Phase Composition and Microstructure of Grain Boundaries in Nd-Fe-B Strip Casting Flakes
The grain-boundary(GB)phase composition and microstructure have an important influence on the coercivity of sintered Nd-Fe-B magnets.In order to clarify the evolution mechanism of the GB phase composition and microstructure of sintered magnets,it is necessary to study the GB characteristics of Nd-Fe-B strip casting(SC)flakes as the precursor of the magnets.The Nd-Fe-B SC flakes were prepared by vacuum induction melting SC technology.TEM investigation of the phase composition and microstructure of grain boundaries in Nd-Fe-B SC flakes were carried out.It is found that there are two areas with obvious different contrast existing in a trian-gular Nd-rich grain boundary.The area with uniform contrast is amorphous/crystallite.The other area with obvi-ous light and dark contrast contains the double hexagonal close-packed Nd phase(dhcp-Nd),the low melting-point eutectic"Nd+Nd2Fe14B"and a trace amount of amorphous/crystallite.The EDS line scan results shows that the atomic ratio of Nd to Fe in the GB regions is not a constant value.In the amorphous/crystallite area,Fe con-tent is higher with an average value of more than 60％;In the crystalline area,the composition fluctuates in a large scale,accompanied with obvious light and dark contrast change,which is probably because the area is a mixture of dhcp-Nd,and eutectic"Nd+Nd2Fe14B"and a trace amount of amorphous/crystallite.Amorphous phase exists commonly in the GB regions of Nd-Fe-B alloys,which is because Nd-Fe-B alloy belongs to deep eutectic system,the alloy liquid tends to solidify earlier than eutectic point during cooling process and form disordered amorphous phase,the meta-stable amorphous remains until room temperature.
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