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Crystallization sequences and magnetic properties of melt-spun Nd2Fe14B-based nanocomposites containing Co and Cr
Journal of Alloys and Compounds 270 (1998) 265–274
L
Crystallization sequences and magnetic properties of melt-spun Nd 2 Fe 14 Bbased nanocomposites containing Co and Cr a, 1 ,a 2 ,a b L.H. Lewis *, K. Gallagher , B. Hoerman , V. Panchanathan a
Materials Science Division, Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973 -5000, USA b Magnequench International, Inc. ( MQI), 6435 Scatterfield Road, Anderson, IN 46013, USA Received 16 February 1998; received in revised form 18 March 1998
Abstract The physical, chemical and microstructural characteristics of the two-phase nanocrystalline alloy Nd 2 [Co 0.06 (Fe 12x Crx ) 0.94 ] 23.2 B 1.48 (0#x,0.9) made by melt-spinning methods were investigated to ascertain their effects on room-temperature magnetic properties, with the ultimate goal of understanding, controlling and optimizing the alloys’ coercivity and exchange-enhanced remanence. Detailed investigations were made into the phase content, character and grain size of the alloys using synchrotron X-ray diffraction. The technical magnetic properties were studied with SQUID and VSM magnetometry. Differential thermal analysis was performed to monitor the crystallization sequences of the as-quenched alloys as a function of composition, as well as to identify the Curie temperatures of annealed alloys. Most annealed samples consist of a phase isostructural with Nd 2 Fe 14 B (denoted Nd 2 M 14 B or 2-14-1) and a bcc Fe-rich phase (denoted bcc-(FeCoCr)). The replacement of Fe by Cr (up to Cr content x|0.03) causes an initial increase in the lattice parameters of the constituent phases found in the annealed samples. Further Cr additions cause a sharp drop in the lattice parameters. The calculated average grain sizes in the annealed samples exhibit an abrupt increase for larger chromium contents, and the grain size of the Nd 2 M 14 B phase is larger than that of the bcc-(FeCoCr) phase for all samples studied. The Curie temperatures of the 2-14-1- and bcc Fe-type phases are found to decrease with increasing Cr content. Thermal analysis and laboratory X-ray diffraction experiments performed on selected annealed samples suggest that the enrichment of Fe in the nanocomposite alloy allows the system to access the metastable non-stoichiometric Nd 2 M 17 -type phase prior to transformation to the 2-14-1 phase. Increased Cr content in the starting alloy stabilizes the amorphous→Nd 2 M 17 -type (metastable) phase transition and the formation of the Nd 2 M 14 B phase, shifting them to higher temperatures. The transformation behavior of the alloys as a function of Cr content ultimately controls the phase constitution, phase chemistry and the grain size, which in turn control the room-temperature hysteretic magnetic properties. 1998 Elsevier Science S.A. Keywords: Intermetallic compounds; Magnetic properties; Metastable phases; Nanocomposite; Rapid solidification
1. Introduction Nanocomposite exchange-coupled magnetic materials are anticipated to comprise the next generation of permanent magnet materials [1]. Ideally synthesized from intimately-mixed grains of a magnetically-hard phase and a magnetically-soft phase, the overall microstructure should possess a grain size on the order of twice the domain wall width in the hard phase for optimum exchange coupling, to yield optimum technical magnetic properties [2]. To date, *Corresponding author. Tel.: 11 516 344 2861; fax: 11 516 344 4071; e-mail: [email protected] 1 Present address: Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA. 2 Present address: Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, USA. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00529-5
nanocomposite permanent magnet materials with enhanced remanence ratios have not yet exhibited the outstanding performance predicted by theoretical models of these materials [3,4]; specifically, the remanence enhancement Br /Ms and the coercivity Hci are often not as large as predicted. In nanocomposite materials based on a-Fe or Fex B (x52, 3) and (RE) 2 Fe 14 B (2-14-1), where RE is a rare-earth element such as Nd or Pr, these observations are true regardless of the method of fabrication (ball milling or rapid solidification) [5,6] or elemental additives [7]. It is true that certain elemental additions, or combinations of elemental additions, do improve the room-temperature magnetic hysteretic properties. For example Hirosawa et al. [8] have found remanences up to 1.2 T and coercivities up to 8600 Oe in different samples with compositions near Nd 5.5 Fe 66 B 18.5 Cr 5 Co 5 . Jurczyk et al. [9] found that the addition of Zr to 2-14-1 / a-Fe nanocomposites produced
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by high-energy ball milling increases the final coercivity over that of the Zr-free composition, but the reasons underlying the phenomenon are not clear. In general, such alloying additions further complicate an already extremely complex, nanoscale, non-equilibrium system, obscuring the underlying reasons for the property enhancement. The remanence enhancement and coercivity are dependent upon the degree of interphase exchange coupling present within the nanocomposite system. The remanence enhancement is itself a function of the magnetic properties of the phases present in the nanostructure as well as the grain sizes and homogeneity of these phases in relationship to the domain wall width in the system. Alloying additions to a rapidly-solidified system may have any or all of the following effects: promotion or suppression of metastable transitional phases and stoichiometries, stabilization of either a glassy or a crystalline phase (which in turn may produce disparate grain sizes and shapes), inhibition of grain growth and the production of alignment of the constituent grains [10]. In order to assess the efficacy of any particular combination of elemental additions in producing optimal magnetic properties in nanocomposite alloys, it is necessary to first identify the effects it has upon the final nanostructure. To this end, detailed materials characterization studies of the low-boron melt-spun nanocomposite alloy Nd 2 [Co 0.06 (Fe 12x Cr x ) 0.94 ] 23.2 B 1.48 (0#x,0.9) were performed. The phase constitution and character of the annealed samples were assessed with X-ray diffraction techniques. The crystallization sequence experienced by the as-quenched samples during the standard annealing treatment was followed by differential thermal analysis (DTA). The Curie temperatures of the constituent phases in the annealed specimens were determined to obtain information concerning the partitioning of the transition metal species in each phase as a function of Cr content. Such information is not easily obtainable by X-ray scattering methods, due to the similarity of the electronic structure of the transition metal species. It is found that subtle changes in the nominal starting chemistry
of the alloy may produce significant changes in the stability of the metastable phases present in the system, which in turn determine the magnetic properties of the annealed nanocomposite.
2. Experimental procedures The samples were made by standard melt-quenching techniques from commercial-grade alloys. The compositions are obtained from the starting alloy compositions, listed in Table 1. They are expressed in both wt% and stoichiometry, to illustrate the amount of transition-metal enrichment over that of the stoichiometric Nd 2 M 14 B base alloy, where M denotes a mixture of the transition-metal species Fe, Co and Cr. The samples are identified by the atomic percentage of chromium replacing iron in the nominal total transition-metal content; i.e. x in Nd 2 (Co 0.06 (Fe 12x Cr x ) 0.94 ) 23 B 1.5 (average composition). For comparative purposes, some chromium- and cobaltfree compositions were also studied; their compositions are also included in Table 1. Melt-spun ribbons were overquenched from the melt at a wheel speed of 22 m s 21 . A portion of each sample was left in its as-quenched state, while a separate portion was vacuum annealed at 6908C for 4 min to obtain optimal technical magnetic properties. The samples were characterized by a variety of techniques. X-ray diffraction was performed in the transmission mode at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, using X-ray ˚ ˚ The wavelength was wavelengths 0.90 A#l#1.18 A. chosen so as to avoid the excitation of Fe fluorescence. Such a technique not only unambiguously distinguishes an amorphous phase content from fluorescence, but also results in very good Bragg peak resolution that improves the detection of minor phases otherwise obscured by fluorescence. Additional laboratory reflection X-ray diffraction was performed as needed using standard Cu Ka radiation. Crystallite grain sizes were determined from the
Table 1 Sample identification x denoting Cr content in Nd 2 (Co 0.6 (Fe 12x Cr x ) 94 ) 23 B 1.5 (approximate composition) and sample compositions in both stoichiometry and weight percentage Sample ID/ comment
x50 x50.007 x50.017 x50.029 x50.048 x50.060 x50.068 x50.087 Cobalt-free alloy Cobalt- and chromium-free alloy Commercial MQ-A powder
Composition Stoichiometry
(wt%)
(Nd) 2 (Co 0.062 Fe 0.938 ) 22.70 B 1.46 (Nd) 2 (Co 0.060 Fe 0.934 Cr 0.007 ) 22.70 B 1.46 (Nd) 2 (Co 0.061 Fe 0.922 Cr 0.017 ) 23.34 B 1.55 (Nd) 2 (Co 0.061 Fe 0.910 Cr 0.029 ) 23.06 B 1.44 (Nd) 2 (Co 0.061 Fe 0.891 Cr 0.048 ) 23.09 B 1.46 (Nd) 2 (Co 0.062 Fe 0.878 Cr 0.060 ) 23.26 B 1.48 (Nd) 2 (Co 0.075 Fe 0.856 Cr 0.068 ) 22.19 B 1.44 (Nd) 2 (Co 0.064 Fe 0.849 Cr 0.087 ) 21.94 B 1.44 (Nd) 2 (Cr 0.032 Fe 0.968 ) 23.14 B 1.47 (Nd) 2 Fe 23.25 B 1.47 Nd 2.39 Fe 14 B 0.95
(Nd) 18.3 Fe 76.4 Co 5.3 B 1.00 (Nd) 17.7 Fe 75.7 Co 5.1 Cr 0.5 B 1.00 (Nd) 17.9 Fe 74.6 Co 5.2 Cr 1.3 B 1.04 (Nd) 18.1 Fe 73.5 Co 5.2 Cr 2.2 B 0.98 (Nd) 18.1 Fe 72.1 Co 5.2 Cr 3.6 B 0.99 (Nd) 18.1 Fe 71.2 Co 5.3 Cr 4.5 B 1.00 (Nd) 18.7 Fe 68.6 Co 6.4 Cr 5.1 B 1.01 (Nd) 18.9 Fe 68.6 Co 5.4 Cr 6.5 B 1.02 (Nd) 18.1 Fe 78.5 Cr 2.4 B 1.00 (Nd) 18.0 Fe 81.01 B 0.99 (Nd) 30.5 Fe 68.6 B 0.90
L.H. Lewis et al. / Journal of Alloys and Compounds 270 (1998) 265 – 274
half-width of the Bragg peak of the NSLS XRD data at the half-maximum intensity position, corrected for the intrinsic broadening of the synchrotron beam, using the Scherrer formula [11]. Lattice parameters were obtained using a least-squares-fit algorithm. Room-temperature magnetic measurements were performed using a vibrating sample magnetometer (VSM) and a SQUID magnetometer. The hysteretic properties remanence Br , intrinsic coercivity Hci and energy product (BH ) max were measured on samples that were premagnetized at 4 T. Room-temperature demagnetization curves were obtained using a maximum applied field of 5 T on isotropic cylinders formed from packed powder set in epoxy. The data were corrected for demagnetization effects. In order to calculate the remanence ratios Br /Ms , the saturation magnetization Ms was obtained by fitting the high-field portion (Hint $20 000 G) of the demagnetization curve to the expression 2
M(H ) 5 Ms (H 5 `)(1 2 (a /H ) 2 (b /H ))
(1)
where H is the internal field, and a and b are arbitrary constants. To monitor the crystallization sequence of the asquenched alloys as well as to monitor the Curie temperatures (T c ) of the various magnetic phases present in the annealed alloys, differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed using a TA Instruments Model SDT 2960 Simultaneous DTA / TGA. Measurements were made using a ramp rate of 208 min 21 in the typical temperature range 375 K#T # 900 K in an atmosphere of high-purity Ar with 1% hydrogen. The 0.2% average weight gain of the samples during the course of the approximately 4 h measurement indicates that there was no significant uptake of hydrogen and / or oxygen. The hydrogen acts as an oxygen getter; under the same conditions, the use of pure Ar as a purging gas results in a weight gain of over five times that obtained with 1% hydrogen. To monitor the crystallization sequence, at least two separate, approximately 100 mg, quantities of sample were measured. Fig. 1 depicts a typical crystallographic transformation sequence for an unannealed alloy, where the exothermic transformations are marked by three peaks in the curve. In some instances the transformation curves overlapped one another, making the determination of peak temperatures difficult. In such cases a multi-peak Gaussian curve was used to fit the data. The Curie temperature transition produces a change in the slope of the DTA curve with both heating and cooling. This transition is illustrated for an annealed sample by the dashed line in Fig. 2. The Curie temperatures were determined as the minimum in the temperature derivative of the DTA cooling curve, the solid line of Fig. 2. The T c transitions were determined separately using two new 100 mg quantities of sample; each sample was cycled up and down in temperature a minimum of two times. The
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Fig. 1. DTA data collected from an as-quenched sample, typical of those measured in this study. Three exothermic peaks are noted.
error in T c determined by the thermal method is taken as the standard deviation of the measurements, and is usually less than 18. In some cases portions of selected asquenched alloys were annealed at intermediate temperatures for short times in the DTA, in preparation for laboratory X-ray diffraction. This procedure was done so as to insure minimal oxidation of the sample.
3. Results
3.1. Phase constitution and characterization of annealed samples The lattice parameters and related properties of the samples show complex trends with composition, as might be expected for composite specimens of complicated composition produced in a non-equilibrium manner. For all structural parameters measured – lattice parameters and average grain sizes – there appears to be a discontinuity in,
Fig. 2. DTA data collected from an annealed sample (- - -); the derivative of the DTA cooling curve (———) emphasizes the change in the signal produced by the Curie transition.
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Fig. 3. A synchrotron X-ray diffraction pattern typical of those collected in this study. The Nd 2 Fe 14 B reference Bragg peaks are marked by vertical dashed lines, whereas the bcc-phase Bragg reflections are marked by arrows.
or anomalous value of, the measured parameter in the intermediate compositional range. Fig. 3 shows a typical synchrotron transmission X-ray diffraction scan obtained in this study; the Nd 2 Fe 14 B phase and the (FeCoCr) phase Bragg reflections are identified in the figure. All annealed samples, except for that with the highest chromium concentration (x50.087), were found to consist only of a Nd 2 Fe 14 B-type phase and a bcc-(FeCoCr) phase. It is estimated that synchrotron X-ray diffraction would detect minor phases of 0.1–1.0 wt% [12]. The most Cr-rich composition studied (x50.087) consists of three phases: a non-stoichiometric metal-rich Nd 2 (FeCoCr) 17 type phase with the approximate composition RE 2 M 19.2 [13], the bcc-(FeCrCo) phase and a small amount of the 2-14-1 phase. Figs. 4 and 5 illustrate the variation of lattice parameters and the average grain sizes of the phases with compositional change. The Nd 2 Fe 14 B-type phase displays an overall increasing lattice parameter trend with composition (Fig. 4a) up to the middle of the compositional range studied. For greater Cr concentrations there is a precipitous drop in the lattice parameters. (The data points for 2-14-1 lattice parameters of the [Cr]50.017 sample appear to be anomalous.) This general trend with increasing Cr content is mirrored in the bcc-(FeCoCr) phase lattice parameters and the constituent grain sizes. The lattice parameters of the bcc phase (Fig. 4b) show the a-parameter increasing up to a chromium content of x5 0.029 and then discontinuously dropping to a more constant value for x$0.048. The average grain size determinations of both the 2-14-1 phase and the bcc Fe phase are shown in Fig. 5, with lines drawn to guide the eye. It can be seen that the average grain size of the Nd 2 Fe 14 B phase is larger than that of the bcc Fe phase for all compositions studied. As was found for the lattice parameters, a discontinuity in the average grain size of both phases occurs in the vicinity of the composition x$0.048.
Fig. 4. (a) Variation with Cr content x of the measured Nd 2 M 14 B lattice parameters in the annealed samples. (b) Variation with Cr content x of the measured lattice parameter of the bcc-(FeCoCr) phase in the annealed samples.
3.2. Magnetic properties 3.2.1. Technical magnetic properties of the annealed samples The room-temperature technical magnetic properties of the annealed samples (Br , Hci , Ms and (BH ) max ) and the remanence ratios Br /Ms are illustrated in Fig. 6a–e. In general, the addition of Cr to the alloy causes both the saturation magnetization Ms and the remanence Br to decrease. The saturation magnetization reaches a high value of 1.84 T at x50.007, while the remanence varies
L.H. Lewis et al. / Journal of Alloys and Compounds 270 (1998) 265 – 274
269
by the same method for other sample compositions made by melt-spinning and subsequent annealing. These samples have the following nominal compositions: single transition-metal Nd 2 Fe 23.25 B 1.47 , Cr-free Nd 2 (Fe 0.938 Co 0.062 ) and Co-free Nd 2 (Fe 0.968 Cr 0.032 ). The measured Curie temperatures of the phases present in these samples are shown in Table 2.
3.3. Crystallographic transformation sequences
Fig. 5. Variation of average grain size with Cr content of the annealed samples.
from a high value of 0.99 T at x50 to a low value of approximately 0.85 T at x50.087. The decrease in the remanence is significant for x$0.029. The coercivity Hci , on the other hand, does not show a monotonic trend with increasing chromium concentration, but always remains below 4.1 kOe. The variation of the energy products (BH ) max with chromium concentration mimics that experienced by remanence Br , exhibiting values near 12 MGOe for low-Cr-content samples. All samples are modestly remanence-enhanced, with a peak in the remanence enhancement of 0.59 for the x50.017 specimen (Fig. 6e). A remanence ratio value in isotropic alloys equal to 0.5 implies that no remanence enhancement is present [14].
3.2.2. Curie temperatures of the phases in the annealed samples Fig. 7a shows the Curie temperatures as a function of composition for the Nd 2 (FeCoCr) 14 B phase, whereas Fig. 7b shows determinations of the same parameter for the bcc Fe-rich phase. In both figures, the line drawn between points is to guide the eye. The data for the two crystallographic phases exhibit similar trends: an increased Cr content causes the Curie temperatures to decrease, with the Curie temperature of the 2-14-1 phase falling from a high of 3728C at x50 down to 3488C at x50.087. This change is approximately twice that of the analogous drop in the bcc phase, which falls from a high value of 8218C to 7938C in the same compositional range. Comparisons with the published Curie-temperature dependence of bcc-Fe on cobalt content [15] indicate that there is between 4 and 5 at% Co in the bcc Fe-rich phase of the Cr-free (x50) sample. However, the calculated unit cell volume for the bcc phase at that composition is approximately 1% larger than that for the standard reference JCPDS bcc Fe lattice parameter [16]. These data suggest the presence of retained Nd, as will be discussed further in Section 4.1. For comparative purposes, Curie temperatures were determined
The exothermic peak temperatures measured by DTA in as-quenched samples as a function of Cr content [x] are shown in Fig. 8. The dashed lines are drawn between data points to guide the eye. The standard annealing temperature used to heat-treat the quenched alloys in this study (6908C) is marked by a horizontal line in Fig. 8. All DTA peaks are broad, commonly spanning a temperature range of 808; this feature implies that the phase transformations under consideration take place over a considerable temperature range. The low-temperature and high-temperature peaks denote crystallographic transformation temperatures that increase with increasing chromium content, while the intermediate temperature transformation is independent of composition to a large degree. The hypothesized transformations, based on results described below, are included in Fig. 8. Selected samples were subjected to intermediate heat treatments and subsequently investigated with standard laboratory Cu Ka X-ray diffraction to identify the metastable transition phase. It is concluded that the metastable transition phase present at intermediate temperatures is the non-stoichiometric Nd 2 (FeCoCr) 17 -type phase with the approximate metal-rich stoichiometry Co 16 Fe 35.6 Y 5.38 , which may be roughly written as RE 2 M 19.2 [13]. No evidence of other complex intermetallic phases such as the cubic Nd 2 Fe 23 B 3 , the hexagonal NdFe 12 B 6 and the cubic Y 3 Fe 62 B 14 -type phases was found, as has been by other authors in samples of similar composition [8,17]. It appears that a poorly crystallized precursor to the metalrich 2-17-type phase is present even in the quenched alloys, in addition to very fine grained bcc-(FeCoCr). Fig. 9 illustrates X-ray diffraction data of sample x50.048, typifying the overall crystallographic transformation sequence. It is apparent that a 1 min anneal of this sample at T55858C produces a reduction in the glassy component of the alloy, to form more of the Fe-rich bcc phase and the non-stoichiometric Fe-rich Nd 2 (FeCoCr) 17 -type phase. A subsequent 5 min anneal at 6508C of the same alloy shows a significant reduction in the 2-17-type phase content of the alloy. Higher-temperature anneals produce the Nd 2 M 14 B phase and presumably more bcc Fe as decomposition products of the 2-17-type phase. It is to be noted that as the bcc-type phase is present at all stages of the crystallographic transformation, it is postulated to undergo a continuous nucleation and growth process throughout the annealing sequence.
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Fig. 6. Room-temperature magnetic properties of the annealed alloys. (a) Saturation magnetization Ms ; (b) remanence Br ; (c) coercivity Hci ; (d) energy product (BH ) max ; (e) remanence ratio Br /Ms (estimated using extrapolation to H5`, as described in the text).
4. Discussion of results It is apparent that the room-temperature technical magnetic behavior of the magnetic compounds in this study results from a very complex interplay of atomic structure, microstructural and chemical details. The chemical details of the constituent phases within the samples are illuminated by both the lattice parameter trends and the Curie temperature evolution of the phases with respect to chromium content. The extrinsic microstructural factors, characterized by X-ray diffraction and by the crystallization sequences identified by DTA, clarify the trends
underlying the structure-sensitive technical magnetic properties.
4.1. Compositional character of the annealed samples Elemental segregation and diffusion occur in the quenched alloys as the crystallization process proceeds under the influence of the standard annealing treatment. All three transition metals, as well as the Nd and the B present in the alloys, simultaneously diffuse through the system and affect the magnetic and crystallographic properties of the phases present.
L.H. Lewis et al. / Journal of Alloys and Compounds 270 (1998) 265 – 274
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Fig. 8. Trends of the DTA exothermic peak temperatures with increasing Cr content x. The standard annealing temperature of T ann 56908C is marked by a double-headed arrow for reference.
Fig. 7. Curie temperatures measured by DTA: (a) Nd 2 M 14 B; (b) bcc(FeCoCr).
An analysis of information from the lattice parameter data (Fig. 4b), the Curie temperature data (Fig. 7b), and Table 2 leads to the conclusion that the bcc Fe-rich phase in the Cr-free (x50) alloy contains a small amount of Nd, in addition to Fe and Co. This conclusion results from the following observations. The lattice parameter of this bcc phase is larger than that of pure a-Fe [16], outside of experimental uncertainty, by 0.3%, and the magnetic transition temperature is approximately 508 higher than that of pure a-Fe. Elevated Curie temperatures in the bcc phase relative to that of pure a-Fe can only be produced by the presence of cobalt: the substitution of Co for Fe in the bcc lattice increases the Curie temperature at a rate of
Fig. 9. (Laboratory) X-ray diffraction data illustrating the transformation sequence of the metastable phases found in the x50.048 sample. The figure is described more fully in the text.
approximately 2.88 per at% Co up to 15 at% Co [15]. However, the substitution of cobalt for Fe in the bcc lattice has little effect on the lattice parameter up to approximately 20 at% Co [18]; for Co concentrations greater than 20 at% the lattice parameter of a-Fe decreases rapidly. It is
Table 2 Lattice parameters and measured Curie temperatures of nanocomposite samples containing only one or two transition-metal species Sample
Nd 2 (Co 0.062 Fe 0.938 ) 22.70 B 1.46 Nd 2 (Cr 0.032 Fe 0.968 ) 23.14 B 1.47 Nd 2 Fe 23.25 B 1.47 a
Nd 2 M 14 B phase
bcc Fe-rich phase
˚ a-parameter (A)
˚ c-parameter (A)
Curie temperature (8C)
˚ a-parameter (A)
Curie temperature (8C)
8.803860.0023 8.813060.0027 8.80560.002
12.205460.0037 12.226260.0044 12.21460.005
372.260.8 300.763.5 313.160.35
2.87560.004 2.87860.003 2.88 a
821 773 767
Uncertainty in parameter unavailable because only one peak was measured.
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unlikely that boron is residing in the bcc phase: the introduction of boron into the a-Fe lattice both lowers the Curie temperature of the phase and produces a contraction in the lattice parameter [19]. Therefore, the most likely scenario is that the bcc Fe-rich phase in the x50 specimen contains Nd in addition to Co. The binary Fe–Nd phase diagram [20] reveals a single-phase solid solubility of up to approximately 3 at% Nd in a-Fe, in the temperature range 780 K#T #1536 K, the melting point of the pure phase. It is likely that the rapid solidification process results in retained Nd impurities that become trapped within the bcc lattice. A similar conclusion was reached by Sagawa et al. [21], who detected a bcc Fe-rich phase in melt-spun binary Fe–Nd alloys that contained approximately 14 at% Nd and possessed the lattice parameter ˚ While Sagawa et al. estimated the Curie a52.87 A. temperature of this phase to be in the range 400–450 K, it is likely that a smaller retained Nd content would produce a smaller reduction in Curie temperature from that of pure a-Fe. The measured increase in the bcc lattice parameter with increasing Cr content suggests that Cr enhances Nd segregation in the phase up to a critical Cr concentration of x|0.03. The Curie temperatures of the bcc Fe-rich phase in the alloys decrease slowly for 0$x.0.06, and then decrease rapidly down to the most Cr-rich specimen investigated in this study. Out of all the elemental components in the system, only Co is capable of raising the Curie temperature of the bcc Fe-rich phase beyond its pure a-Fe value of 7708C. The substitution of Cr for Fe in a-Fe causes a drop in the Curie temperature at a rate of approximately 48 per at% Cr in the compositional range 5 at%#[Cr]#18 at% [22]. As discussed above, the expanding bcc lattice parameter for x,0.048 strongly suggests that Nd is segregating to the bcc phase as the overall Cr content in the alloy increases in the compositional range 0#x#0.048. Such a segregation of both Cr and Nd to the bcc phase would likely cause a rapid decrease in the Curie temperature, which is not observed. Thus it is postulated that all three elements – Cr, Co and Nd – are segregating in various increasing amounts to the bcc phase, until the Cr concentration in the alloy exceeds a mole fraction equal to 0.07. The observed decrease in the Curie temperature of Nd 2 M 14 B with increasing Cr content phase signifies the occupancy of Cr in the 2-14-1 lattice. It is possible that Cr displaces Co in the 2-14-1 lattice; such behavior might be expected if both elements compete for the same site in the Nd 2 Fe 14 B lattice. As discussed by Herbst [23], both chromium and cobalt show a preference for the 8j 2 site when substituted for iron in the Nd 2 Fe 14 B structure, although studies on the site occupancy of cobalt-only substituted Nd 2 Fe 14 B have yielded conflicting conclusions. In the Nd 2 Fe 14 B lattice, the substitution of Co for Fe is known to raise the Curie temperature as well as lower the anisotropy, while the substitution of chromium is known to
lower the Curie temperature [24]. However, cobalt is still present in the 2-14-1 lattice at all compositions studied, as evidenced by the anomalously-high 2-14-1 Curie temperature values for the x50.060 and x50.068 samples (these two samples possess starting compositions that are richer in cobalt than those of the other samples, Table 1). Increasing amounts of Co and Cr substituted for Fe in the 2-14-1 lattice decrease both the saturation magnetization and the remanence [24], as confirmed by the data of Fig. 6.
4.2. Phase transformations in as-spun alloys In general, it is extremely difficult to suppress the formation of a primary properitectic bcc Fe-rich phase during rapid solidification from a melt with a composition close to Nd 2 Fe 14 B [25]. This fact is borne out in the synchrotron XRD data of selected as-quenched alloys in this study, in which extremely fine-grained crystalline a-Fe is clearly visible. There have been a few studies on the transformation sequences found in rapidly-solidified nanocomposite alloys with compositions analogous to those in this study [8,17,26–29]. In general, most authors identify two or three exothermic crystallographic transformations in nanocomposite 2-14-1 / a-Fe alloys: the lower-temperature transformations, in the range 570–5908C, are associated with the crystallization and growth of a-Fe and Fe-borides, while the highest-temperature transformations are invariably associated with the crystallization of the RE 2 M 14 B phase. Peaks that are intermediate in temperature have been connected to the formation of complex intermetallic phases, such as the cubic Nd 2 Fe 23 B 3 , the hexagonal NdFe 12 B 6 and the cubic Y 3 Fe 62 B 14 -type phases [8,17]. The fact that no metastable complex intermetallic-boride phases such as Nd 3 Fe 62 B 14 or Nd 2 Fe 23 B 3 are found in these alloys might be attributed to their relatively low boron content, as well as to the presence of Co and Cr. The presence of significantly more iron in the nanocomposite alloys relative to the single-phase 2-14-1 alloy allows the system to access an additional, lower free-energy Fe-rich Nd 2 M 17 -type phase prior to the formation of Nd 2 M 14 B. The nominally-stoichiometric melt-spun compound Nd 2 Fe 14 B exhibits a single exothermic peak at T5578.58C (Table 3) which marks the crystallization of the 2-14-1 phase from the largely-amorphous matrix. However, nanocomposite alloys containing two or more transition-metal species do not exhibit the 2-14-1 phase
Table 3 Exothermic peak temperatures as measured by DTA for Cr- and Co, Cr-free Nd 2 M 14 B-based samples Sample
Peak 1
Peak 2
Peak 3
Nd 2.39 Fe 14 B 0.95 Nd 2 Fe 23.3 B 1.47 Nd 2 (Fe 0.968 Cr 0.032 ) 23.14 B 1.47
578.58C 578.48C 583.58C
657.28C 661.28C
683.48C
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crystallization until significantly higher temperatures, in the vicinity of 6608C. The presence of either Co or Cr in the nanocomposite alloy stabilizes the non-stoichiometric Nd 2 M 17 -type phase against decomposition. This observation echoes that of Chang et al., who also find that the crystallization temperatures of their alloys increase with increasing chromium content [28]. It is noteworthy that the Co- and Cr-free nanocomposite composition Nd 2 Fe 23.3 B 1.45 exhibits only two exothermic peaks during crystallization (Table 3), instead of the expected three peaks. Standard laboratory X-ray diffraction studies performed on this sample reveal identical phase transformation behavior to that illustrated in Fig. 9 for the x50.048 composition, albeit occurring at the lower temperatures listed in Table 3. It is postulated that only two exothermic events are detected in this composition because the crystallization from the amorphous phase and the decomposition of the Nd 2 M 17 -type phase occur in the same temperature range. In general, the considerable broadness of the DTA exothermic peaks found in this study suggest that the phases formed occupy a range of chemical variability.
4.3. Crystallographic /structural considerations The discontinuities present in both the measured lattice parameters and the grain sizes near the composition x| 0.03 reflect chemical differences in the constituent phases that may be attributed to the phase transformation behavior of the system. The lower crystallization temperatures of compositions containing smaller amounts of chromium promote an increased nucleation rate, which results in a smaller grain size relative to those alloy compositions with higher Cr content. The increasing Cr content in the bcc phase appears to produce an increased affinity for Nd, up to a critical chromium concentration of x|0.03. The phases of compositions containing higher amounts of chromium require higher temperatures to form, which results in larger Nd 2 M 14 B and bcc-(FeCrCo) grain sizes that presumably cannot accommodate such a high concentration of Nd. For Cr concentrations .0.03 the bcc lattice parameters are still larger than that of pure a-Fe, so the bcc phase likely contains Nd in the entire composition range studied. Both the a- and c-lattice parameters of the RE 2 (FeCoCr) 14 B phase (excluding the anomalous data points at x50.29) exhibit an analogous trend to that found for the bcc-(FeCoCr) phase, in which the lattice parameters rise to their highest level at x|0.03 and then fall precipitously for larger chromium concentrations. Although Nd 2 M 14 B is considered to be a line compound [30], the combined effects of a nanoscale microstructure, nonequilibrium processing and low-level elemental variation on the defect character of Nd 2 M 14 B, specifically atomic site substitutions, vacancies and interstitial occupancies, has to date not been thoroughly investigated. Thus it is difficult to ascertain what atomic / crystallographic features
273
have produced the observed lattice parameter trends in Nd 2 M 14 B. It is postulated that subtle processing differences in the x50.02 specimen have resulted in a defect character different from that of the other samples and thereby produced anomalously low lattice parameters.
5. Relations to technical magnetic properties The results of the investigations into the Nd 2 [Co 0.06 (Fe 12x Cr x ) 0.94 ] 23.2 B 1.48 nanocomposite alloys reveal structure–property relationships pertinent to their performance as remanence-enhanced permanent magnets, and therein suggest processing improvements. The three main factors that impact the technical magnetic properties of the alloys in this study are phase constitution, phase chemistry, and constituent phase grain size. All three factors are interrelated. The nanocomposite phase constitution present after the annealing treatment is the main factor affecting the technical magnetic properties. Comparisons between the metastable phases identified in the present study and those in the literature underscore the importance of the starting chemistry, especially the boron content, in the determination of the final phases in the nanocomposite. Because the prototypical metastable phase identified in the alloys of this study, Nd 2 Fe 17 , not only has a Curie temperature that is just slightly above room temperature but also has basalplane anisotropy and is therefore magnetically soft [31], it is necessary to insure the complete decomposition of the metastable Nd 2 Fe 17 -type phase and subsequent complete formation of the Nd 2 M 14 B phase to obtain optimum results. As illustrated in Fig. 8, an increasing Cr content pushes the 2-14-1 formation temperature significantly above the standard annealing temperature. Indeed, the annealed sample with the greatest amount of Cr, (Nd) 2 (Co 0.064 Fe 0.849 Cr 0.087 ) 21.94 B 1.44 , contains the metalrich Nd 2 Fe 17 -type phase as the major component. This sample also possesses the poorest room-temperature magnetic properties of the compositions studied. In addition to influencing the kinetics of phase formation in the system, the presence of chromium reduces the magnetization of the constituent phases, contributing to the decline of the room-temperature magnetic properties with increasing Cr content. The detailed composition of the Nd 2 M 14 B and bcc-(FeCoCr) phases comprising the nanocomposite determines the Curie temperatures of the phases. Broadly speaking, the Curie temperatures determine the operating temperatures of the final nanocomposite, and thus higher Curie temperatures are preferred. Higher Curie temperatures also favorably affect the interphase exchange coupling and thereby influence the remanence enhancement. A reduction in the magnetic moment of the compound, such as that produced by an increase in the temperature or, equivalently, a decrease in the Curie temperature, would produce a decrease in the strength of
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intergrain exchange coupling. This phenomenon would be expected to produce a lower remanence enhancement but an increased coercivity [32]. The remanence enhancement, as well as the coercivity, is also strongly affected by the average grain size [3] of the nanocomposite, with greater values anticipated for smaller grain sizes. This principle is roughly followed by the data obtained in this study: samples with a smaller Cr content possess relatively smaller grain sizes (between 200 and ˚ and larger room-temperature remanence ratios and 300 A) coercivities. The sample with the largest grain size (composition of x50.68) does indeed exhibit the smallest remanence ratio. The higher formation temperature of the Nd 2 M 14 B phase containing relatively more chromium promotes larger grain sizes, due to accelerated growth kinetics at higher temperature. It appears that the phase transformation kinetics indirectly affect the chemistry of the system, resulting in an Nd-enriched bcc phase for lower values of Cr content, as discussed in Section 4.2. For lower chromium concentrations (x#0.03) the growth of the grains may be limited by the availability of Nd, contributing to the smaller microstructural scale of alloys with relatively less Cr. Improvement of the magnetic properties of the alloys containing higher amounts of chromium might be achieved by the addition of grain growth inhibitors.
Acknowledgements This research was performed under the auspices of the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences, under Contract No. DE-AC02-76CH00016. Research was carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Science and Division of Chemical Sciences.
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[5] W.F. Miao, P.G. McCormick, R. Street, Solid State Commun. 101(9) (1997) 687. [6] L. Withanawasam, A.S. Murphy, G.C. Hadjipanayis, R.F. Krause, J. Appl. Phys. 76(10) (1994) 7065. [7] S. Hirosawa, H. Kanekiyo, M. Uehara, J. Appl. Phys. 73(10) (1993) 6488. [8] S. Hirosawa, H. Kanekiyo, in: 13th International Workshop on Rare-Earth Magnets and Their Applications, Birmingham, UK, 1994, p. 87. [9] M. Jurczyk, S.J. Colloctt, J.B. Dunlop, P.B. Gwan, J. Phys. D: Appl. Phys. 29 (1996) 2284. [10] M.J. Kramer, C.P. Li, K.W. Dennis, R.W. McCallum, For presentation at the Spring 1998 TMS Symposium on Rapid Solidification. [11] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley, Reading, MA, 1978, p. 102. [12] D. Cox, personal communication. [13] R.S. Perkins, P. Fischer, Solid State Commun. 20 (1976) 1013. [14] E.C. Stoner, E.P. Wohlfarth, Philos. Trans. R. Soc. London A 240 (1948) 599. [15] W.G. Moffatt, The Handbook of Binary Phase Diagrams, Vol. 2, Genium, Schenectady, NY, 1986. [16] JCPDS powder file [6-696, JCPDS-ICDD copyright, 1991. [17] L. Withanawasam, I. Panagiotopoulos, G.C. Hadjipanayis, IEEE Trans. Magn. 32(5) (1996) 4422. [18] R.M. Bozorth, Ferromagnetism, IEEE Press, New York, 1978, p. 192. [19] R. Ray, R. Hasegawa, Solid State Commun. 27 (1978) 471. [20] W.G. Moffatt, The Handbook of Binary Phase Diagrams, Vol. 2, Genium, Schenectady, NY, 1986. [21] M. Sagawa, S. Hirosawa, H. Yamamoto, Y. Matsuura, S. Fujimura, H. Tokuhara, K. Hiraga, IEEE Trans. Magn. MAG-22(5) (1986) 910. [22] M. Hansen, Constitution of Binary Alloys, 2nd ed., McGraw-Hill, New York, 1958, p. 527. [23] J.F. Herbst, Rev. Mod. Phys. 63(4) (1991) 819. [24] H.C. Ku, L.S. Yen, J. Less-Common Met. 127 (1987) 43. [25] L.H. Lewis, C.H. Sellers, V. Panchanathan, in: R.G. Bautista, C.O. Bounds, T.W. Ellis, T. Kilbourn (Eds.), Rare Earths, Science Technology and Applications III, TMS, 1997, p. 119. [26] Kh.Ya. Mulyukov, R.Z. Vaoiev, G.F. Korznikova, V.V. Stolyarov, Phys. Status Solidi (a) 112 (1989) 137. [27] W.C. Chang, S.H. Wu, B.M. Ma, C.O. Bounds, J. Magn. Magn. Mater. 167 (1997) 65. [28] W.C. Chang, S.H. Wu, B.M. Ma, C.O. Bounds, J. Appl. Phys. 81(8) (1997) 4453. [29] Shan-dong Li, Wen-sheng Sun, Ming-xiu Quan, Mater. Lett. 30 (1997) 351. [30] K.H.J. Buschow, D.B. De Mooij, J.L.C. Daams, H.M. Van Noort, J. Less-Common Met. 115 (1986) 357. [31] J.M.D. Coey, in: G.F. Chiarotti, F. Fumi, M.P. Tosi (Eds.), International School of Physics ‘‘Enrico Fermi’’, Current Trends in the Physics of Materials, Villa Marigola, Italy, 20 June–8 July 1988, p. 265. [32] G. Herzer, J. Magn. Magn. Mater. 112 (1992) 258.
L
Crystallization sequences and magnetic properties of melt-spun Nd 2 Fe 14 Bbased nanocomposites containing Co and Cr a, 1 ,a 2 ,a b L.H. Lewis *, K. Gallagher , B. Hoerman , V. Panchanathan a
Materials Science Division, Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973 -5000, USA b Magnequench International, Inc. ( MQI), 6435 Scatterfield Road, Anderson, IN 46013, USA Received 16 February 1998; received in revised form 18 March 1998
Abstract The physical, chemical and microstructural characteristics of the two-phase nanocrystalline alloy Nd 2 [Co 0.06 (Fe 12x Crx ) 0.94 ] 23.2 B 1.48 (0#x,0.9) made by melt-spinning methods were investigated to ascertain their effects on room-temperature magnetic properties, with the ultimate goal of understanding, controlling and optimizing the alloys’ coercivity and exchange-enhanced remanence. Detailed investigations were made into the phase content, character and grain size of the alloys using synchrotron X-ray diffraction. The technical magnetic properties were studied with SQUID and VSM magnetometry. Differential thermal analysis was performed to monitor the crystallization sequences of the as-quenched alloys as a function of composition, as well as to identify the Curie temperatures of annealed alloys. Most annealed samples consist of a phase isostructural with Nd 2 Fe 14 B (denoted Nd 2 M 14 B or 2-14-1) and a bcc Fe-rich phase (denoted bcc-(FeCoCr)). The replacement of Fe by Cr (up to Cr content x|0.03) causes an initial increase in the lattice parameters of the constituent phases found in the annealed samples. Further Cr additions cause a sharp drop in the lattice parameters. The calculated average grain sizes in the annealed samples exhibit an abrupt increase for larger chromium contents, and the grain size of the Nd 2 M 14 B phase is larger than that of the bcc-(FeCoCr) phase for all samples studied. The Curie temperatures of the 2-14-1- and bcc Fe-type phases are found to decrease with increasing Cr content. Thermal analysis and laboratory X-ray diffraction experiments performed on selected annealed samples suggest that the enrichment of Fe in the nanocomposite alloy allows the system to access the metastable non-stoichiometric Nd 2 M 17 -type phase prior to transformation to the 2-14-1 phase. Increased Cr content in the starting alloy stabilizes the amorphous→Nd 2 M 17 -type (metastable) phase transition and the formation of the Nd 2 M 14 B phase, shifting them to higher temperatures. The transformation behavior of the alloys as a function of Cr content ultimately controls the phase constitution, phase chemistry and the grain size, which in turn control the room-temperature hysteretic magnetic properties. 1998 Elsevier Science S.A. Keywords: Intermetallic compounds; Magnetic properties; Metastable phases; Nanocomposite; Rapid solidification
1. Introduction Nanocomposite exchange-coupled magnetic materials are anticipated to comprise the next generation of permanent magnet materials [1]. Ideally synthesized from intimately-mixed grains of a magnetically-hard phase and a magnetically-soft phase, the overall microstructure should possess a grain size on the order of twice the domain wall width in the hard phase for optimum exchange coupling, to yield optimum technical magnetic properties [2]. To date, *Corresponding author. Tel.: 11 516 344 2861; fax: 11 516 344 4071; e-mail: [email protected] 1 Present address: Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA. 2 Present address: Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108, USA. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00529-5
nanocomposite permanent magnet materials with enhanced remanence ratios have not yet exhibited the outstanding performance predicted by theoretical models of these materials [3,4]; specifically, the remanence enhancement Br /Ms and the coercivity Hci are often not as large as predicted. In nanocomposite materials based on a-Fe or Fex B (x52, 3) and (RE) 2 Fe 14 B (2-14-1), where RE is a rare-earth element such as Nd or Pr, these observations are true regardless of the method of fabrication (ball milling or rapid solidification) [5,6] or elemental additives [7]. It is true that certain elemental additions, or combinations of elemental additions, do improve the room-temperature magnetic hysteretic properties. For example Hirosawa et al. [8] have found remanences up to 1.2 T and coercivities up to 8600 Oe in different samples with compositions near Nd 5.5 Fe 66 B 18.5 Cr 5 Co 5 . Jurczyk et al. [9] found that the addition of Zr to 2-14-1 / a-Fe nanocomposites produced
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by high-energy ball milling increases the final coercivity over that of the Zr-free composition, but the reasons underlying the phenomenon are not clear. In general, such alloying additions further complicate an already extremely complex, nanoscale, non-equilibrium system, obscuring the underlying reasons for the property enhancement. The remanence enhancement and coercivity are dependent upon the degree of interphase exchange coupling present within the nanocomposite system. The remanence enhancement is itself a function of the magnetic properties of the phases present in the nanostructure as well as the grain sizes and homogeneity of these phases in relationship to the domain wall width in the system. Alloying additions to a rapidly-solidified system may have any or all of the following effects: promotion or suppression of metastable transitional phases and stoichiometries, stabilization of either a glassy or a crystalline phase (which in turn may produce disparate grain sizes and shapes), inhibition of grain growth and the production of alignment of the constituent grains [10]. In order to assess the efficacy of any particular combination of elemental additions in producing optimal magnetic properties in nanocomposite alloys, it is necessary to first identify the effects it has upon the final nanostructure. To this end, detailed materials characterization studies of the low-boron melt-spun nanocomposite alloy Nd 2 [Co 0.06 (Fe 12x Cr x ) 0.94 ] 23.2 B 1.48 (0#x,0.9) were performed. The phase constitution and character of the annealed samples were assessed with X-ray diffraction techniques. The crystallization sequence experienced by the as-quenched samples during the standard annealing treatment was followed by differential thermal analysis (DTA). The Curie temperatures of the constituent phases in the annealed specimens were determined to obtain information concerning the partitioning of the transition metal species in each phase as a function of Cr content. Such information is not easily obtainable by X-ray scattering methods, due to the similarity of the electronic structure of the transition metal species. It is found that subtle changes in the nominal starting chemistry
of the alloy may produce significant changes in the stability of the metastable phases present in the system, which in turn determine the magnetic properties of the annealed nanocomposite.
2. Experimental procedures The samples were made by standard melt-quenching techniques from commercial-grade alloys. The compositions are obtained from the starting alloy compositions, listed in Table 1. They are expressed in both wt% and stoichiometry, to illustrate the amount of transition-metal enrichment over that of the stoichiometric Nd 2 M 14 B base alloy, where M denotes a mixture of the transition-metal species Fe, Co and Cr. The samples are identified by the atomic percentage of chromium replacing iron in the nominal total transition-metal content; i.e. x in Nd 2 (Co 0.06 (Fe 12x Cr x ) 0.94 ) 23 B 1.5 (average composition). For comparative purposes, some chromium- and cobaltfree compositions were also studied; their compositions are also included in Table 1. Melt-spun ribbons were overquenched from the melt at a wheel speed of 22 m s 21 . A portion of each sample was left in its as-quenched state, while a separate portion was vacuum annealed at 6908C for 4 min to obtain optimal technical magnetic properties. The samples were characterized by a variety of techniques. X-ray diffraction was performed in the transmission mode at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, using X-ray ˚ ˚ The wavelength was wavelengths 0.90 A#l#1.18 A. chosen so as to avoid the excitation of Fe fluorescence. Such a technique not only unambiguously distinguishes an amorphous phase content from fluorescence, but also results in very good Bragg peak resolution that improves the detection of minor phases otherwise obscured by fluorescence. Additional laboratory reflection X-ray diffraction was performed as needed using standard Cu Ka radiation. Crystallite grain sizes were determined from the
Table 1 Sample identification x denoting Cr content in Nd 2 (Co 0.6 (Fe 12x Cr x ) 94 ) 23 B 1.5 (approximate composition) and sample compositions in both stoichiometry and weight percentage Sample ID/ comment
x50 x50.007 x50.017 x50.029 x50.048 x50.060 x50.068 x50.087 Cobalt-free alloy Cobalt- and chromium-free alloy Commercial MQ-A powder
Composition Stoichiometry
(wt%)
(Nd) 2 (Co 0.062 Fe 0.938 ) 22.70 B 1.46 (Nd) 2 (Co 0.060 Fe 0.934 Cr 0.007 ) 22.70 B 1.46 (Nd) 2 (Co 0.061 Fe 0.922 Cr 0.017 ) 23.34 B 1.55 (Nd) 2 (Co 0.061 Fe 0.910 Cr 0.029 ) 23.06 B 1.44 (Nd) 2 (Co 0.061 Fe 0.891 Cr 0.048 ) 23.09 B 1.46 (Nd) 2 (Co 0.062 Fe 0.878 Cr 0.060 ) 23.26 B 1.48 (Nd) 2 (Co 0.075 Fe 0.856 Cr 0.068 ) 22.19 B 1.44 (Nd) 2 (Co 0.064 Fe 0.849 Cr 0.087 ) 21.94 B 1.44 (Nd) 2 (Cr 0.032 Fe 0.968 ) 23.14 B 1.47 (Nd) 2 Fe 23.25 B 1.47 Nd 2.39 Fe 14 B 0.95
(Nd) 18.3 Fe 76.4 Co 5.3 B 1.00 (Nd) 17.7 Fe 75.7 Co 5.1 Cr 0.5 B 1.00 (Nd) 17.9 Fe 74.6 Co 5.2 Cr 1.3 B 1.04 (Nd) 18.1 Fe 73.5 Co 5.2 Cr 2.2 B 0.98 (Nd) 18.1 Fe 72.1 Co 5.2 Cr 3.6 B 0.99 (Nd) 18.1 Fe 71.2 Co 5.3 Cr 4.5 B 1.00 (Nd) 18.7 Fe 68.6 Co 6.4 Cr 5.1 B 1.01 (Nd) 18.9 Fe 68.6 Co 5.4 Cr 6.5 B 1.02 (Nd) 18.1 Fe 78.5 Cr 2.4 B 1.00 (Nd) 18.0 Fe 81.01 B 0.99 (Nd) 30.5 Fe 68.6 B 0.90
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half-width of the Bragg peak of the NSLS XRD data at the half-maximum intensity position, corrected for the intrinsic broadening of the synchrotron beam, using the Scherrer formula [11]. Lattice parameters were obtained using a least-squares-fit algorithm. Room-temperature magnetic measurements were performed using a vibrating sample magnetometer (VSM) and a SQUID magnetometer. The hysteretic properties remanence Br , intrinsic coercivity Hci and energy product (BH ) max were measured on samples that were premagnetized at 4 T. Room-temperature demagnetization curves were obtained using a maximum applied field of 5 T on isotropic cylinders formed from packed powder set in epoxy. The data were corrected for demagnetization effects. In order to calculate the remanence ratios Br /Ms , the saturation magnetization Ms was obtained by fitting the high-field portion (Hint $20 000 G) of the demagnetization curve to the expression 2
M(H ) 5 Ms (H 5 `)(1 2 (a /H ) 2 (b /H ))
(1)
where H is the internal field, and a and b are arbitrary constants. To monitor the crystallization sequence of the asquenched alloys as well as to monitor the Curie temperatures (T c ) of the various magnetic phases present in the annealed alloys, differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed using a TA Instruments Model SDT 2960 Simultaneous DTA / TGA. Measurements were made using a ramp rate of 208 min 21 in the typical temperature range 375 K#T # 900 K in an atmosphere of high-purity Ar with 1% hydrogen. The 0.2% average weight gain of the samples during the course of the approximately 4 h measurement indicates that there was no significant uptake of hydrogen and / or oxygen. The hydrogen acts as an oxygen getter; under the same conditions, the use of pure Ar as a purging gas results in a weight gain of over five times that obtained with 1% hydrogen. To monitor the crystallization sequence, at least two separate, approximately 100 mg, quantities of sample were measured. Fig. 1 depicts a typical crystallographic transformation sequence for an unannealed alloy, where the exothermic transformations are marked by three peaks in the curve. In some instances the transformation curves overlapped one another, making the determination of peak temperatures difficult. In such cases a multi-peak Gaussian curve was used to fit the data. The Curie temperature transition produces a change in the slope of the DTA curve with both heating and cooling. This transition is illustrated for an annealed sample by the dashed line in Fig. 2. The Curie temperatures were determined as the minimum in the temperature derivative of the DTA cooling curve, the solid line of Fig. 2. The T c transitions were determined separately using two new 100 mg quantities of sample; each sample was cycled up and down in temperature a minimum of two times. The
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Fig. 1. DTA data collected from an as-quenched sample, typical of those measured in this study. Three exothermic peaks are noted.
error in T c determined by the thermal method is taken as the standard deviation of the measurements, and is usually less than 18. In some cases portions of selected asquenched alloys were annealed at intermediate temperatures for short times in the DTA, in preparation for laboratory X-ray diffraction. This procedure was done so as to insure minimal oxidation of the sample.
3. Results
3.1. Phase constitution and characterization of annealed samples The lattice parameters and related properties of the samples show complex trends with composition, as might be expected for composite specimens of complicated composition produced in a non-equilibrium manner. For all structural parameters measured – lattice parameters and average grain sizes – there appears to be a discontinuity in,
Fig. 2. DTA data collected from an annealed sample (- - -); the derivative of the DTA cooling curve (———) emphasizes the change in the signal produced by the Curie transition.
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Fig. 3. A synchrotron X-ray diffraction pattern typical of those collected in this study. The Nd 2 Fe 14 B reference Bragg peaks are marked by vertical dashed lines, whereas the bcc-phase Bragg reflections are marked by arrows.
or anomalous value of, the measured parameter in the intermediate compositional range. Fig. 3 shows a typical synchrotron transmission X-ray diffraction scan obtained in this study; the Nd 2 Fe 14 B phase and the (FeCoCr) phase Bragg reflections are identified in the figure. All annealed samples, except for that with the highest chromium concentration (x50.087), were found to consist only of a Nd 2 Fe 14 B-type phase and a bcc-(FeCoCr) phase. It is estimated that synchrotron X-ray diffraction would detect minor phases of 0.1–1.0 wt% [12]. The most Cr-rich composition studied (x50.087) consists of three phases: a non-stoichiometric metal-rich Nd 2 (FeCoCr) 17 type phase with the approximate composition RE 2 M 19.2 [13], the bcc-(FeCrCo) phase and a small amount of the 2-14-1 phase. Figs. 4 and 5 illustrate the variation of lattice parameters and the average grain sizes of the phases with compositional change. The Nd 2 Fe 14 B-type phase displays an overall increasing lattice parameter trend with composition (Fig. 4a) up to the middle of the compositional range studied. For greater Cr concentrations there is a precipitous drop in the lattice parameters. (The data points for 2-14-1 lattice parameters of the [Cr]50.017 sample appear to be anomalous.) This general trend with increasing Cr content is mirrored in the bcc-(FeCoCr) phase lattice parameters and the constituent grain sizes. The lattice parameters of the bcc phase (Fig. 4b) show the a-parameter increasing up to a chromium content of x5 0.029 and then discontinuously dropping to a more constant value for x$0.048. The average grain size determinations of both the 2-14-1 phase and the bcc Fe phase are shown in Fig. 5, with lines drawn to guide the eye. It can be seen that the average grain size of the Nd 2 Fe 14 B phase is larger than that of the bcc Fe phase for all compositions studied. As was found for the lattice parameters, a discontinuity in the average grain size of both phases occurs in the vicinity of the composition x$0.048.
Fig. 4. (a) Variation with Cr content x of the measured Nd 2 M 14 B lattice parameters in the annealed samples. (b) Variation with Cr content x of the measured lattice parameter of the bcc-(FeCoCr) phase in the annealed samples.
3.2. Magnetic properties 3.2.1. Technical magnetic properties of the annealed samples The room-temperature technical magnetic properties of the annealed samples (Br , Hci , Ms and (BH ) max ) and the remanence ratios Br /Ms are illustrated in Fig. 6a–e. In general, the addition of Cr to the alloy causes both the saturation magnetization Ms and the remanence Br to decrease. The saturation magnetization reaches a high value of 1.84 T at x50.007, while the remanence varies
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by the same method for other sample compositions made by melt-spinning and subsequent annealing. These samples have the following nominal compositions: single transition-metal Nd 2 Fe 23.25 B 1.47 , Cr-free Nd 2 (Fe 0.938 Co 0.062 ) and Co-free Nd 2 (Fe 0.968 Cr 0.032 ). The measured Curie temperatures of the phases present in these samples are shown in Table 2.
3.3. Crystallographic transformation sequences
Fig. 5. Variation of average grain size with Cr content of the annealed samples.
from a high value of 0.99 T at x50 to a low value of approximately 0.85 T at x50.087. The decrease in the remanence is significant for x$0.029. The coercivity Hci , on the other hand, does not show a monotonic trend with increasing chromium concentration, but always remains below 4.1 kOe. The variation of the energy products (BH ) max with chromium concentration mimics that experienced by remanence Br , exhibiting values near 12 MGOe for low-Cr-content samples. All samples are modestly remanence-enhanced, with a peak in the remanence enhancement of 0.59 for the x50.017 specimen (Fig. 6e). A remanence ratio value in isotropic alloys equal to 0.5 implies that no remanence enhancement is present [14].
3.2.2. Curie temperatures of the phases in the annealed samples Fig. 7a shows the Curie temperatures as a function of composition for the Nd 2 (FeCoCr) 14 B phase, whereas Fig. 7b shows determinations of the same parameter for the bcc Fe-rich phase. In both figures, the line drawn between points is to guide the eye. The data for the two crystallographic phases exhibit similar trends: an increased Cr content causes the Curie temperatures to decrease, with the Curie temperature of the 2-14-1 phase falling from a high of 3728C at x50 down to 3488C at x50.087. This change is approximately twice that of the analogous drop in the bcc phase, which falls from a high value of 8218C to 7938C in the same compositional range. Comparisons with the published Curie-temperature dependence of bcc-Fe on cobalt content [15] indicate that there is between 4 and 5 at% Co in the bcc Fe-rich phase of the Cr-free (x50) sample. However, the calculated unit cell volume for the bcc phase at that composition is approximately 1% larger than that for the standard reference JCPDS bcc Fe lattice parameter [16]. These data suggest the presence of retained Nd, as will be discussed further in Section 4.1. For comparative purposes, Curie temperatures were determined
The exothermic peak temperatures measured by DTA in as-quenched samples as a function of Cr content [x] are shown in Fig. 8. The dashed lines are drawn between data points to guide the eye. The standard annealing temperature used to heat-treat the quenched alloys in this study (6908C) is marked by a horizontal line in Fig. 8. All DTA peaks are broad, commonly spanning a temperature range of 808; this feature implies that the phase transformations under consideration take place over a considerable temperature range. The low-temperature and high-temperature peaks denote crystallographic transformation temperatures that increase with increasing chromium content, while the intermediate temperature transformation is independent of composition to a large degree. The hypothesized transformations, based on results described below, are included in Fig. 8. Selected samples were subjected to intermediate heat treatments and subsequently investigated with standard laboratory Cu Ka X-ray diffraction to identify the metastable transition phase. It is concluded that the metastable transition phase present at intermediate temperatures is the non-stoichiometric Nd 2 (FeCoCr) 17 -type phase with the approximate metal-rich stoichiometry Co 16 Fe 35.6 Y 5.38 , which may be roughly written as RE 2 M 19.2 [13]. No evidence of other complex intermetallic phases such as the cubic Nd 2 Fe 23 B 3 , the hexagonal NdFe 12 B 6 and the cubic Y 3 Fe 62 B 14 -type phases was found, as has been by other authors in samples of similar composition [8,17]. It appears that a poorly crystallized precursor to the metalrich 2-17-type phase is present even in the quenched alloys, in addition to very fine grained bcc-(FeCoCr). Fig. 9 illustrates X-ray diffraction data of sample x50.048, typifying the overall crystallographic transformation sequence. It is apparent that a 1 min anneal of this sample at T55858C produces a reduction in the glassy component of the alloy, to form more of the Fe-rich bcc phase and the non-stoichiometric Fe-rich Nd 2 (FeCoCr) 17 -type phase. A subsequent 5 min anneal at 6508C of the same alloy shows a significant reduction in the 2-17-type phase content of the alloy. Higher-temperature anneals produce the Nd 2 M 14 B phase and presumably more bcc Fe as decomposition products of the 2-17-type phase. It is to be noted that as the bcc-type phase is present at all stages of the crystallographic transformation, it is postulated to undergo a continuous nucleation and growth process throughout the annealing sequence.
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Fig. 6. Room-temperature magnetic properties of the annealed alloys. (a) Saturation magnetization Ms ; (b) remanence Br ; (c) coercivity Hci ; (d) energy product (BH ) max ; (e) remanence ratio Br /Ms (estimated using extrapolation to H5`, as described in the text).
4. Discussion of results It is apparent that the room-temperature technical magnetic behavior of the magnetic compounds in this study results from a very complex interplay of atomic structure, microstructural and chemical details. The chemical details of the constituent phases within the samples are illuminated by both the lattice parameter trends and the Curie temperature evolution of the phases with respect to chromium content. The extrinsic microstructural factors, characterized by X-ray diffraction and by the crystallization sequences identified by DTA, clarify the trends
underlying the structure-sensitive technical magnetic properties.
4.1. Compositional character of the annealed samples Elemental segregation and diffusion occur in the quenched alloys as the crystallization process proceeds under the influence of the standard annealing treatment. All three transition metals, as well as the Nd and the B present in the alloys, simultaneously diffuse through the system and affect the magnetic and crystallographic properties of the phases present.
L.H. Lewis et al. / Journal of Alloys and Compounds 270 (1998) 265 – 274
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Fig. 8. Trends of the DTA exothermic peak temperatures with increasing Cr content x. The standard annealing temperature of T ann 56908C is marked by a double-headed arrow for reference.
Fig. 7. Curie temperatures measured by DTA: (a) Nd 2 M 14 B; (b) bcc(FeCoCr).
An analysis of information from the lattice parameter data (Fig. 4b), the Curie temperature data (Fig. 7b), and Table 2 leads to the conclusion that the bcc Fe-rich phase in the Cr-free (x50) alloy contains a small amount of Nd, in addition to Fe and Co. This conclusion results from the following observations. The lattice parameter of this bcc phase is larger than that of pure a-Fe [16], outside of experimental uncertainty, by 0.3%, and the magnetic transition temperature is approximately 508 higher than that of pure a-Fe. Elevated Curie temperatures in the bcc phase relative to that of pure a-Fe can only be produced by the presence of cobalt: the substitution of Co for Fe in the bcc lattice increases the Curie temperature at a rate of
Fig. 9. (Laboratory) X-ray diffraction data illustrating the transformation sequence of the metastable phases found in the x50.048 sample. The figure is described more fully in the text.
approximately 2.88 per at% Co up to 15 at% Co [15]. However, the substitution of cobalt for Fe in the bcc lattice has little effect on the lattice parameter up to approximately 20 at% Co [18]; for Co concentrations greater than 20 at% the lattice parameter of a-Fe decreases rapidly. It is
Table 2 Lattice parameters and measured Curie temperatures of nanocomposite samples containing only one or two transition-metal species Sample
Nd 2 (Co 0.062 Fe 0.938 ) 22.70 B 1.46 Nd 2 (Cr 0.032 Fe 0.968 ) 23.14 B 1.47 Nd 2 Fe 23.25 B 1.47 a
Nd 2 M 14 B phase
bcc Fe-rich phase
˚ a-parameter (A)
˚ c-parameter (A)
Curie temperature (8C)
˚ a-parameter (A)
Curie temperature (8C)
8.803860.0023 8.813060.0027 8.80560.002
12.205460.0037 12.226260.0044 12.21460.005
372.260.8 300.763.5 313.160.35
2.87560.004 2.87860.003 2.88 a
821 773 767
Uncertainty in parameter unavailable because only one peak was measured.
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unlikely that boron is residing in the bcc phase: the introduction of boron into the a-Fe lattice both lowers the Curie temperature of the phase and produces a contraction in the lattice parameter [19]. Therefore, the most likely scenario is that the bcc Fe-rich phase in the x50 specimen contains Nd in addition to Co. The binary Fe–Nd phase diagram [20] reveals a single-phase solid solubility of up to approximately 3 at% Nd in a-Fe, in the temperature range 780 K#T #1536 K, the melting point of the pure phase. It is likely that the rapid solidification process results in retained Nd impurities that become trapped within the bcc lattice. A similar conclusion was reached by Sagawa et al. [21], who detected a bcc Fe-rich phase in melt-spun binary Fe–Nd alloys that contained approximately 14 at% Nd and possessed the lattice parameter ˚ While Sagawa et al. estimated the Curie a52.87 A. temperature of this phase to be in the range 400–450 K, it is likely that a smaller retained Nd content would produce a smaller reduction in Curie temperature from that of pure a-Fe. The measured increase in the bcc lattice parameter with increasing Cr content suggests that Cr enhances Nd segregation in the phase up to a critical Cr concentration of x|0.03. The Curie temperatures of the bcc Fe-rich phase in the alloys decrease slowly for 0$x.0.06, and then decrease rapidly down to the most Cr-rich specimen investigated in this study. Out of all the elemental components in the system, only Co is capable of raising the Curie temperature of the bcc Fe-rich phase beyond its pure a-Fe value of 7708C. The substitution of Cr for Fe in a-Fe causes a drop in the Curie temperature at a rate of approximately 48 per at% Cr in the compositional range 5 at%#[Cr]#18 at% [22]. As discussed above, the expanding bcc lattice parameter for x,0.048 strongly suggests that Nd is segregating to the bcc phase as the overall Cr content in the alloy increases in the compositional range 0#x#0.048. Such a segregation of both Cr and Nd to the bcc phase would likely cause a rapid decrease in the Curie temperature, which is not observed. Thus it is postulated that all three elements – Cr, Co and Nd – are segregating in various increasing amounts to the bcc phase, until the Cr concentration in the alloy exceeds a mole fraction equal to 0.07. The observed decrease in the Curie temperature of Nd 2 M 14 B with increasing Cr content phase signifies the occupancy of Cr in the 2-14-1 lattice. It is possible that Cr displaces Co in the 2-14-1 lattice; such behavior might be expected if both elements compete for the same site in the Nd 2 Fe 14 B lattice. As discussed by Herbst [23], both chromium and cobalt show a preference for the 8j 2 site when substituted for iron in the Nd 2 Fe 14 B structure, although studies on the site occupancy of cobalt-only substituted Nd 2 Fe 14 B have yielded conflicting conclusions. In the Nd 2 Fe 14 B lattice, the substitution of Co for Fe is known to raise the Curie temperature as well as lower the anisotropy, while the substitution of chromium is known to
lower the Curie temperature [24]. However, cobalt is still present in the 2-14-1 lattice at all compositions studied, as evidenced by the anomalously-high 2-14-1 Curie temperature values for the x50.060 and x50.068 samples (these two samples possess starting compositions that are richer in cobalt than those of the other samples, Table 1). Increasing amounts of Co and Cr substituted for Fe in the 2-14-1 lattice decrease both the saturation magnetization and the remanence [24], as confirmed by the data of Fig. 6.
4.2. Phase transformations in as-spun alloys In general, it is extremely difficult to suppress the formation of a primary properitectic bcc Fe-rich phase during rapid solidification from a melt with a composition close to Nd 2 Fe 14 B [25]. This fact is borne out in the synchrotron XRD data of selected as-quenched alloys in this study, in which extremely fine-grained crystalline a-Fe is clearly visible. There have been a few studies on the transformation sequences found in rapidly-solidified nanocomposite alloys with compositions analogous to those in this study [8,17,26–29]. In general, most authors identify two or three exothermic crystallographic transformations in nanocomposite 2-14-1 / a-Fe alloys: the lower-temperature transformations, in the range 570–5908C, are associated with the crystallization and growth of a-Fe and Fe-borides, while the highest-temperature transformations are invariably associated with the crystallization of the RE 2 M 14 B phase. Peaks that are intermediate in temperature have been connected to the formation of complex intermetallic phases, such as the cubic Nd 2 Fe 23 B 3 , the hexagonal NdFe 12 B 6 and the cubic Y 3 Fe 62 B 14 -type phases [8,17]. The fact that no metastable complex intermetallic-boride phases such as Nd 3 Fe 62 B 14 or Nd 2 Fe 23 B 3 are found in these alloys might be attributed to their relatively low boron content, as well as to the presence of Co and Cr. The presence of significantly more iron in the nanocomposite alloys relative to the single-phase 2-14-1 alloy allows the system to access an additional, lower free-energy Fe-rich Nd 2 M 17 -type phase prior to the formation of Nd 2 M 14 B. The nominally-stoichiometric melt-spun compound Nd 2 Fe 14 B exhibits a single exothermic peak at T5578.58C (Table 3) which marks the crystallization of the 2-14-1 phase from the largely-amorphous matrix. However, nanocomposite alloys containing two or more transition-metal species do not exhibit the 2-14-1 phase
Table 3 Exothermic peak temperatures as measured by DTA for Cr- and Co, Cr-free Nd 2 M 14 B-based samples Sample
Peak 1
Peak 2
Peak 3
Nd 2.39 Fe 14 B 0.95 Nd 2 Fe 23.3 B 1.47 Nd 2 (Fe 0.968 Cr 0.032 ) 23.14 B 1.47
578.58C 578.48C 583.58C
657.28C 661.28C
683.48C
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crystallization until significantly higher temperatures, in the vicinity of 6608C. The presence of either Co or Cr in the nanocomposite alloy stabilizes the non-stoichiometric Nd 2 M 17 -type phase against decomposition. This observation echoes that of Chang et al., who also find that the crystallization temperatures of their alloys increase with increasing chromium content [28]. It is noteworthy that the Co- and Cr-free nanocomposite composition Nd 2 Fe 23.3 B 1.45 exhibits only two exothermic peaks during crystallization (Table 3), instead of the expected three peaks. Standard laboratory X-ray diffraction studies performed on this sample reveal identical phase transformation behavior to that illustrated in Fig. 9 for the x50.048 composition, albeit occurring at the lower temperatures listed in Table 3. It is postulated that only two exothermic events are detected in this composition because the crystallization from the amorphous phase and the decomposition of the Nd 2 M 17 -type phase occur in the same temperature range. In general, the considerable broadness of the DTA exothermic peaks found in this study suggest that the phases formed occupy a range of chemical variability.
4.3. Crystallographic /structural considerations The discontinuities present in both the measured lattice parameters and the grain sizes near the composition x| 0.03 reflect chemical differences in the constituent phases that may be attributed to the phase transformation behavior of the system. The lower crystallization temperatures of compositions containing smaller amounts of chromium promote an increased nucleation rate, which results in a smaller grain size relative to those alloy compositions with higher Cr content. The increasing Cr content in the bcc phase appears to produce an increased affinity for Nd, up to a critical chromium concentration of x|0.03. The phases of compositions containing higher amounts of chromium require higher temperatures to form, which results in larger Nd 2 M 14 B and bcc-(FeCrCo) grain sizes that presumably cannot accommodate such a high concentration of Nd. For Cr concentrations .0.03 the bcc lattice parameters are still larger than that of pure a-Fe, so the bcc phase likely contains Nd in the entire composition range studied. Both the a- and c-lattice parameters of the RE 2 (FeCoCr) 14 B phase (excluding the anomalous data points at x50.29) exhibit an analogous trend to that found for the bcc-(FeCoCr) phase, in which the lattice parameters rise to their highest level at x|0.03 and then fall precipitously for larger chromium concentrations. Although Nd 2 M 14 B is considered to be a line compound [30], the combined effects of a nanoscale microstructure, nonequilibrium processing and low-level elemental variation on the defect character of Nd 2 M 14 B, specifically atomic site substitutions, vacancies and interstitial occupancies, has to date not been thoroughly investigated. Thus it is difficult to ascertain what atomic / crystallographic features
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have produced the observed lattice parameter trends in Nd 2 M 14 B. It is postulated that subtle processing differences in the x50.02 specimen have resulted in a defect character different from that of the other samples and thereby produced anomalously low lattice parameters.
5. Relations to technical magnetic properties The results of the investigations into the Nd 2 [Co 0.06 (Fe 12x Cr x ) 0.94 ] 23.2 B 1.48 nanocomposite alloys reveal structure–property relationships pertinent to their performance as remanence-enhanced permanent magnets, and therein suggest processing improvements. The three main factors that impact the technical magnetic properties of the alloys in this study are phase constitution, phase chemistry, and constituent phase grain size. All three factors are interrelated. The nanocomposite phase constitution present after the annealing treatment is the main factor affecting the technical magnetic properties. Comparisons between the metastable phases identified in the present study and those in the literature underscore the importance of the starting chemistry, especially the boron content, in the determination of the final phases in the nanocomposite. Because the prototypical metastable phase identified in the alloys of this study, Nd 2 Fe 17 , not only has a Curie temperature that is just slightly above room temperature but also has basalplane anisotropy and is therefore magnetically soft [31], it is necessary to insure the complete decomposition of the metastable Nd 2 Fe 17 -type phase and subsequent complete formation of the Nd 2 M 14 B phase to obtain optimum results. As illustrated in Fig. 8, an increasing Cr content pushes the 2-14-1 formation temperature significantly above the standard annealing temperature. Indeed, the annealed sample with the greatest amount of Cr, (Nd) 2 (Co 0.064 Fe 0.849 Cr 0.087 ) 21.94 B 1.44 , contains the metalrich Nd 2 Fe 17 -type phase as the major component. This sample also possesses the poorest room-temperature magnetic properties of the compositions studied. In addition to influencing the kinetics of phase formation in the system, the presence of chromium reduces the magnetization of the constituent phases, contributing to the decline of the room-temperature magnetic properties with increasing Cr content. The detailed composition of the Nd 2 M 14 B and bcc-(FeCoCr) phases comprising the nanocomposite determines the Curie temperatures of the phases. Broadly speaking, the Curie temperatures determine the operating temperatures of the final nanocomposite, and thus higher Curie temperatures are preferred. Higher Curie temperatures also favorably affect the interphase exchange coupling and thereby influence the remanence enhancement. A reduction in the magnetic moment of the compound, such as that produced by an increase in the temperature or, equivalently, a decrease in the Curie temperature, would produce a decrease in the strength of
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intergrain exchange coupling. This phenomenon would be expected to produce a lower remanence enhancement but an increased coercivity [32]. The remanence enhancement, as well as the coercivity, is also strongly affected by the average grain size [3] of the nanocomposite, with greater values anticipated for smaller grain sizes. This principle is roughly followed by the data obtained in this study: samples with a smaller Cr content possess relatively smaller grain sizes (between 200 and ˚ and larger room-temperature remanence ratios and 300 A) coercivities. The sample with the largest grain size (composition of x50.68) does indeed exhibit the smallest remanence ratio. The higher formation temperature of the Nd 2 M 14 B phase containing relatively more chromium promotes larger grain sizes, due to accelerated growth kinetics at higher temperature. It appears that the phase transformation kinetics indirectly affect the chemistry of the system, resulting in an Nd-enriched bcc phase for lower values of Cr content, as discussed in Section 4.2. For lower chromium concentrations (x#0.03) the growth of the grains may be limited by the availability of Nd, contributing to the smaller microstructural scale of alloys with relatively less Cr. Improvement of the magnetic properties of the alloys containing higher amounts of chromium might be achieved by the addition of grain growth inhibitors.
Acknowledgements This research was performed under the auspices of the U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Sciences, under Contract No. DE-AC02-76CH00016. Research was carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Science and Division of Chemical Sciences.
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