Crystallographic and magnetic alignment in die-upset Nd-Fe-B magnets

Abstract

Fully dense and aligned Nd-Fe-B permanent magnets can be prepared from melt-spun amorphous ribbons by a two-step process involving hot compaction and hot deformation. It is the latter processing step that produces alignment. During hot deformation, the randomly oriented and equiaxed Nd₂Fe₁₄B grains change to platelet shaped grains such that their crystallographicc axes lie normal to the flat surfaces. The final microstructure of the die-upset magnet, consisting of closely stacked flat Nd₂Fe₁₄B grains separated by a Nd-rich boundary phase, is produced by a combination of grain boundary sliding and grain boundary migration. Factors such as temperature, strain rate, grain size, etc. which affect these processes also affect deformation and alignment. Processing optimum die-upset magnets involves optimizing such variables as composition (to obtain maximum percentage of magnetic phase while retaining an appropriate intergranular phase to aid in grain boundary sliding and migration), alignment (to obtain maximum remanence) and microstructure (to provide for domain wall pinning sites).