Maocai Zhang

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.

Effects of quenching rate on the phase transformation and magnetic properties of melt-spun Pr8Fe86B6 ribbons during annealing

Abstract Phase transformations and magnetic properties of overquenched Pr 8 Fe 86 B 6 ribbons during annealing treatment have been investigated. The as-quenched structure of the ribbons was varied by changing the quenching rate during the melt spinning process. It was found that, depending on their as-quenched structures, the phase transformations of the ribbons during annealing may take place in one of the following sequences: (1) Amorphous phase (Am)+Pr 2 Fe 14 B+α-Fe→Pr 2 Fe 14 B+α-Fe; (2) Am+α-Fe→ Am′+α-Fe→α-Fe+Pr 2 Fe 23 B 3+ Pr 2 Fe 14 B→Pr 2 Fe 14 B+α-Fe; and (3) Am→Am′+α-Fe→Pr 2 Fe 23 B 3 +α-Fe→ Pr 2 Fe 14 B+α-Fe. However, for all the ribbons, the microstructure after optimal annealing was found to consist only of two magnetic phases: Pr 2 Fe 14 B and α-Fe. The optimum magnetic properties (intrinsic coercivity H ci and remanent polarization J r ), and the squareness of the demagnetization curves of the annealed ribbons deteriorated drastically with increasing quenching rate of unannealed precursors. This deterioration can be attributed to the formation of a coarser and more irregular microstructure during annealing of the samples initially melt spun with higher wheel speeds, which was confirmed by our TEM observations, analysis of the temperature dependence of coercivity and measurement of irreversible susceptibility.

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.

Phase transformations and magnetic properties of melt-spun Pr7Fe88B5 ribbons during annealing

Abstract X-ray diffraction and Mossbauer measurements indicate that melt spinning at different wheel velocities causes as-quenched Pr7Fe88B5 ribbons to have distinctive structures. Depending on their as-quenched structures, the phase transformations 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. However, for all the ribbon samples, the microstructures after optimal annealing with respect to magnetic properties were found to only consist of two magnetic phases: Pr2Fe14B and α-Fe. The optimum magnetic properties of H c j and Jr, and the squareness of the demagnetization curves of the annealed ribbons deteriorate drastically with increasing quenching rate of unannealed precursors. Taking into account TEM results, the values of αex and Neff derived from the temperature dependence of the coercivity, these deteriorated effects can be attributed to the formation of a coarser and more irregular microstructure during annealing in the samples initially melt spun with higher wheel speeds.

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...

Magnetic Properties and Phase Transformation of Tb1-x PrxFe1.96 (x = 0 to 0.7) Compounds

The magnetic properties and the phase Transformation of Tb1-xPrxFe1.96(x = 0 to 0.7) compounds were studied by means of vibrating sample magnetometer(VSM), X-ray diffraction(XRD) and SEM back-scattered electron (BSE). The result indicates that the saturation magnetization σs of compounds along an easy axis decreases with the addition of Pr contents, which reduces from 77.24 Am2 · kg−1 (x = 0) to 11.84 Am2·kg−1(x = 0.5) and then returns to 37.14 Am2·kg−1 (x = 0.7). The non-cubic phases appear when x exceeds 0.2, and the matrix of Tb1-xPrxFe1.96 compounds changes from (Tb, Pr)Fe2 phase with x = 0 to (Tb, Pr)Fe3 phase with x = 0.4, and at last to (Tb, Pr)2Fe17 phase in Tb0.3Pr0.7Fe1.96. Moreover, the structure of the compounds may become more complex with the increase of Pr content.