Influence of W Addition on Phase Constitution, Microstructure and Magnetic Properties of the Nanocrystalline Pr9Fe65WxB26-x (Where: x = 2, 4, 6, 8) Alloy Ribbons

The aim of the present work was to investigate an influence of W addition on the phase constitution, microstructure and magnetic properties of the Pr9Fe65WxB26-x (where: x = 2, 4, 6, 8) alloy ribbons. Ribbons were obtained using the melt-spinning technique under low pressure of Ar. The as-cast samples were fully amorphous and revealed soft magnetic properties. These facts were confirmed by X-ray diffractometry, Mössbauer spectroscopy and magnetic measurements. Differential scanning calorimetry and differential thermal analysis allowed us to determine the thermal stability parameters of the amorphous phase. The Kissinger plots were constructed in order to calculate the activation energies for crystallization. Heat treatment carried out at various temperatures caused changes in the phase constitution and magnetic properties of the alloys. The phase analysis has shown the presence of the hard magnetic Pr2Fe14B and paramagnetic Pr1+xFe4B4 phases. Additionally, for the x = 2 and x = 6 alloys, a crystallization of soft magnetic Fe2B and α-Fe phases was observed. The Mössbauer spectroscopy allowed us to determine the volume fractions of constituent phases formed during annealing. The microstructure of annealed ribbons was observed using transmission electron microscopy.


Introduction
For over half a century, much attention has been paid to the development of various types of high-performance permanent magnets based on rare-earth (RE) transition-metal (TM) intermetallic compounds. Their exceptional magnetic properties can be attributed to the favorable combination of properties such as high magnetocrystalline anisotropy, which is provided by the RE elements and high magnetic moments of TM elements also present in the unit cells of these compounds [1]. The most important advance in RE hard magnetic materials was made in 1984 when a new Nd 2 Fe 14 B ternary compound was announced by two independent groups-Sagawa et al. [2] at Sumitomo Special Metals Analysis of the Mössbauer spectra measured for the as-cast Pr9Fe65WxB26-x (where: x = 2, 4, 6, 8) ( Figure 2) ribbons confirmed their amorphous structure. In all cases, large broadenings of the Mössbauer lines were measured. In the fitting procedure, the hyperfine field distributions were calculated and experimental spectra were fitted by broad lines corresponding to these distributions. The bimodal character of the distributions suggests the existence of nonequivalent surroundings of Fe atoms in the amorphous phase. Furthermore, there is no significant shift in the maxima of the hyperfine field distributions with the increase of the tungsten content. This indicates similar magnetic properties of the amorphous phase for all alloy compositions.  Analysis of the Mössbauer spectra measured for the as-cast Pr 9 Fe 65 W x B 26-x (where: x = 2, 4, 6, 8) ( Figure 2) ribbons confirmed their amorphous structure. In all cases, large broadenings of the Mössbauer lines were measured. In the fitting procedure, the hyperfine field distributions were calculated and experimental spectra were fitted by broad lines corresponding to these distributions. The bimodal character of the distributions suggests the existence of nonequivalent surroundings of Fe atoms in the amorphous phase. Furthermore, there is no significant shift in the maxima of the hyperfine field distributions with the increase of the tungsten content. This indicates similar magnetic properties of the amorphous phase for all alloy compositions. Analysis of the Mössbauer spectra measured for the as-cast Pr9Fe65WxB26-x (where: x = 2, 4, 6, 8) ( Figure 2) ribbons confirmed their amorphous structure. In all cases, large broadenings of the Mössbauer lines were measured. In the fitting procedure, the hyperfine field distributions were calculated and experimental spectra were fitted by broad lines corresponding to these distributions. The bimodal character of the distributions suggests the existence of nonequivalent surroundings of Fe atoms in the amorphous phase. Furthermore, there is no significant shift in the maxima of the hyperfine field distributions with the increase of the tungsten content. This indicates similar magnetic properties of the amorphous phase for all alloy compositions.  The DSC and DTA scans measured for the amorphous ribbon samples of all investigated compositions are presented in Figures 3 and 4, respectively. These measurements allowed us to determine the glass transition temperature T g , onset crystallization temperature T x , melting temperature T m , supercooled liquid region ∆T x = T x − T g , reduced glass transition temperature T rg = T g /T m as well the glass forming ability parameter γ = T x /(T g + T m ) (Table 1) [41,42].
Materials 2020, 13, x FOR PEER REVIEW 5 of 15 The DSC and DTA scans measured for the amorphous ribbon samples of all investigated compositions are presented in Figures 3 and 4, respectively. These measurements allowed us to determine the glass transition temperature Tg, onset crystallization temperature Tx, melting temperature Tm, supercooled liquid region ΔTx = Tx − Tg, reduced glass transition temperature Trg = Tg/Tm as well the glass forming ability parameter γ = Tx/(Tg + Tm) (Table 1) [41,42].     The DSC and DTA scans measured for the amorphous ribbon samples of all investigated compositions are presented in Figures 3 and 4, respectively. These measurements allowed us to determine the glass transition temperature Tg, onset crystallization temperature Tx, melting temperature Tm, supercooled liquid region ΔTx = Tx − Tg, reduced glass transition temperature Trg = Tg/Tm as well the glass forming ability parameter γ = Tx/(Tg + Tm) (Table 1) [41,42].     Table 1. Thermal stability parameters determined for amorphous ribbons of Pr 9 Fe 65 W x B 26-x (where: x = 2, 4, 6, 8) alloys: T g -glass transition temperature, T x 1 and T x 2 -first and second onset crystallization temperature, T m 1 and T m 2 -first and second melting temperatures, ∆T x = T x − T g -supercooled liquid region, T rg = T g /T m -reduced glass transition temperature, γ = T x /(T g +T m )-glass forming ability parameter.
x (at. %) The changes of T g and T x , as well as ∆T x , were observed with the change of W content. However, other parameters (T m , T rg and γ) did not significantly change. The values of ∆T x suggest that the Pr 9 Fe 65 W x B 26-x (where: x = 2, 4, 6, 8) alloys should not exhibit good glass forming abilities. On the other hand, high values of T rg (~0.6) and γ (~0.3) indicate good glass forming abilities and a possibility of manufacturing amorphous samples with significant geometric dimensions [43][44][45]. However, our previous studies [22] of the Pr 9 Fe 65 W x B 26-x (where: x = 2, 4, 6, 8) alloys in the form of 1 mm rods and 0.5 mm thick plates produced using the suction-casting technique have shown partial crystallization of the as-cast specimens.
The activation energies E a for crystallization for each alloy compositions were calculated using the Kissinger analysis [46] base on the plots shown in Figure 5. The values of E a were collected in Table 2.
Error analysis was carried out using standard methods, taking into account apparatus input, apparatus calibration quality and subjective assessment of uncertainty associated with the determination of characteristic temperatures based on DSC curves. The results indicate that for Pr 9 Fe 65 W 4 B 22 and Pr 9 Fe 65 W 8 B 18 alloys, the crystallization processes feature lower values of E a compared to those for the Pr 9 Fe 65 W 2 B 24 and Pr 9 Fe 65 W 6 B 20 alloys. These differences can be attributed to the crystallization of additional soft magnetic Fe 2 B or α-Fe phases during heat treatment of alloys containing 2 and 6 at. % of W, respectively (see later).  [43][44][45]. However, our previous studies [22] of the Pr9Fe65WxB26-x (where: x = 2, 4, 6, 8) alloys in the form of 1 mm rods and 0.5 mm thick plates produced using the suction-casting technique have shown partial crystallization of the as-cast specimens. The activation energies Ea for crystallization for each alloy compositions were calculated using the Kissinger analysis [46] base on the plots shown in Figure 5. The values of Ea were collected in Table 2. Error analysis was carried out using standard methods, taking into account apparatus input, apparatus calibration quality and subjective assessment of uncertainty associated with the determination of characteristic temperatures based on DSC curves. The results indicate that for Pr9Fe65W4B22 and Pr9Fe65W8B18 alloys, the crystallization processes feature lower values of Ea compared to those for the Pr9Fe65W2B24 and Pr9Fe65W6B20 alloys. These differences can be attributed to the crystallization of additional soft magnetic Fe2B or α-Fe phases during heat treatment of alloys containing 2 and 6 at. % of W, respectively (see later).  The XRD scans measured for Pr9Fe65WxB26-x (where x = 2, 4, 6, 8) alloy ribbons subjected to annealing are shown in Figure 6. Short-time annealing of the specimens resulted in significant changes in their phase constitution. The crystalline phases detected for all annealed samples are the hard magnetic Pr2Fe14B and paramagnetic Pr1+xFe4B4. Furthermore, the phase analysis for the Pr9Fe65W2B24 alloy ribbons has shown the presence of the reflexes corresponding to the soft magnetic Fe2B phase (Figure 6a), while for the Pr9Fe65W6B20 alloy, the α-Fe phase was detected (Figure 6d). Heat treatment at 943 K and higher temperatures resulted in nucleation and growth of the crystalline phases. The increase of annealing temperature caused changes in the volume fractions of the constituent phases, which was reflected in their magnetic properties. Moreover, for the Pr9Fe65W2B24 alloy, the widening of the diffraction peaks is related to low volume fractions and smaller grain sizes of the crystalline phases.  The XRD scans measured for Pr 9 Fe 65 W x B 26-x (where x = 2, 4, 6, 8) alloy ribbons subjected to annealing are shown in Figure 6. Short-time annealing of the specimens resulted in significant changes in their phase constitution. The crystalline phases detected for all annealed samples are the hard magnetic Pr 2 Fe 14 B and paramagnetic Pr 1+x Fe 4 B 4 . Furthermore, the phase analysis for the Pr 9 Fe 65 W 2 B 24 alloy ribbons has shown the presence of the reflexes corresponding to the soft magnetic Fe 2 B phase (Figure 6a), while for the Pr 9 Fe 65 W 6 B 20 alloy, the α-Fe phase was detected (Figure 6d). Heat treatment at 943 K and higher temperatures resulted in nucleation and growth of the crystalline phases. The increase of annealing temperature caused changes in the volume fractions of the constituent phases, which was reflected in their magnetic properties. Moreover, for the Pr 9 Fe 65 W 2 B 24 alloy, the widening of the diffraction peaks is related to low volume fractions and smaller grain sizes of the crystalline phases. Materials 2020, 13, x FOR PEER REVIEW 7 of 15 Based on the XRD studies of annealed Pr9Fe65WxB26-x (where: x = 2, 4, 6, 8) ribbons, one can deduce that the wide crystallization peaks on the DSC and DTA curves corresponding to T (Figures 3 and  4) indicate the formation of the Pr2Fe14B and Pr1+xFe4B4 phases. It is expected that the T may be related to the decomposition of the hard magnetic Pr2Fe14B phase. This might be related to some oxidation processes leading to a reduction of the volume fraction of the hard magnetic phase. This might be a reason for the decrease of JHc in ribbon annealed at 1023 K (see Table 4). Based on the XRD studies of annealed Pr 9 Fe 65 W x B 26-x (where: x = 2, 4, 6, 8) ribbons, one can deduce that the wide crystallization peaks on the DSC and DTA curves corresponding to T x 1 (Figures 3  and 4) indicate the formation of the Pr 2 Fe 14 B and Pr 1+x Fe 4 B 4 phases. It is expected that the T x 2 may be related to the decomposition of the hard magnetic Pr 2 Fe 14 B phase. This might be related to some oxidation processes leading to a reduction of the volume fraction of the hard magnetic phase. This might be a reason for the decrease of J H c in ribbon annealed at 1023 K (see Table 4).
The transmission Mössbauer spectra measured for the Pr 9 Fe 65 W x B 26-x (where: x = 2, 4, 6, 8) ribbons annealed at 1003 K for 5 min are shown in Figure 7. The contribution from the hard magnetic Pr 2 Fe 14 B phase was represented by six sextets each of them corresponding to a distinct Fe site in this phase. They correspond to six nonequivalent positions of Fe atoms in its elementary cell. Base on the Wyckoff notation they are labeled as 16k 1 , 16k 2 , 8j 1 , 8j 2 , 4e and 4c, for which the relative intensity ratios of 4:4:2:2:1:1 were chosen. The standard fitting procedure for the Pr 2 Fe 14 B phase was based on assumptions introduced by Pinkerton et al. [47]. In order to match theoretical spectra to the experimental data, additional spectral components were added. Particularly, the doublet line corresponding to the paramagnetic Pr 1+x Fe 4 B 4 crystalline phase was appended to the fitting model [48]. In the case of the Pr 9 Fe 65 W 2 B 24 alloy (Figure 7a), the presence of the soft magnetic Fe 2 B phase was also considered [49]. The Fe 2 B phase was represented by a single sextet. The quantitative analysis of the spectrum has shown that the Pr 9 Fe 65 W 2 B 24 ribbon sample contains low volume fractions of the hard magnetic Pr 2 Fe 14 B (9 vol. %) and paramagnetic Pr 1+x Fe 4 B 4 (9 vol. %) phases. These calculations have also shown that the largest volume fraction corresponds to the soft magnetic Fe 2 B phase (82 vol. %) thus explaining relatively weak magnetic properties of the annealed Pr 9 Fe 65 W 2 B 24 ribbons. The shape of the Mössbauer spectrum measured for the Pr 9 Fe 65 W 4 B 22 alloy ribbon suggests the presence of a major fraction of crystalline phases (Figure 7b). In order to obtain a good fit an additional broad sextet line, defined by the hyperfine field distribution, was included. As there are limitations regarding the number of sub-spectra that can be independently fitted by the Mössbauer spectra analysis software, the parameters for the low-intensity sub-spectra corresponding to 4e and 4c positions of Fe atoms were arbitrary set. One could expect that the short-time heat treatment (for 5 min) led to the presence of a remnant amorphous phase. However, the broad sextet line more likely can be attributed to the highly disordered hard magnetic Pr 2 Fe 14 B phase, formed during short-time annealing and then rapid quenching in icy water. A narrow hyperfine field distribution (instead of a broad one, which is typical for the amorphous phase) was calculated. It can be an argument in favor of the formation of a highly disordered crystalline    Table 3. Maximum uncertainties for the calculated hyperfine parameters are ∆B hf = ± 0.1 T for hyperfine field, ∆QS = ± 0.05 mm/s for the isomer shift, ∆IS = ± 0.002 mm/s for quadrupole splitting.
The selected hysteresis loops J(H) measured for the Pr 9 Fe 65 W x B 26-x (where: x = 2, 4, 6, 8) alloy ribbons are shown in Figure 8. The hysteresis loops were measured as a dependence of magnetic polarization J vs. external magnetic field H, therefore, the determined coercive fields correspond to the magnetic field at which J reaches zero. J(H) measured for as-cast samples are characteristic of the soft magnetic materials. Annealing of ribbons containing 2 at. % of W did not cause significant changes in their magnetic properties. Low coercivity J H c indicate a low volume fraction of a hard magnetic Pr 2 Fe 14 B phase, which was previously confirmed by the Mössbauer spectra analysis. A change of W content resulted in an evolution of microstructure and phase constitution of annealed specimens, thus leading to significant changes in their magnetic properties. Magnetic measurements carried out on ribbons containing 4, 6 and 8 at. % of W have shown their hard magnetic properties. Especially, an increase of the coercivity J H c and maximum magnetic energy product (BH) max with the increase of the annealing temperature was observed. However, for the x = 6 and x = 8 alloys annealed at 943 K, the wasp-waisted shapes of the hysteresis loops were measured. This suggests the formation of the hard magnetic Pr 2 Fe 14 B phase within the soft magnetic amorphous matrix. For the alloys containing 4 and 8 at. % of W annealed at higher temperatures, the presence of large volume fractions of the hard magnetic Pr 2 Fe 14 B phase resulted in wide hysteresis loops. Based on our Mössbauer studies (Table 3) one can assume that the improvement of J H c is caused by the larger volume fraction of the hard magnetic phases in the vicinity of PrFe 4 B 4 type borides in the x = 4 alloy, while the presence of α-Fe and Fe 2 B for the annealed samples of the x = 2 and x = 6 alloys, respectively, resulted in a higher J r value. Higher (BH) max for the x = 8 alloy might be somewhat related to a more homogeneous microstructure of the annealed specimens (see Figure 10). It was shown that the increase of W contents at the expense of Fe resulted in a decrease of the saturation polarization J s of the as-cast specimens. The magnetic parameters measured for the Pr 9 Fe 65 W x B 26-x (where: x = 2, 4, 6, 8) alloy ribbons are collected in Table 4. Based on technical specifications of the LakeShore VSM 7307 magnetometer, uncertainties for the measurement of the magnetic field were taken as 1% of the readings from the apparatus. The maximum coercivity of J H c = (1148 ± 12) kA/m was measured for the Pr 9 Fe 65 W 4 B 22 alloy ribbon annealed at 1003 K, while the largest (BH) max = (23 ± 3) kJ/m 3 was determined for the Pr 9 Fe 65 W 8 B 18 alloy ribbon annealed at 983 K.   Table 4. Magnetic parameters: coercivity JHc, remanence polarization Jr, maximum magnetic energy product (BH)max and saturation polarization Js for annealed ribbon samples.   The microstructure of samples subjected to annealing was observed using a transmission electron microscope. A heterogeneous microstructure was observed for the Pr 9 Fe 65 W 4 B 22 sample (Figure 9).

x (at. %) T (K) JHc (kA/m) Jr (T] (BH)max (kJ/m 3 ) Js (T)
Here, next to the large grains (diameter of~70 nm), some small crystallites having diameters lower than 20 nm, were measured. Moreover, the distribution of grain sizes obtained for the Pr 9 Fe 65 W 4 B 22 alloy ribbon differs from those obtained for the Pr 9 Fe 65 W 8 B 18 alloy. For the Pr 9 Fe 65 W 8 B 18 alloy, the TEM studies have revealed the existence of agglomerates having the same crystallographic orientation that can also impact the magnetic properties of the sample (Figure 10). In the case of the Pr 9 Fe 65 W 8 B 18 alloy ribbon annealed at 1003 K, the grain size distribution was relatively wide with a maximum at 30 nm (Figure 10b). It has to be pointed out that except for the phase composition the microstructure can also play a significant role in shaping the magnetic properties. The electron diffraction patterns obtained for the investigated samples confirmed the presence of crystalline phases and indicated their nanocrystalline structure. The presence of a diffused electron diffraction pattern for the x = 8 alloy was caused by a large fraction of crystallites with the sizes lower than 10 nm. In the case of the x = 4 alloy, the presence of crystallites larger than 80 nm may give an additional point diffraction pattern superimposed on the diffused diffraction pattern. The microstructure of samples subjected to annealing was observed using a transmission electron microscope. A heterogeneous microstructure was observed for the Pr9Fe65W4B22 sample ( Figure 9). Here, next to the large grains (diameter of ∼70 nm), some small crystallites having diameters lower than 20 nm, were measured. Moreover, the distribution of grain sizes obtained for the Pr9Fe65W4B22 alloy ribbon differs from those obtained for the Pr9Fe65W8B18 alloy. For the Pr9Fe65W8B18 alloy, the TEM studies have revealed the existence of agglomerates having the same crystallographic orientation that can also impact the magnetic properties of the sample (Figure 10). In the case of the Pr9Fe65W8B18 alloy ribbon annealed at 1003 K, the grain size distribution was relatively wide with a maximum at ~30 nm (Figure 10b). It has to be pointed out that except for the phase composition the microstructure can also play a significant role in shaping the magnetic properties. The electron diffraction patterns obtained for the investigated samples confirmed the presence of crystalline phases and indicated their nanocrystalline structure. The presence of a diffused electron diffraction pattern for the x = 8 alloy was caused by a large fraction of crystallites with the sizes lower than 10 nm. In the case of the x = 4 alloy, the presence of crystallites larger than 80 nm may give an additional point diffraction pattern superimposed on the diffused diffraction pattern.

Conclusions
In the present study, the phase constitution, microstructure and magnetic properties of the nanocrystalline Pr9Fe65WxB26-x (where: x = 2, 4, 6, 8) alloy ribbons in an as-cast state and subjected to annealing were investigated. XRD and Mössbauer spectroscopy have shown that the as-cast ribbons of all alloy compositions were fully amorphous. Moreover, the analysis of the Mössbauer spectra has shown the bimodal character of the hyperfine field distribution, suggesting the existence of The microstructure of samples subjected to annealing was observed using a transmission electron microscope. A heterogeneous microstructure was observed for the Pr9Fe65W4B22 sample ( Figure 9). Here, next to the large grains (diameter of ∼70 nm), some small crystallites having diameters lower than 20 nm, were measured. Moreover, the distribution of grain sizes obtained for the Pr9Fe65W4B22 alloy ribbon differs from those obtained for the Pr9Fe65W8B18 alloy. For the Pr9Fe65W8B18 alloy, the TEM studies have revealed the existence of agglomerates having the same crystallographic orientation that can also impact the magnetic properties of the sample (Figure 10). In the case of the Pr9Fe65W8B18 alloy ribbon annealed at 1003 K, the grain size distribution was relatively wide with a maximum at ~30 nm (Figure 10b). It has to be pointed out that except for the phase composition the microstructure can also play a significant role in shaping the magnetic properties. The electron diffraction patterns obtained for the investigated samples confirmed the presence of crystalline phases and indicated their nanocrystalline structure. The presence of a diffused electron diffraction pattern for the x = 8 alloy was caused by a large fraction of crystallites with the sizes lower than 10 nm. In the case of the x = 4 alloy, the presence of crystallites larger than 80 nm may give an additional point diffraction pattern superimposed on the diffused diffraction pattern.

Conclusions
In the present study, the phase constitution, microstructure and magnetic properties of the nanocrystalline Pr9Fe65WxB26-x (where: x = 2, 4, 6, 8) alloy ribbons in an as-cast state and subjected to annealing were investigated. XRD and Mössbauer spectroscopy have shown that the as-cast ribbons of all alloy compositions were fully amorphous. Moreover, the analysis of the Mössbauer spectra has shown the bimodal character of the hyperfine field distribution, suggesting the existence of

Conclusions
In the present study, the phase constitution, microstructure and magnetic properties of the nanocrystalline Pr 9 Fe 65 W x B 26-x (where: x = 2, 4, 6, 8) alloy ribbons in an as-cast state and subjected to annealing were investigated. XRD and Mössbauer spectroscopy have shown that the as-cast ribbons of all alloy compositions were fully amorphous. Moreover, the analysis of the Mössbauer spectra has shown the bimodal character of the hyperfine field distribution, suggesting the existence of nonequivalent surroundings of Fe atoms in the amorphous phase. The hysteresis loops measured for the as-cast samples were characteristic for soft magnetic materials. Heat treatment of the samples led to precipitation of various crystalline phases. The phase constitution changes with the composition of the alloys. However, for all annealed samples, the crystallization of the hard magnetic Pr 2 Fe 14 B and the paramagnetic Pr 1+x Fe 4 B 4 phases took place. Additionally, for Pr 9 Fe 65 W 2 B 24 and Pr 9 Fe 65 W 6 B 20 alloy ribbons, the soft magnetic Fe 2 B and the α-Fe phases were observed. The crystallization of these additional crystalline phases during heat treatment can be related to higher values of the activation energy E a for crystallization. An increase in tungsten content, as well as the rise of annealing temperature, resulted in the improvement of the magnetic properties. The maximum value of coercivity J H c = (1148 ± 12) kA/m was measured for the Pr 9 Fe 65 W 4 B 22 alloy ribbon annealed at 1003 K, whereas the largest maximum magnetic energy product (BH) max = (23 ± 3) kJ/m 3 was determined for the Pr 9 Fe 65 W 8 B 18 alloy ribbon annealed at 983 K. The TEM studies revealed a heterogeneous mixture consisting of large (of diameter of~70 nm) and small (of the diameter < 20 nm) crystallites for the Pr 9 Fe 65 W 4 B 22 alloy. For the Pr 9 Fe 65 W 8 B 18 ribbon, the largest number of grain reached sizes of~30 nm. TEM dark-field studies of the Pr 9 Fe 65 W 8 B 18 alloy specimen revealed a presence of agglomerates having the same crystallographic orientation. This might be a reason for the highest (BH) max measured for the ribbon annealed at 1003 K.