The Influence of Heat Treatment on the Phase Structure of the Nanocrystalline (Nd 10 Fe 67 B 23 ) 95 Nb 5 Alloy Ribbons

The main goal of present paper was to study of phase composition of nanocrystalline (Nd 10 Fe 67 B 23 ) 95 Nb 5 ribbons prepared by melt-spinning technique with different velocity of the copper wheel. Samples in the as-cast state obtained using speed 5 and 10 m/s had nanocrystalline structure. In case of samples prepared by with speed of copper Wheel 15 and 35 m/s were fully amorphous. In the DSC curve, the two minima corresponding to crystallization of the Nd 2 Fe 23 B 3 and Nd 2 Fe 14 B phases were detected. The coexistence of three phases: Nd 2 Fe 14 B, Nd 2 Fe 23 B 3 and Nd 1+ε Fe 4 B 4 were detected. Higher temperatures of annealing caused crystallization of α-Fe phase. The XRD results were confirmed by the Mössbauer studies.

The technology of materials production has strong influence on their microstructure and magnetic properties. Such problem is the fundamental of materials engineering. During materials obtaining process it is necessary to produce sample with required chase composition, which is basic condition to form microstructure. Magnetic properties of material partially depend on phase composition and are formed by microstructure. Proper phase composition guaranties to obtain an optimal magnetic properties and is reached during controlled crystallization in low temperature annealing process [1][2][3].
The Nd-Fe-B-type compound, in the Fe-rich range, contains following phases [4]: -hard magnetic Nd2Fe14B phase -B-rich Nd1-εFe4B4 phase -Nd-rich Nd-Fe phase [5], -soft magnetic: α-Fe, Fe2B, Nd2Fe17 phases. The Nd2Fe14B compound is characterized by high coercivity JHc, high density of magnetic energy (BH)max, and relatively low the Curie temperature TC=585 K. The main objective of modification of chemical composition of this compound was an increase of the Curie point. Partial substitution of Fe by Nb causes increasing the TC of the Nd2Fe14B phase. However, its saturation magnetisation Js is decreased while anisotropy field HA raises. The chemical compositions of composite permanent magnets are chosen in order to produce biphasic structrure based on the α-Fe -Nd2Fe14B. [6][7][8]. The main goal of present paper was to study of the (Nd10Fe67B23)95Nb5 alloy.

Experimental part
Materials to studies was the (Nd10Fe67B23)95Nb5 alloy. The ingots were prepared by arc-melting of the high purity of constituent elements (more than 3N purity) under low pressure of Ar. Then ribbons were prepared Rusing melt spinning technique under Ar atmosphere. The several linear speeds of copper wheel were used and they equaled 5, 10, 15, and 35 m/s. In order to produce nanocrystalline microstructure, the prepared ssamples were sealed off in quartz tubes under low pressure of Ar and annealed in the range of temperatures from 923 to 1063K (with step 20K) for 5 min. and then cooled in ice water [9,10].
Phase structure was investigated using Bruker D8 Advance X0-ray diffractometer with CuKα radiation and semiconductor Lynxeye detector. Mössbauer spectra were measured at room temperature using Polon Mössbauer spectrometer working in a transmission geometrywith the 57Co source in the Rh matrix (of the activity of 50 mCi). The studies were carried out on samples crushed to the powder in order to obtain a representation of the entire volume. The analysis was performed using a thin absorber approximation. The Mössbauer spectra were fitted with the WinNORMOS for Igor 6.04 package. Error bars equaled ± 0,1T for hyperfine fields (Bhf), ± 0,02 mm/s for isomeric shifts (IS) oraz ± 0,05 mm/s for quadrupole splitting (QS). [Cite your source here.]

Results and discussions
In order to prepare ribbons with different thickness, four different speeds of copper wheel were used Turing melt spinning process. The increase of speed of cooper wheel caused to obtain lower thickness of ribbon: 5m/smean ribbon thickness d=(602) m, 10m/s -mean ribbon thickness d=(352) m, 15m/s -mean ribbon thickness d=(252) m and 35 m/s -mean ribbon thickness d=(202) m.
The X-ray diffraction studies were carried out for all prepared samples of the (Nd10Fe67B23)95Nb5 alloy in as cast state. The analysis of X-ray pattern ( Fig. 1) revealed that in samples with thickness 60, 35, 25 and 20 μm, some reflexes with low intensity are observed, which allowed us to conclude that in samples some crystalline phases present. However, due to low intensity the phases identification was not possible. Moreover, visible braodering of these peaks is related to low volume fraction of these phases and low dimension of grains. Based on the analysis of the XRD patterns of samples in the as-cast state, the sample obtained using the highest speed of the copper wheel was taken for further studies.
The DSC curve was measured under flow of inert gas (Ar) with a rate of temperature changes 10K/s (Fig. 2). Based on this curve, the temperatures of crystallization begin was revealed Tx1 = 964 K corresponding to formation of the metastable Nd2Fe23B3 phase and Tx2 = 979 K related to Nd2Fe14B phase. Moreover, some other unrecognized peaks were observed. It was helped to reveal temperature range of annealing from 923K to 1063K. Based on results of Zhanga et al [11] and Tamura et al [12], a time of annealing was defined as 5 min. Then, the samples were annealed in the range of temperatures from 923K to 1063K with step 20K for 5min. in order to produce nanocrystalline microstructure with hard magnetic Nd2Fe14B phase. In the next step, we carried out X-ray diffraction of the (Nd10Fe67B23)95Nb5 alloy ribbons. The analysis of the XRD patterns (Fig. 3) collected for samples annealed at 923K and 943 allowed for identification of hard magnetic Nd2Fe14B [13], paramagnetic Nd1+εFe4B4 [11] and metastable Nd2Fe23B3 [14,15].
The annealing of samples at 963K and higher temperatures caused to nanocrystalization and growth of grains Nd2Fe14B, Nd2Fe23B3 , Nd1+εFe4B4 phases, while the changes of phase structure was not observed. The phase composition was changed at 1023, 1043 and 1063K (Fig. 4), when additional reflexes corresponding to α-Fe were detected [11,12]. The Mössabuer studies confirmed an occurrence of α-Fe phase in sample annealed at 1063K. The presence of soft magnetic α-Fe phase in sample annealed at higher temperatures could be related to degradation of the hard magnetic Nd2Fe14B phase in this temperature range.  Fig. 3. The XRD patterns of the (Nd10Fe67B23)95Nb5 alloy ribbons Fig. 4. The XRD patterns (Nd10Fe67B23)95Nb5 alloy Ribbon in as-cast state and annelaed at 923 and 943K for 5 min.
The XRD studies allowed to calculate mean grain size of recognized phases (annealed at 1023K, 1043K and 1063K). In order to reveal mean grain size, the Scherrers method was used [17]. The broadening of diffraction reflexes was used. Scherrers equation was used in following form: is the Scherrers constant [18] and 54051 , 1   Å is wavelength of X-ray radiation [14] . Results of grain size investigation for all samples were collected in Table 1. Based on these results it was revealed that the temperature of annealing has not an influence on mean grain size of ferromagnetic Nd2Fe14B, Nd2Fe23B3 and α-Fe phases. In case of the Nd1+εFe4B4 phase, a decrease of grain size is observed with an increase of temperature annealing. For sample annealed at 1063 was characterized by the smallest grain size. Al-Khafaji and coworkers [19] investigated the influence of phase composition on magnetic properties of the Nd-Fe-B type alloys produced by melt-spinning. Studies carried out for the Nd11,8Fe82,3B5,9, Nd9,5Fe84,5B6 and Nd18Fe76B6 alloys revealed that mean grain size was 35 nm for Nd2Fe14B phase and 15 nm for -Fe, which corresponds well with results delivered in present work for the (Nd10Fe67B23)95Nb5 alloy. The nanocrystalline structure of produced samples was confirmed.
The Mössabuer spectroscopy allowed to do more precise identyficatation of the Nd2Fe14B phase, paramagnetic Nd1+εFe4B4 phase and metastable Nd2Fe23B3 phase in investigated samples. Correct identification based on XRD studies was not possibile due to ovaerlapping peaks corresponding to different phases.
In Fig. 5, the Mössbauer spectrum of the (Nd10Fe67B23)95Nb5 alloy ribbon with thickness 20 m in the as-cast state. Sample was powdered, in order to investigate its whole volume. The Mössbauer studies were carried out at room temperature. The experimental spectrum was fitted by three subsectrums corresponding to following phases: Nd2Fe14B, NdFe4B4 and amorphous. http://www.revistadechimie.ro The Mössbauer studies of the Nd9,5Fe63,65B21,85Nb5 alloy ribbon revealed that material is not fully amorphous The analysis showed the presence of the hard magnetic Nd2Fe14B [20][21][22]. Moreover, one paramagnetic doublet was assigned to the Nd1+Fe4B4 phase [23][24][25].
All parameters of hyperfine structure delivered by the Mössbauer studies were collected in Tab. 2. Moreover, the analysis of the spectrum allowed to determine weight contents of recognized phases. An investigated sample was built by following phases: amorphous (85 wt.%)and the rest one was Nd2Fe14B (3.5 wt.%) phase and Nd1+Fe4B4 (11.1 wt.%).
In Fig. 6, the Mössbauer spectra registered for the (Nd10Fe67B23)95Nb5 alloy ribbon, annealed at 1043K (a) and 1063K (b) for 5 min. together with distribution of hyperfine fields P(Bhf) corresponding to theoretical curve. During spectrum analysis, contributions from the hard magnetic Nd2Fe14B, paramagnetic Nd1+εFe4B4 and metastable Nd2Fe23B3 were considered. The paramagnetic Nd1+εFe4B4 phase was described by single doublet. The presence of the Nd2Fe14B phase is represented by six Zeeman sextets corresponding to six nonequivalent positions of Fe atoms in the elementary cell. Moreover, during calculations, three Zeeman sextets were assigned to the metastable soft magnetic Nd2Fe23B3 phase [26][27][28]. In order to increase accuracy of fitting, the additional line corresponding to continous distribution of the hyperfine fields was added. This Line is related to the presence of disordered phase. Fig. 6. The Mössbauer spectra of the (Nd10Fe67B23)95Nb5 alloy Ribbon annealed at 1043K (a) and 1063K (b) for 5 min. tog heter with distribution of hyperfine fields P(Bhf) corresponding to phases with continous distribution of hyperfine fields.
In Table 2 volume contents of recognized phases in the sample of (Nd10Fe67B23)95Nb5 were collected. An increase of annealing temperature, the rise of volume content of crystalline phases is observed at the expense of disordered phase. In case of sample with temperature of heat treatment 1063K, the presence of small amount of the soft magnetic -Fe phase (about 3 wt.%) [23,[29][30][31], which was not observed at lower temperatures. The Mössbauer studies revealed changes in phase constitution during heat treatment. Percentage content of the hard magnetic Nd2Fe14B phase has a strong influence on values of magnetic parameters of produced sample. Moreover, the presence of paramagnetic phase in sample affects on the saturation magnetization and maximum density of magnetic energy.

Conclusions
The analysis of the XRD patterns revealed that heat treatment causes changed in the structure of crystalline ribbon. The recognized phases were Nd2Fe14B, Nd1+εFe4B4, α-Fe (this phase was observed at 1063K). Moreover, short time annealing allowed to form metastable Nd2Fe23B3 phase, which proves nanocrystalline structure. An increase of temperature caused an increase of the percentage content of crystalline phases. The sample in the as-cast state was generally built by amorphous phase (85 wt.%), with small amount of the hard magnetic Nd2Fe14B phase (3.5 wt.%) and paramagnetic Nd1+Fe4B4 phase (11.1 %wag.). For sample annealed at 1063K, the content of desired hard magnetic Nd2Fe14B phase rises to ~30.1 wt.% to 3.5 wt.% for sample in as-cast state.