Shouzeng Zhou

Effects of as-quenched structures on the phase transformations and magnetic properties of melt-spun Pr7Fe88B5 ribbons

Phase transformations and magnetic properties of overquenched Pr7Fe88B5 ribbons during annealing have been investigated. X-ray diffraction and Mossbauer measurement indicate that melt spinning at different wheel velocities caused the as-quenched ribbons to have distinctive structures. Depending on their as-quenched structure, the phase transformation of the ribbons during annealing may take place in one of the following sequences: (1) amorphous phase (Am)+Pr2Fe14B+α-Fe→Pr2Fe14B+α-Fe; (2) Am+α-Fe→Am′+α-Fe→α-Fe+1:7 phase+Pr2Fe14B→Pr2Fe14B+α-Fe; and (3) Am→Am′+α-Fe→1:7 phase+α-Fe→Pr2Fe14B+α-Fe. In all cases, the microstructure of the ribbons after optimal annealing was found to only consist of two magnetic phases: Pr2Fe14B and α-Fe. However, with increasing initial quenching rate, the microstructure of optimally heat treated ribbons becomes coarser and more irregular, and the magnetic properties of them deteriorated drastically. The δM plots, irreversible susceptibility, and the temperature dependence of coe...

Magnetostriction and microstructure of the melt-spun Fe83Ga17 alloy

The ribbons with different thicknesses of Fe83Ga17 alloy were prepared by melt spun. The maximum magnetostriction of −2100ppm has been obtained in the ribbon with the thickness of 75μm. The microstructures of the ribbons were determined by x-ray diffraction. It was found that DO3 structure emerges in those ribbons melt spun at a higher cooling rate. This special DO3 structure is favorable to the enhancement of magnetostriction. It is considered that more short-range ordering of Ga atoms appeared when liquid alloy was solidified with a certain extreme cooling rate. Such short-range ordering of Ga atoms brings a local stress and results in the giant magnetostriction. The large demagnetizing magnetic energy in the normal direction of the ribbons causes the magnetic moments parallel to the ribbon plane. When an applied magnetic field is perpendicular to the ribbon plane, the magnetic moments turn 90° and generate giant magnetostriction.

High-performance α-Fe/Pr2Fe14B-type nanocomposite magnets produced by hot compaction under high pressure

The α-Fe/R2Fe14B-type nanocomposite magnets have been prepared by hot pressing melt spun Pr8Dy1Fe74.5Co10Nb0.5B6 flakes under a conventional pressure P of 125 MPa and high pressures ranging from 1 to 7 GPa. It was found that increasing compaction pressure from 125 MPa to 5 GPa led to marked grain refinement in the magnet and consequently resulted in significant improvement of magnetic properties. When hot pressing under even higher pressure (P>5 GPa), however, the crystallization was constrained and the hot pressed magnets retained a certain amount of amorphous phase besides the Pr2Fe14B-type phase and α-(FeCo) phase, which resulted in the deterioration of the magnetic properties. A remanence of 11.1 kG, coercivity of 10.2 kOe, and maximum energy product of 23.6 MGOe have been achieved in the magnet hot pressed under a pressure of 5 GPa.

High-coercivity (NdDy)2(FeNb)14B–α–Fe nanocrystalline alloys

High coercivity, high remanence, and high energy product (NdDy)2(FeNb)14B–α–Fe nanocrystalline alloys containing 0 to 30 wt % α–Fe have been prepared by melt spinning and subsequent annealing. The best magnetic properties of remanence (Br), coercivity (Hci), and maximum energy product [(BH)max] are 1.02 T, 702 kA/m, and 134 kJ/m3, respectively, for Nd8.16Dy1Fe85.26Nb1B4.58. The microstructure consists of a two phase nanocomposite of hard magnetic (NdDy)2(FeNb)14B and soft magnetic α–Fe with an average size of about 30 nm. These small dimensions allow effective exchange coupling between hard and soft magnetic grains and result in the simultaneous enhancement of Br, Hci, and (BH)max. A systematic study on the effect of annealing temperature and time on the microstructure and magnetic properties has been carried out.

Beneficial effects of Cu substitution on the microstructures and magnetic properties of Pr2(FeCo)14B/α-(FeCo) nanocomposites

Abstract We studied the phase composition, microstructures and magnetic properties of melt-spun Pr 8.5 (Fe 0.8 Co 0.2 ) 86.5− x Cu x B 5 ( x =0, 0.5, 1.0, 2.0, 3.0) nanocomposites. It was found that Cu addition suppresses the formation of Pr 2 (CoFe) 17 phase and results in a two-phase mixture of α-(FeCo)/Pr 2 (FeCo) 14 B. Transmission electron microscopy (TEM) and energy-dispersive X-ray analysis (EDX) demonstrate that Cu strongly segregates at intergranular regions, leading to drastic decrease of the grain sizes of both 2:14:1 phase and α-(FeCo) phase. Wohlfarth remanence analysis indicates that the impact of Cu addition on the strength of exchange coupling interactions between hard and soft magnetic phases in the ribbons is determined by two opposite factors: (1) grain refinement and (2) grain isolation especially at higher Cu content. A small amount of Cu addition ( x =0–3.0) has little effect on the Curie temperature of 2:14:1 phase. The magnetic properties of B r , i H c , and ( BH ) max of optimally processed Pr 8 5 (Fe 0 8 Co 0.2 ) 86.5− x Cu x B 5 ribbons initially increase with increasing Cu content from x =0 to 0.5 but all of them decrease drastically with further increasing Cu content, which can be mainly attributed to the variation of exchange coupling effect between hard and soft magnetic grains and the decrease in saturation magnetization. A combination of i H c =7.4 kOe, B r =10.9 kG and ( BH ) max =20.1 MGOe has been obtained in Pr 8.5 (Fe 0.8 Co 0.2 ) 86 Cu 0.5 B 5 ribbon. Moreover, minor addition of Cu ( x =0.5) is effective in reducing the irreversible loss of induction ( h irr ) of the studied Pr 2 (FeCo) 14 B/α-(FeCo) nanocomposite magnets.

Preparation and magnetic properties of melt-spun Nd2Fe14(BC)/α-Fe nanocomposite magnets

Phase evolution and magnetic properties of melt-spun Nd8Fe86B6−xCx (x=0, 2, 4, 5, 6) alloy ribbons have been investigated. Increasing the C content was found to decrease the glass-forming tendency of as-spun ribbons. For samples with low C content (x⩽4), the best magnetic properties were achieved by directly quenching at an optimum roll speed, by which a uniform exchange coupled nanocomposite Nd2Fe14(BC)/α-Fe structure was developed. Moreover, within this composition range, C substitution did not significantly affect the microstructure of the optimally quenched ribbons. However, for the samples with higher C content (x>4), the composite 2:14:1/α-Fe structure can only be obtained by a phase transformation from the mixture of 2:17:Cx+2:14:1+α-Fe phases during annealing treatment. Moreover, the resultant 2:14:1/α-Fe composite structure was irregular, with a coarse grain size, which strongly degraded the exchange interaction between hard and soft magnetic phases. For optimally processed samples, replacement o...

Microstructure evolution and magnetic properties of overquenched Pr8Fe86B6 ribbons during annealing

Microstructure evolution and magnetic properties of overquenched Pr8Fe86B6 ribbons during annealing have been investigated. The results showed that, in as-quenched state, the microstructure of the ribbons consists of a mixture of amorphous phase (Am)+Pr2Fe14B+a-Fe, Am+a-Fe and only amorphous phase, respectively, for the wheel speed of 22, 26, and 30 m/s. Depending on the overquenched precursor, the microstructure evolution of the ribbons during annealing can be classified into: (1) Am+Pr2Fe14B+a-Fe→Pr2Fe14B+a-Fe; (2) Am+α-Fe→Am′+a-Fe→a-Fe+Pr2Fe23B3+Pr2Fe14B→Pr2Fe14B+a-Fe; and (3) Am→Am+a-Fe→Pr2Fe23B3+a-Fe→Pr2Fe14B+a-Fe. In all cases, the microstructure of optimally annealed ribbon samples consist of magnetically hard Pr2Fe14B and soft magnetic a-Fe phases. The magnetic properties achieved by optimal annealing were found to be strongly dependent upon the initial quenching rate of the unannealed precursor. A fairly significant drop in both Hci and Br was observed with the increase of quenching rate of the p...

Effects of C content on the formation and magnetic properties of Nd2Fe14(BC)/α-Fe nanocomposite magnets

The phase evolution, microstructure, and magnetic properties of Nd8Fe86B6−xCx (x=0, 2, 4, 5, 6) melt-spun ribbons were systematically studied as a function of C content. It was found that the addition of C decreases the glass-forming tendency of the as-spun ribbons significantly. A uniform nanoscale exchange coupled Nd2Fe14(BC)/α-Fe microstructure with an average grain size of 20–25 nm can be developed in the directly quenched ribbons with C contents up to 4 at. %. Further increase of C content to x=5 leads to, in the optimally quenched ribbons, the presence of an undesirable Nd2Fe17Cx phase in addition to the 2:14:1 and α-Fe phases, whereas the alloy ribbon containing 6 at. % C consists almost entirely of the soft magnetic Nd2Fe17Cx and α-Fe phases. Subsequent annealing induces a transformation of the 2:17:Cx phase to the 2:14:1 phase +α-Fe in the ribbons with x=5 and 6, resulting in the formation of a composite 2:14:1/α-Fe structure having relatively large crystallite sizes. Magnetic measurements reveal...

Microstructure and magnetic properties of Sm2(Fe1 − xGax)17 and nitrides

Abstract The crystal structure and microstructure of Sm 2 (Fe 1 − x Ga x ) 17 ( x = 0−0.12) have been studied by X-ray diffraction and scanning electron microscopy analysis techniques. Results indicate that the alloys have a single phase with Th 2 Zn 17 -type structure for x ⩽ 0.02; when x = 0.05−0.12, two phases, namely Sm 2 (Fe, Ga) 17 and (Ga, Fe)Sm, exist and, the higher the gallium substitution the more (Ga, Fe)Sm phase occurs. The lattice parameters increase with Ga addition. The Curie temperature and saturation magnetization of the nitrides decrease with increasing gallium substitution; however, the anisotropy field reaches a maximum value of 18.4 T for x = 0.02 at room temperature. The hard magnetic properties of the nitrides decrease also except for x = 0.02. The optimum hard magnetic properties obtained are B r = 12.3 kG, i H c = 10.0 kOe and ( BH ) max = 26.2 MG Oe with x = 0.02. The thermomagnetic curves measured of all the nitrides have shown that the onset decomposition temperatures are 760–780 K, which are slightly affected by gallium substitution.

Intrinsic and hard magnetic properties of Sm2(Fe1 − xAlx)17 nitrides

Abstract The intrinsic and hard magnetic properties of Sm 2 (Fe 1 − x Al x ) 17 nitrides were investigated. The Curie temperature and saturation magnetization decrease linearly with increasing Al concentration, as also does the anisotropy field at room temperature. The X-ray diffraction patterns have shown that all the samples exist in a nearly single phase with Th 2 Zn 17 -type structure for x up to 0.12. The lattice parameters increase with the substitution of Al for Fe atoms. The hard magnetic properties of the nitrides obtained indicate that the addition of Al has an unfavorable effect. The onset decomposition temperature of all nitrides derived from the thermomagnetic curves is around 750 K, implying that Al substitution could not improve the thermal stability of 2:17 nitrides.