Magnetic hysteresis properties and structure of melt-spun (Nd0.8-xCexZr0.2)(Fe0.75Co0.25)11.3Ti0.35V0.35 alloys (x = 0-0.3) after annealing and nitriding

We have studied the effect of the arc-melting, melt-spinning, annealing, nitriding and doping with cerium on the structure and magnetic hysteresis properties at room temperature of alloys based on the compound (Nd0.8Zr0.2)(Fe0.75Co0.25)11.3Ti0.35V0.35 with the ThMn12-type. We found that the optimal magnetic hysteresis properties were obtained for (Nd0.8Ce0.2Zr0.2)(Fe0.75Co0.25)11.3Ti0.35V0.35 after combinated treatment that consists in melt-spinning, annealing, and nitriding.


Experimental
Alloys of the compositions (Nd0.8-xCexZr0.2)(Fe0.75Co0.25)11.3Ti0.35V0.35 (x = 0-0.3) 110-120 g in weight were obtained by arc-melting in an argon atmosphere. Rapidly quenched ribbons were obtained from these ingots by melt-spinning in an argon atmosphere using a DVX-II set up equipped with a rotating copper wheel. The linear rotation speed of the copper wheel was 30 m/s. A portion of these ribbons was annealed in vacuum at 700 °C for 15 min and was pulverized into particles less than 45 µm in diameter. The chemical compositions of samples were determined by X-ray fluorescent analysis using a Rigaku ZSX Primus II X-ray fluorescence wave-dispersion spectrometer. X-ray diffraction (XRD) studies were performed on a Rigaku Ultima IV diffractometer (CoKα radiation, λ = 1.79021 Å). X ray diffraction patterns were processed by the Rietveld method using the Rigaku PDXL 2 software. The microstructure of the cast alloys was studied on a Tescan Vega 3SB scanning electron microscope (SEM) equipped with an Oxford Instruments EDX detector for elemental microanalysis. The micro- 2 structure of the rapidly quenched samples was studied by transmission electron microscopy (TEM) using a JEOL JEM 1400 electron microscope. The magnetic hysteresis properties of samples were measured at room temperature in magnetic fields up to 1.6 MA/m (20 kOe) using a VSM-250 system. Table 1 shows chemical compositions of the as-cast samples, which are very close to the (Nd0.8-  Table 2. Samples after arc-melting were multi-phase: (Nd, Ce, Zr)(Fe, Co, Ti, V)12 (I4/mmm) (60-72 %), α-(Fe, Co) (Im-3m) (20-27 %), (Fe, Co)2(Ti, V) (P63/mmc) (0-5 %) and (Nd, Ce)2(Fe, Co)17 (R-3m) (5-10 %). As seen from Table 2, as the Ce content increases from x = 0 to 0.3, the lattice parameter a of the main 1-12 phase decreases from 8.456 Å to 8.413 Å and the lattice parameter c decreases from 4.894 Å to 4.879 Å. The results of quantitative XRD analysis of phases observed for the Ce side of the alloy series in cast state are confirmed by the SEM-EDS data. SEM micrographs taken in backscattered electron mode demonstrate the presence of three phases in the sample with x = 0.0 and four phases in other samples ( Fig. 1). According to data of SEM-EDS analysis, the chemical composition of the main 1-12 phase was found to be (Nd0.80Zr0.20)(Fe0.74Co0.26)9.8Ti0.23V0.20 for

Melt-spun, annealed and nitrided alloys
Results of the XRD analysis for samples after melt-spinning and after annealing at 700 °C for 15 min and nitriding are shown in Table 3. The volume fraction of the compound with the ThMn12-type structure after melt-spinning increases non-monotonically from 84 % to 88 % with increasing Ce content from x = 0 to 0.3. The maximum volume fraction of the 1:12 phase (93 %) was obtained for the sample with x = 0.2. It was found that the melt-spinning leads to the formation of a small content of α-(Fe, Co) phase. The unit cell volume (V) of the 1:12 phase in the as-spun samples also increases nonmonotonically with increasing Ce content. The maximum value of V = 350.02 Å 3 was obtained for sample with x =0.2. Fig. 3 shows a bright-field TEM image and electron diffraction pattern for the (Nd0.6Ce0.2Zr0.2)(Fe0.75Co0.25)11.3Ti0.35V0.35 as-spun alloy. The results of TEM analysis agree with the XRD data. The general microstructure of the alloy is the 1:12 phase in an equiaxed polycrystalline state with an average grain size of 120-180 nm. An amorphous phase was found to be present along grain boundaries of the main phase.
As seen from Table 3, the unit cell volume of the main 1:12 phase in annealed samples nonmonotonically increases from 344.08 Å 3 to 348.32 Å 3 with increasing Ce content from x = 0 to 0.3. The maximum value of V = 351.06 Å 3 was obtained at x =0.2. The maximum volume fraction (84 %) of the 1:12 phase was obtained for annealed sample with x = 0.2. The unit cell volume of the main 1:12 phase for samples after nitriding non-monotonically increases from 350.02 Å 3 to 357.24 Å 3 with increasing Ce content from x = 0 to 0.3. The maximum value of V = 360.41 Å 3 was obtained at x =0.2. The maximum volume fraction (79 %) of the 1:12 phase was also obtained for annealed sample with x = 0.2 (see Table 3). The average value of the volume effect (the increase in the unit cell volume after nitriding) was obtained to be ~2.5 %. For example, Figure  2 shows X-ray diffraction patterns of the (Nd0.6Ce0.2Zr0.2)(Fe0.75Co0.25)11.3Ti0.35V0.35 sample after arc-melting, melt-spinning, annealing and nitriding (in different structural state). Fig. 2

Magnetic hysteresis properties
Magnetic hysteresis properties for the (Nd0.8-xCexZr0.2)(Fe0.75Co0.25)11.3Ti0.35V0.35 alloys in various structural states are shown in Table 4. The arc-melted alloys were characterized by coarse-grained state and had the low both coercive force iHc (7.0-8.8 kA/m) and remanence magnetization σr (5.0-5.5 А•m 2 /kg). The saturation magnetization σs decreases from 154 to 150 А•m 2 /kg as the Ce content increases from x = 0 to x = 0.3. As seen from Table 4, the coercive force, remanence magnetization, the saturation magnetization of the samples after melt-spinning with the Ce content of in the range x = 0-0. The increase in the magnetic hysteresis properties of the melt-spun samples at room temperature as compared to those of as-cast alloys is related to the fact that the main 1:12 phase has nanocrystalline grains which are magnetically coupled with each other or with α-(Fe, Co) grains (trace amount).
The coercive force, remanence magnetization, saturation magnetization of the samples with the content Ce x = 0-0.3 after annealing non-monotonically increase from 20. 6  As is seen from Table 4, the coercive force, remanence magnetization, saturation magnetization of the samples after nitriding with the content Ce x = 0-0.3 non-monotonically increase from 28.8 to 40.5 kA/m, from 24.6 to 28.5 A•m 2 /kg, from 150 to 154 A•m 2 /kg, respectively. The maximum values of iHc = 56.8 kA/m, σr = 34.2 A•m 2 /kg and σs = 157 A•m 2 /kg were obtained for sample with x = 0.2. The increase in the magnetic hysteresis properties at room temperature of samples after nitriding is likely to be related to the volume effect of the main 1:12 phase. 40.5 (509) 28.5 154 The observed differences in the magnetic hysteresis characteristics of the alloys in various states are associated with their features of the microstructure (the volume fractions of constituent phases and their chemical compositions) and substructure (dispersity of grains and level of microdeformations).