Ming-Xi Mao

EFFECT OF HEAT TREATMENT ON THE PHASES OF MELT-SPUN NDFEB ALLOYS

The phase composition of crystallized NdxFe81.5−xB18.5 (x=3, 4, 4.5, 6, 12, and 15) alloys annealed under optimal condition are investigated by means of zero‐field spin‐echo nuclear magnetic resonance (NMR) and Mossbauer effect (ME), together with x‐ray diffraction (XRD). It is found that the phase composition depends on the Nd concentration. The samples with 12–16 at. %‐Nd contain Nd2Fe14B magnetically hard phase and Nd1.1Fe4B4 paramagnetic phase. Their large coercive fields are due to the very fine size of Nd2Fe14B crystallite below the critical size of single domain particles, and some minor phases precipitated at grain boundaries, which have a pinning domain wall effect. The samples with a lower Nd concentration (x=3–6 at. %) consist of body‐centered‐tetragonal (bct)‐Fe3B and a small amount of α‐Fe, and no Nd2Fe14B and Nd1.1Fe4B4. Furthermore, the NMR results show that the spectra corresponding to bct‐Fe3B in the samples annealed at 670 °C distort and broaden more seriously than that of pure bct‐Fe3B ...

On the short-range order in the Fe84B10C6 amorphous alloy

Abstract Differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) and Mossbauer effect (ME) experiments have been carried out in order to study the short-range order (SRO) in the Fe 84 B 10 C 6 amorphous alloy. NMR provides clear evidence for the existence of both body-centered tetragonal Fe 3 B-like and orthorhombic Fe 3 B-like SRO in the as-quenched alloy. The NMR results are used as a reference to determine the optimal heat treatment conditions under which the SRO existing in the as-quenched alloy nucleates and grows without undergoing a phase transition. It was found that when the alloy is annealed at the onset temperature for the second stage of the crystallization process, which corresponds to the crystallization initiation point for the SRO in the amorphous alloys, the crystallized precipitates have structures similar to that of the SRO originally existing in the as-quenched alloy.

THE ORIGIN OF HARD MAGNETIC-PROPERTIES IN RAPIDLY QUENCHED ND4FE77.5B18.5 ALLOY

Until now, the origin of hard magnetic properties of rapidly quenched Nd-Fe-B alloys with lower Nd concentration is not clear. In this paper, the phase compositions of rapidly quenched Nd4Fe77.5B18.5 alloys annealed under different conditions have been studied by using zero-field spin-echo nuclear magnetic resonance (NMR) and Mossbauer effect (ME) techniques. It is found that there exists a certain amount of Nd2Fe14B phase in the samples annealed at 960-degrees-C and ingot alloy, which have poor hard magnetic properties; while, the sample annealed under optimal condition consists only of bct-Fe3B as the main phase and a small amount of a-Fe. However, the ME result indicates that about 5 at. % Fe atoms in Fe(III) (8 g) site of bct-Fe3B have been replaced by Nd atoms; the NMR result demonstrates that B-11 NMR spectrum is the characteristic peak of bct-Fe3B, but it broadens asymmetrically to the high frequency side, which is due to the bct-Fe3B influenced by Nd atoms. The amplitude of radio frequency (rf) excitation field required to get the maximum B-11 spin-echo signal from bct-Fe3B in the sample annealed at 839-degrees-C is only about one third as much as that required to excite the B-11 in the bct-Fe3B influenced by Nd atoms in the sample annealed at 670-degrees-C for a short time, which implies the latter has a larger coercivity field than the former. It is concluded that the origin of hard magnetic properties of Nd4Fe77.5B18.5 alloy is not related to the 2:14:1 phase, but to the change of bct-Fe3B itself

NUCLEAR-MAGNETIC-RESONANCE AND MOSSBAUER STUDIES OF RAPIDLY QUENCHED AND HEAT-TREATED ND4FE77.5B18.5 ALLOYS

Using the Mossbauer effect and NMR, and phase compositions of rapidly quenched Nd4Fe77.5B18.5 alloy under different heat treatment conditions were studied. It was shown that the sample which was annealed at 670 degrees C for a short time and has the optimal magnetic properties has a main BCT Fe3B phase containing Nd atoms and a small amount of alpha -Fe (6%). It is concluded that the hard magnetic properties of crystallized melt-quenched Nd-Fe-B with a low Nd concentration do not originate from the presence of the hard magnetic Nd2Fe14B phase but from the BCT Fe3B phase containing Nd atoms.

Preparation and Mössbauer study of Gd2Fe17Cx (x≤2) with higher carbon concentration by melt quenching

Carbides Gd2Fe17Cx with composition up to x=2 were successfully prepared. The single phase Gd2Fe17Cx (x=1.5, 2.0) carbides with higher carbon concentration were first obtained by the melt quenching technique. 57Fe Mossbauer spectra of all compounds were measured and the results show that the average hyperfine field and isomer shift increase with increasing carbon concentration in the studied composition range. The effects of the interstitial carbon atoms on the hyperfine parameters are discussed. According to the result of the quadrupole splitting, the easy magnetization direction at room temperature for all carbides was determined.

Magnetic hyperfine fields in Y2Fe17?x Si x compounds

Y2Fe17−xSix compounds withx=0, 0.5, 1.0, 1.5, 2.0, 2.5 and3.0 were investigated by magnetic measurement andNMR. It is found that with increasing Si content the Curie temperatureTC increases while the average Fe magnetic moment μFe decreases. NMR study indicates that Si preferentially substitute the Fe atoms at 4f sites, which is responsible for the increase ofTC.

Magnetism and hyperfine fields in melt-spun R-Fe-B alloys with a low R concentration (R≡Y, Pr, Nd, Gd or Dy)

The hyperfine fields and magnetic properties of amorphous and crystallized R(x)Nd(4-x)Fe(77.5)B(18.5) (R = Y, Pr, Gd or Dy; 0 less than or equal to x less than or equal to 4) alloys have been investigated by means of magnetization measurements, zero-field spin-echo nuclear magnetic resonance (NMR) and the Mossbauer effect (ME). It is found that the Curie temperatures of amorphous alloys change slightly on the addition of R, but the coercive fields of crystallized alloys decrease monotonically with increasing x for R = Y, Pr and Gd. A small addition of Dy increases the coercivity; full substitution of Nd by Dy leads to low coercivity owing to the formation of the magnetically soft Dy3Fe62B14. NMR, ME and x-ray diffraction indicate that the samples with R = Pr and Gd for 0 less than or equal to x less than or equal to 4 and R = Y and Dy for 0 less than or equal to x less than or equal to 2 consist of Fe3B with a body-centred tetragonal (BCT) structure and a small amount of alpha-Fe. Furthermore, the NMR results indicate that the B-11 hyperfine fields of sn Fe3B increase linearly from 25.3 kOe (34.7 MHz) to 26.3 kOe (36.0 MHz) on the addition of Dy or Gd, but that of alpha-Fe does not change with increasing R concentration; the Mossbauer spectra show that the relative intensity of the subspectrum corresponding to Fe-57 at Fe-III(8g) sites in BCT Fe3B is about 5% weaker than those of the other two, implying that about 5 at.% Fe atoms in this site are substituted by other atoms. According to this, it may be reasonable to assume that R atoms enter into BCT Fe3B.

MAGNETISM AND HYPERFINE FIELDS IN YFE10V2 - A COMBINED NUCLEAR-MAGNETIC-RESONANCE AND MOSSBAUER STUDY

A combined nuclear magnetic resonance (NMR) and Mossbauer effect study of YFe10V2 has been carried out. Based on the 57Fe hyperfine fields (HFS) obtained from the Mossbauer results, the resonance lines of the NMR spectrum are assigned and the site preference of V atoms is determined. On the other hand, according to the site preference of vanadium atoms determined by NMR, together with neutron diffraction results, the 57Fe Mossbauer spectrum is fitted. It can be deduced from the NMR results that the existence of one nearest-neighbour Fe atom leads to an increase in the V HF of about 0.35, and the core electron polarization contribution Hcp, the 4s conduction electron polarization contribution Hs and the effect of the second-nearest-neighbour Fe atoms on the HF sites cannot be negligible.

Development of short-range order in Fe-B-C amorphous alloys with thermal treatment: an NMR study

Abstract In an earlier work, 11 B NMR spectra obtained from a series of as-quenched Fe-B-C amorphous alloys revealed two distinct peaks centered at 34.7 and 36.3 MHz, which were attributed to regions with short-range order (SRO) characteristic of body-centered tetragonal Fe 3 B (bct-Fe 3 B) and orthorhombic Fe 3 B (o-Fe 3 B), respectively. It was determined that the addition of C favors the formation of o-Fe 3 B-like SRO and this was correlated with the enhancement of the soft magnetic properties. In this report, 11 B NMR is utilized to study the evolution of the above phases, along with bcc α-Fe, in the Fe 80 B 20 - x C x alloys ( x = 0, 2, 4, 7 and 9) for various thermal treatments. Consistent with the earlier work, it is found that the addition of C is necessary for o-Fe 3 B-like SRO formation upon thermal treatment. In particular, annealing the x = 0 alloy at both 460°C and 840°C results in bct-Fe 3 B precipitates plus bcc α-Fe. However, for the alloys with x > 0, the annealing treatment at 460°C results in some o-Fe 3 B precipitates along with bct-Fe 3 B and bcc α-Fe; the amount of o-Fe 3 B increases with the C content. The same situation occurs for the 840°C heat treatment with the o-Fe 3 B precipitates occurring to a greater degree and the bct-Fe 3 B SRO being completely suppressed for x ≥ 4. These results show that the C atoms play a crucial role in the formation and stability of the o-Fe 3 B SRO which is developed during the thermal treatments.

SHORT-RANGE ORDER OF THE AMORPHOUS FE-B POWDERS PREPARED BY BOROHYDRIDE REDUCTION

Ultrafine Fe−B amorphous alloy powders were prepared by reducing Fe2+ ions using KBH4 and NaBH4 in aqueous solution. Adjusting technological factors, the amorphous powders around the composition of Fe65B35 can be easily obtained, but in the vicinity of eutectic point (Fe80B20) a certain amount of α-Fe often appears in the samples. From the Mössbauer spectrum, the crystallization products of the Fe63B37 amorphous powder are α-Fe and Fe2B phases. The measurement of11B spin echo nuclear magnetic resonance (NMR) at 8K showed that Fe2B-like and Fe3B-like short range orders (SRO) exist in the amorphous powder of Fe76B24.