Radially oriented Nd–Fe–B rings are prepared by backward extrusion of fine grained melt-spun powder. Meltspun powder with the nominal composition of Nd 30.5 Fe bal. Co 6.0 Ga0.6 Al0.2 B0.9 (wt%) is used as starting material. The effects of process variables, such as deformation temperature (Td ), strain rate (ε) and height reduction ( h%), on the magnetic properties of the rings are investigated. A scanning electron microscope (SEM) equipped with an energy spectrum device is used to study the metallograph and microfracture of the extruded rings. The B r and (BH) max reach the optimum values at Td = 800 C, ε˙ = 0.01 mm/s, and h% = 70%. It is found by SEM observations that the particle boundaries, which seemingly correspond to the interfaces of the starting melt-spun powders, emerge after the corrosion of metallography specimens. This is helpful for studying the effects of powder–powder interface on the local deformation and deformation homogeneity in the rings. For different spatial positions of the extruded rings, there are characteristic metallographies and microfractures. The upper end of the rings has the least deformation and worst texture, and therefore the worst magnetic properties. The magnetic properties in the radial direction increase slightly along the axis from the bottom to the middle, then steeply decrease at the upper end of the ring. The deformation and the formation-of-texturing processes are discussed. The deformation and the texturing formation of melt-spun Nd–Fe–B alloys probably involve grain boundary sliding and grain rotation, the solution-precipitation process and preferential growth of Nd 2 Fe 14 B nanograins along the easy growth a-axis.
Isotropic magnets were prepared from melt-spun powders at different hot pressing temperatures from 550 to 700 oC,then upset into fully dense anisotropic magnets at the same die-upsetting temperature of 850 oC.Die-upset magnets had the characteristics of inhomogeneous microstructure,including well-aligned grains structure and nonaligned grains layers transverse to press direction,which was quasi-periodic layer structure with a total length of 5-15 m.Nonaligned grains layers were mainly made of large grains and had higher Nd content.To clearly understand the formation of layer structure,the microstructure of isotropic precursors with different hot pressing temperatures and their subsequent die-upset magnets was investigated.A new interpretation for the formation of layer structure was proposed in this paper: the layer structure was correlated to the original ribbon interface which was divided into three types based on the contact forms.Because of the incomplete contact of neighboring ribbons,concentration of stress occurred in the contacted points and the Nd-rich phase was squeezed into interspaces at high temperature under stress.Due to the release of interfacial energy and the fluidity of enough Nd-rich liquid phases,the nonaligned layers with large grains formed both in hot compaction and subsequent hot deformation process.The layer structure affected the magnetic properties of die-upset magnets.With increase of the hot pressing temperature,the nonaligned grains layers became thicker,and the magnetic performance of die-upset magnets decreased.It was necessary to reduce the thickness of large grains layers for the preparation of high-performance die-upset magnets.
