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

Abstract

Phase transformations and magnetic properties of overquenched Pr7Fe88B5 ribbons during annealing have been investigated. X-ray diffraction and Mössbauer 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 coercivity of the annealed ribbons were measured and analyzed. It was found that the coarser-grained and more irregular microstructures, as mentioned above, would lead to the weakening of the exchange coupling effect between the hard and soft phase, the inhomogeneous nucleation of reverse domain, a lower critical nucleation field, and a decrease in αex and Neff. These may be the reasons that the optimal magnetic properties of annealed ribbons reduce significantly with increasing initial quenching rate.

The role of sub-micron grain size in the development of rare earth hard magnetic alloys

“…Hence, fewer, but larger, ␣-Fe crystals would be expected in the former than in the latter [24]. TEM studies on these alloys [17,21] provide evidence to support this proposition the OQA alloys having a unimodal distribution of grain sizes centred at ∼40 nm, in contrast to the DQ alloys which showed a clearly bimodal distribution with one peak centred at ∼15 nm, corresponding to ␣-Fe, and another centred typically at ∼40-45 nm, corresponding to the RE 2 Fe 14 B grains. The coarser ␣-Fe crystallites in the OQA samples may not be completely exchange coupled to the adjacent 2/14/1 grains which would account for the inferior second quadrant loop shapes observed in those alloys.…”

“…The influence of the RE concentrations on J r , i H c and (BH) max for Nd y Fe 94−y B 6 [9], Pr y Fe 94−y B 6 [15][16][17] and (Nd 0.75 Pr 0.25 ) y Fe 94−y B 6 [18] alloys for approximately constant d g of RE 2 Fe 14 B phase, lying in the range 20-35 nm, are shown in Fig. 1.…”

Effect of wheel speed on the crystallization behavior of as-quenched Nd-Fe-B alloys

“…4,5 Such nanocomposite microstructure is usually prepared by crystallization of amorphous alloys. [7][8][9] However, the crystallization mechanism and phase transition still remain unclear now, because of the involved many factors that make the crystallization behavior quite complex. One most important factor is the wheel speed of rapid quenching, since it affects the initial phase structures of the as-quenched alloys.…”

“…One most important factor is the wheel speed of rapid quenching, since it affects the initial phase structures of the as-quenched alloys. 10 Zhou has studied similar systems and found that the decomposition of metastable phase depended on the wheel speed, 7 but did not have further explanation. We have studied the relationship between the magnetic properties and the wheel speed, and found that samples prepared by mid-speed had the best magnetic properties.…”

Improvement of the magnetic properties of low-neodymium magnets by minor addition of titanium

“…For amorphous RE-Fe-B ribbon alloys (RE contento11.7 at%) the microstructure evolution, occurring in the course of annealing, starts with precipitation and growth of a-Fe [5][6][7], followed by crystallization of the RE 2 Fe 12 B phase. The a-Fe crystallites are large, which leads to reduction of the exchange interactions [5,8].…”