Study On Crystallization Of Melt-spun Nd₉Fe_85.5-xCo_xB_5.5（x=3,5at%） Amorphous Ribbons

Nd-Fe-B nanocrystalline composite permanent magnetic materials with both soft magneticphase and hard one have huge magnetic properties in theory compared with the sintered magnets,but their magnetic properties are very low, one of the reasons is the grain sizes of the two phasesafter crystallization. So in this dissertation, the Nd₉Fe_85.5-xCo_xB_5.5（x=3,5at%） melt-spun ribbonswere prepared with35m/s roller speed. The phases of specimens were examined by X-raydiffraction with Cu-K radiation. The heating curves of two amorphous alloy ribbons weremeasured by DSC protected by argon first under vacuum. The magnetic properties were measuredby VSM with a maximum applied field of3.1T at room temperature. The crystallization activationenergy E, frequency factor Z, K and Avrami exponent n of the two kinds of amorphous alloyribbons were calculated and discussed.XRD analysis results show that: the two kinds of melt-spun ribbons mainly are amorphous.The melt-spun Nd₉Fe_80.5Co₅B_5.5ribbons consist of Nd₃Fe₆B₁₄and α-Fe, with two exothermicpeaks, Nd₉Fe_82.5Co₃B_5.5with only one exothermic peak. So more Co doping can change thecrystallization behavior.And the higher Co content in the alloy, the higher crystallization activation energy E andcrystallization reaction rate constant K of both the initial crystallization temperature Txand thepeak one Tp. This is the reason that the increase of frequency factor Z playes a key role. At theheating speed10k/min, the local Avrami exponent n of Nd₉Fe_82.5Co₃B_5.5decreases sharply at firstand then slow down with the increase of crystallization volume fraction x, but it rises again when xis more than90%. It demonstrates that when x is less than12%, the crystallization nucleationmainly in the three-dimensional nucleation mode, and it changes into the two-dimensionalnucleation one when x is12%³6%, after36%it does in the one-dimensional nucleation one.With the increased of the crystallization temperature from993K to1203K, the saturationmagnetization of magnets gradually improve from1.22T to1.45T. But the remanence, coerciveforce, and maximum magnetic energy product all decrease, especially the coercive force sharplydown about46%, for the large grain sizes of the two phases due to the high crystallization temperature. And the exchange coupling effect between two phases changes from themagnetostatic coupling to the intergranular exchange coupling.