Nanocrystals Formation And Magnetic Properties Of Amorphous Nd_3.6Pr_5.4Fe₈₃Co₃B₅Suffered From Thermomechanical Processings

Nanocomposite exchange coupled magnets consisting of a fine mixture of hard-andsoft-magnetic phases, e.g., R2Fe14B/α-Fe (R=rare earth) with excellent physical andmechanical properties, have attracted much scientific and technological attention.Theoretical calculations have predicted that the maximum energy product of anisotropicnanocomposite magnets is up to1090kJ/m³, higher than any of single phase permanentmagnets, which are promiseful to be the next-generation permanent magnets.Unfortunately, practically obtained maximum energy products in nanocomposite magnetsare only in the range of80-160kJ/m³. The discrepancy between the predicted and actualenergy products is due to the difficulty in obtaining the optimum microstructuresemployed for theoretical models. Theoretical calculations require homogeneouslydistributed nanocrystals with a small size for soft-magnetic phase and a crystallographictexture for hard-magnetic phase. Hence, a fundamental understanding of the formation ofnanocrystallites in amorphous alloy and it's grow behavior are of great significant. In thestudy, Gleeble and six-side anvil cell equipments were used to study the formation ofmicrostructures and magnetic properties in deformed magnets by thermomechanicalprocessings method. And the crystallization behaviors of α-Fe and Nd₂Fe₁₄B phases inamorphous matrix are also been studied.Atomization behaviors of α-Fe and Nd₂Fe₁₄B phases in amorphous matrix have beenstudied by the equipment of Gleeble. The activation volumes for the growth of α-Fe andNd₂Fe₁₄B phases in amorphous matrix are determined in500and550℃. Diffusionmechanism of amorphous crystallization for α-Fe and Nd₂Fe₁₄B phases were discussed.The crystallization of amorphous Nd_3.6Pr_5.4Fe₈₃Co₃B₅alloy has been studied by thermo-mechanical processing method. Nanocomposite exchange coupled bulk magnets withmagnetic anisotropy and texture of (00l) for Nd₂Fe₁₄B phase were successfully prepared.Study shows that the grain sizes, magnetic properties and magnetic anisotropy haverelationship with the strain rate and stress. The grain size of α-Fe and Nd₂Fe₁₄B phases areincrease with the decreasing of strain rate and increase with the decreasing of the pressure.Magnetic properties and magnetic anisotropy are increase with the increasing of strain rate. The magnetic properties first increase and then decrease with the increasing of stress from280to450MPa. The best magnetic properties are obtained at stress of330MPa, and themagnetic anisotropy reaches the maximum at330MPa magnets. The magnets show a littleincrease of Hciat a higher temperature which owing to the easy formation of Nd₂Fe₁₄Bphases at this temperature. But the Brwill decrease more and leading to a decrease of(BH)max. Hence, at higher temperature we can't obtain a magnet with better magneticproperties.To study the influence of initial state to the microstructure and magnetic properties ofdeformed magnets when suffered from the thermomechanical processings, differentmicrostructures were introduced in amorphous matrix. The 'pre-exist' grain has benefit tooptimizing the microstructure of deformed magnets and enhance the magnetic properties.Bulk magnets with magnetic anisotropy were prepared by the thermo-mechanicalprocessing. The magnets made from the ribbons with a full amorphous structure show thelargest magnetic anisotropy. And the optimum magnetic properties are obtained in themagnets made by the hot-deformation of the ribbons with nanocrystals in amorphousmatrix.Nanocrystallization was studied at high pressure and higher temperature by six-sideanvil cell equipments. Nanocomposite exchange coupled bulk magnets with small grainsize were obtained. At appropriate condition, the magnets with (410) texture were yielded.The magnets show enhancement of magnetic properties. The microstructures andmagnetic properties of high pressure magnets and the reason of (410) texture formationwere discussed in the text too.