature range of SSHT, is given in Figure 13. Materials 2020, 13, x FOR PEER REVIEW 15 of 19

(R,Zr)(Co,Cu,Fe)z alloys, obvious melting on the edge of the structural component C (2:7) was observed. Similar melting was observed by Kianvash et al. [17].

5.3. Alloy-Formation in the (Co,Cu,Fe)–Sm–Zr Systems

Taking into account the data given in Figure 6, the phase relations in the Gd1-XZrX(Co,Cu,Fe)Z alloys (Figure 12) easily transform into the corresponding diagram for the Sm1-XZrX(Co,Cu,Fe)Z alloys. Calculations show that the transition from the constructions performed for the Gd-containing compositions to the constructions for the Sm-containing compositions was mainly reached by elongation of the diagram in Figure 12 along the equi-composition axes R–Zr. In this case, it should be taken into account that the diagram for Sm alloys has an additional “arrow” of the 5:19 phase and areas of interaction of the phase with neighboring phases. The obtained sketch of the diagram, which indicates the phase relations in the Sm1- XZrX(Co,Cu,Fe)Z alloys (used for manufacturing of permanent magnets) in the temperature range of SSHT, is given in Figure 13. Figure 14 shows how, in accordance with our results, the binary Co–Sm phase diagram transforms in adding alloying components (Zr,Cu, and Fe) for the section shown by the dashed line Materials 2020 13 in Figure 13., , 3893 15 of 18

Figure 13. Sketch of the 3-dM angle of the quasi-ternary (Co,Cu,Fe)–Sm–Zr phase diagram for a

Figuretemperature 13. Sketch range of of the 1160–1190 3-dM an◦C.gle of the quasi-ternary (Co,Cu,Fe)–Sm–Zr phase diagram for a temperature range of 1160–1190 °С. Figure 14 shows how, in accordance with our results, the binary Co–Sm phase diagram transforms inMaterials adding 2020 alloying, 13, x FOR components PEER REVIEW (Zr,Cu, and Fe) for the section shown by the dashed line in Figure16 of 13 19.

Figure 14. (a) Portion of the binary Co–Sm phase diagram (compilation of Massalski et al. [33]) for the Figure 14. (a) Portion of the binary Co–Sm phase diagram (compilation of Massalski et al. [33]) for the actual composition range and (b) its transformation with adding Zr, Cu, and Fe into the isopleth of the actual composition range and (b) its transformation with adding Zr, Cu, and Fe into the isopleth of (Co,Cu,Fe)–Sm–Zr diagram (dashed line in Figure 13). the (Co,Cu,Fe)–Sm–Zr diagram (dashed line in Figure 13).

As can be seen from Figures 12–14, the phase formation in the five-component (R,Zr)(Co,Cu,Fe)Z As can be seen from Figures 12–14, the phase formation in the five-component (R,Zr)(Co,Cu,Fe)Z system differed substantially from the diagrams given in [25–31] and mainly, does not contradict the system differed substantially from the diagrams given in [25–31] and mainly, does not contradict the data of Morita et al. [22–24]. data of Morita et al. [22–24]. As the Zr content in the alloys increases, the region of the high-temperature 1:7 phase, on the As the Zr content in the alloys increases, the region of the high-temperature 1:7 phase, on the way out the “corridor” of the wide homogeneity range of the derivative 1:(5 + x) phase, “runs” in way out the “corridor” of the wide homogeneity range of the derivative 1:(5 + x) phase, “runs” in two two directions corresponding to the back course of branches of the primary solidification of the alloy directions corresponding to the back course of branches of the primary solidification of the alloy from from liquid. liquid. The first is the direction toward the homogeneity range of the high-temperature hexagonal The first is the direction toward the homogeneity range of the high-temperature hexagonal polymorphous modification of the 2:17 phase (space group P6/mmm). The second is the direction polymorphous modification of the 2:17 phase (space group P6/mmm). The second is the direction toward the homogeneity range of high-temperature hexagonal polymorphous modification of the 2:7 toward the homogeneity range of high-temperature hexagonal polymorphous modification of the 2:7 phase (space group P63/mmc). phase (space group P63/mmc). The logicality, at least, of the first similar closure of high-temperature hexagonal structures was The logicality, at least, of the first similar closure of high-temperature hexagonal structures was indicated repeatedly by Khan in the 18 studies related to the R–Co systems. In particular, he wrote: indicated repeatedly by Khan in the 18 studies related to the R–Co systems. In particular, he wrote: “It should be noted that SmCo5+X and Sm2Co17(h) are isostructural (i.e., of the TbCu7-type with disordered substitutions). It means that we should obtain a homogeneous region from SmCo5+X to Sm2Co17(h) in the phase diagram, at least, at temperatures near to the solidus. What we experimentally obtain is a eutectic in the neighborhood of SmCo7. An explanation for this discrepancy is still being sought” [34]. Thus, at high temperatures, the 1:7-based solid solution range in the quasi-ternary phase diagram is present in the form of an asymmetric biconvex (toward the decreasing temperature) lens adjacent to the high-temperature homogeneity ranges of the 2:17 and 2:7 phases in their wider ranges. It should be noted that the given sketches of the phase diagrams are related to certain relations of 3d elements, which correspond to the formula R1-XZrX(Co0.70Cu0.09Fe0.21)Z.

6. Conclusions We present here an original view on the issues of the structural formation of the (R,Zr)(Co,Cu,Fe)Z alloys in the concentration ranges promising for the production of high-coercivity permanent magnets. These findings were based on a quantitative analysis of the microstructures of the first, most crude level of heterogeneity of these alloys in a highly coercive state. It was shown that:

1. The formula of the alloys R1-XZrX(Co,Cu,Fe)Z is physically most reasonable from the viewpoint of the effect of main elements with the qualitatively different electron shells (4f–R, 4d–Zr and 3d–Co,Cu,Fe) on the structure and properties of the studied alloys, and with allowance for the fact that the initial structural state is the matrix being the single-phase disordered 1:7H solid

Materials 2020, 13, 3893 16 of 18

“It should be noted that SmCo5+X and Sm2Co17(h) are isostructural (i.e., of the TbCu7-type with disordered substitutions). It means that we should obtain a homogeneous region from SmCo5+X to Sm2Co17(h) in the phase diagram, at least, at temperatures near to the solidus. What we experimentally obtain is a eutectic in the neighborhood of SmCo7. An explanation for this discrepancy is still being sought” [34]. Thus, at high temperatures, the 1:7-based solid solution range in the quasi-ternary phase diagram is present in the form of an asymmetric biconvex (toward the decreasing temperature) lens adjacent to the high-temperature homogeneity ranges of the 2:17 and 2:7 phases in their wider ranges. It should be noted that the given sketches of the phase diagrams are related to certain relations of 3d elements, which correspond to the formula R1-XZrX(Co0.70Cu0.09Fe0.21)Z.

6. Conclusions

We present here an original view on the issues of the structural formation of the (R,Zr)(Co,Cu,Fe)Z alloys in the concentration ranges promising for the production of high-coercivity permanent magnets. These findings were based on a quantitative analysis of the microstructures of the first, most crude level of heterogeneity of these alloys in a highly coercive state. It was shown that:

1. The formula of the alloys R1-XZrX(Co,Cu,Fe)Z is physically most reasonable from the viewpoint of the effect of main elements with the qualitatively different electron shells (4f–R, 4d–Zr and 3d–Co,Cu,Fe) on the structure and properties of the studied alloys, and with allowance for the fact that the initial structural state is the matrix being the single-phase disordered 1:7H solid solution, and almost all intermediate and final phases (except the 2:17R phase) formed in the course of phase transformations corresponded better to the formula R1-XZrX(Co,Cu,Fe)Z. 2. The microstructure of the R1-XZrX(Co,Cu,Fe)Z alloys in optical resolutions was formed by three structural components based on the 1:5 (A), 2:17 (B), and 2:7 (C) phases; the qualitative relationships A:B:C varied within wide ranges as the chemical composition changed monotonously.

3. For the studied composition ranges of the Sm1-XZrX(Co,Cu,Fe)Z alloys of all experimental series, the increase in the relative content of 3d elements (z) was accompanied by monotonic changing of the volumes of structural components A and B, corresponding to 90% of the total alloy volume from the dominant volume of structural component A to the dominant volume of structural component B.

4. The hysteretic properties of the Sm1-XZrX(Co,Cu,Fe)Z alloys in the high-coercivity state were strictly controlled by the volume percentage ratio of the structural components A and B. 5. The dominant volume of the structural component A in the alloy ensured the ultimate squareness of the hysteresis loop of the samples; the equality of the volumes VA = VB corresponded to the combination of the high coercive force (HCJ) and ultimate squareness of the hysteresis loop; the predominance of the volume of component B was accompanied by the decrease in the squareness of the hysteresis loop.

6. The increase in the relative Zr content (x) in the Sm1-XZrX(Co,Cu,Fe)Z alloys in the high-coercivity state was accompanied by the decrease in the precision of the dependences of the coercive force on z and the shift of alloy compositions with the equal volumes VA = VB toward the lower relative content of 3d-elements (z). 7. At temperatures corresponding to the supersaturated solid solution treatment, the section of the ternary diagram of the Co–Sm–Zr system undergoes a significant transformation upon introduction of Cu and Fe into the alloy and the transition to the (Co,Cu,Fe)–Sm–Zr quasi-ternary section. 8. In the quasi-ternary phase diagram of (Co,Cu,Fe)–Sm–Zr, at SSHT temperatures, the region of the 1:7-phase-based solid solution had the form of an asymmetric lens, biconvex toward lower Materials 2020, 13, 3893 17 of 18

temperatures; the edges of the lens were adjacent to the homogeneity ranges of the 2:17 and 2:7 phases in their widest, high-temperature regions.

In future studies, we will consider the peculiarities of magnetization reversal and the composition of the structural components of the (Sm,Zr)(Co,Cu,Fe)Z alloys in high-coercivity state for composition ranges that are of importance in the manufacture of permanent magnets.

Author Contributions: Conceptualization, A.G.D.; Methodology, A.G.D.; Formal analysis, N.B.K.; Investigation, N.A.D. and Y.V.M.; Writing—original draft preparation, A.G.D.; Writing—review and editing, N.B.K.; Visualization, N.A.D. and Y.V.M. All authors have read and agreed to the published version of the manuscript. Funding: This study was carried out within the project “Development of the model of formation of high-coercivity phase and structural states of (R,Zr) (Co,Cu,Fe)z alloys and their synthesis for the production anisotropic bonded powder permanent magnets applied in electric machines”, Project no. 20-19-00689 funded by the Russian Science Foundation. Acknowledgments: We sincerely thank Nikolay P. Suponev, (Tver State University) for the attention and provision of data used in the present study. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Ojima, T.; Tomizawa, S.; Yoneyama, T.; Hori, T. Magnetic properties of a new type of rare-earth cobalt magnets. IEEE Trans. Magn. 1977, 13, 1317–1319. [CrossRef] 2. Strnat, K.J. Rare Earth Cobalt Permanent Magnets. Handb. Ferromag. Mater. 1988, 4, 131–209. [CrossRef] 3. Goll, D.; Kronmüller, H.; Stadelmaier, H.H. Micromagnetism and the microstructure of high-temperature permanent magnets. J. Appl. Phys. 2004, 96, 6534–6545. [CrossRef]

4. Stadelmaier, H.H.; Goll, D.; Kronmüller, H. Permanent magnet alloys based on Sm2Co17; phase evolution in the quinary system Sm-Zr-Fe-Co-Cu. Z. Metallkd. 2005, 96, 17–23. [CrossRef] 5. Gopalan, R.; Ohkubo, T.; Hono, K. Identification of the cell boundary phase in the isothermally aged

commercial Sm(Co0.725Fe0.1Cu0.12Zr0.04)7.4 sintered magnet. Scripta Mater. 2006, 54, 1345–1349. [CrossRef] 6. Song, K.; Sun, W.; Chen, H.; Yu, N.; Fang, Y.; Zhu, M.; Li, W. Revealing on metallurgical behavior of iron-rich

Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 sintered magnets. AIP Adv. 2017, 7, 056238. [CrossRef] 7. Popov, A.G.; Golovnia, O.A.; Protasov, A.V.; Gaviko, V.S.; Gopalan, R.; Jiang, C.; Zhang, T. Peculiar kinetics

of coercivity of sintered Sm(Co0.78Fe0.10Cu0.10Zr0.02)7 magnet upon slow cooling. IEEE Trans. Magn. 2018, 54, 2100907. [CrossRef] 8. Xiong, X.Y.; Ohkubo, T.; Koyama, T.; Ohashi, K.; Tawara, Y.; Hono, K. The microstructure of sintered

Sm(Co0.72Fe0.20Cu0.055Zr0.025)7.5 permanent magnet studied by atom probe. Acta Mater. 2004, 52, 737–748. [CrossRef] 9. Ray, A.E. Metallurgical behavior of Sm(Co,Fe,Cu,Zr)z alloys. J. Appl. Phys. 1984, 55, 2094–2096. [CrossRef] 10. Ray, A.E. A revised model for the metallurgical behavior of 2:17-type permanent magnet alloys. J. Appl. Phys. 1990, 67, 4972–4974. [CrossRef] 11. Lyakhova, M.B.; Pushkar’, Y.E.; Shamorikova, E.B.; Babushkin, Y.V. Magnetic Properties, Phase Composition, and Domain Structure of Gd-Zr-Co-Cu-Fe Alloys. In Physics of Magnetic Materials; Kalinin’s State University: Kalinin, USSR, 1985; pp. 90–106. 12. Pushkar’, Y.E.; Lyakhova, M.B.; Guseva, N.N. Effect of d-Element Concentrations on the Microstructure and Magnetic Properties of Gd-Zr-Co-Cu-Fe Alloys. In Physics of Magnetic Materials; Kalinin’s State University: Kalinin, USSR, 1986; pp. 75–81. 13. Okamoto, H. Comment on Co-Sm (Cobalt-Samarium). J. Phase Equilib. 1995, 16, 367–368. [CrossRef]

14. Lefevre, A.; Cohen-Adad, M.T.; Mentzen, B.F. Structural effect of Zr substitution in the Sm2Co17 phase. J. Alloys Comp. 1997, 256, 207–212. [CrossRef] 15. Zinkevich, M.; Mattern, N.; Bacher, I. Experimental and thermodynamic assessment of the Fe–Gd–Zr system. Z. Metallkd. 2002, 93, 186–198. [CrossRef] 16. Mattern, N.; Zinkevich, M.; Handstein, A.; Gruner, W.; Roth, S.; Gutfleisch, O. Influence of Zr addition on

phase formation and magnetic properties of the Fe17Gd2 phase. J. Alloys Comp. 2003, 358, 1–6. [CrossRef] Materials 2020, 13, 3893 18 of 18

17. Kianvash, A.; Harris, I.R. Metallographic studies of a 2-17-Type Sm(Co,Cu,Fe,Zr)8.92 magnetic alloy. J. Less-Common Met. 1984, 98, 93–108. [CrossRef]

18. Delannay, F.; Derkaoui, S.; Allibert, C.H. The influence of zirconium on Sm(Co,Fe,Cu,Zr)7.2 alloys for permanent magnets. I: Identification of the phases by transmission electron microscopy. J. Less-Common Met. 1987, 134, 249–262. [CrossRef]

19. Derkaoui, S.; Allibert, C.H.; Delannay, F.; Laforest, J. The Influence of Zirconium on Sm(Co,Fe,Cu,Zr)7.2 Alloys for Permanent Magnets. II: Composition and lattice Constants of the Phases in Heat-Treated Materials. J. Less-Common Met. 1987, 136, 75–86. [CrossRef] 20. Suponev, N.P.; Shamorikova, E.B.; Dormidontov, A.G.; Titov, Y.V.; Lukin, A.A.; Levandovski, V.V. Structure and Magnetic Properties of Sm-Zr-Co-Cu-Fe Alloys in tхhe High-Coercivity State. 1. Structural Components and Magnetization Reversal Processes. In Physics of Magnetic Materials; Kalinin’s State University: Kalinin, USSR, 1988; pp. 93–106. 21. Gopalan, R.; Muraleedharan, K.; Sastry, T.S.R.K.; Singh, A.K.; Joshi, V.; Sridhara Rao, D.V.; Chandrasekaran, V.

Studies on structural transformation and magnetic properties in Sm2Co17 type alloys. J. Mater. Sci. 2001, 36, 4117–4123. [CrossRef] 22. Morita, Y.; Umeda, T.; Kimura, Y. Phase transformation at high temperatures and coercivity of Sm-Co-Cu-Fe magnet alloys. J. Japan Inst. Meter. 1986, 50, 235–241.x. [CrossRef] 23. Morita, Y.; Umeda, T.; Kimura, Y. Phase transformation at high temperature and coercivity of

Sm(Co,Cu,Fe,Zr)7-9 magnet alloys. IEEE Trans. Magn. 1987, 23, 2702–2704. [CrossRef] 24. Morita, Y.; Umeda, T.; Kimura, Y. Effects of Zr content on phase transformation and magnetic properties of

Sm(Co,Cu,Fe,Zr)7-9 magnet alloys. J. Japan Inst. Meter. 1988, 52, 243–250. [CrossRef] 25. Nishio, T.; Fukui, Y.; Iwama, Y. Effect of zirconium on stabilization for the 1-7 phase in 2-17 type Sm-Co permanent magnet. J. Japan Inst. Meter. 1988, 52, 502–507. [CrossRef] 26. Derkaoui, S.; Valignat, N.; Allibert, C.H. Co corner of the system Sm-Co-Zr: Decomposition of the phase 1:7

and equilibria at 850 ◦C. J. Alloys Comp. 1996, 235, 112–119. [CrossRef] 27. Derkaoui, S.; Valignat, N.; Allibert, C.H. Phase equilibria at 1150 ◦C in the Co-rich alloys Sm-Co-Zr and structure of the 1:7 phase. J. Alloys Comp. 1996, 232, 296–301. [CrossRef] 28. Lefevre, A.; Cataldo, L.; Cohen-Adad, M.T.; Allibert, C.H.; Valignat, N. The ternary system Sm-Co-Zr isoplethic section at Co = 89 at%. J. Alloys Comp. 1996, 241, 210–215. [CrossRef] 29. Lefevre, A.; Cataldo, L.; Cohen-Adad, M.T.; Mentzen, B.F. The ternary system Sm-Co-Zr isoplethic sections at Co=87 and 85 at%. J. Alloys Comp. 1997, 255, 161–168. [CrossRef] 30. Lefevre, A.; Cataldo, L.; Cohen-Adad, M.T.; Mentzen, B.F. A representation of the Sm-Co-Zr-Cu-Fe quinary system: A tool for optimisation of 2/17 permanent magnets. J. Alloys Comp. 1997, 262–263, 129–133. [CrossRef] 31. Lefevre, A.; Cataldo, L.; Cohen-Adad, M.T.; Mentzen, B.F. Optimisation of 2/17 permanent magnets using the quinary Sm–Co–Cu–Fe–Zr phase diagram. J. Alloys Comp. 1998, 275–277, 556–559. [CrossRef]

32. Khan, Y. The crystal structures of R2Co17 intermetallic compounds. Acta Cryst. 1973, B29, 2502–2507. [CrossRef] 33. Okamoto, H.; Massalski, T.B. Binary Alloy Phase Diagrams, 2nd ed.; Massalski, T.B., Ed.; ASM International: Metals Park, OH, USA, 1990. 34. Khan, Y. A Contribution to the Sm-Co phase diagram. Acta Cryst. 1974, B30, 861–864. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).