A comparison of the micromagnetic and microstructural properties of four NdFeB-type materials processed by the HDDR route

Transmission electron microscopy has been used to study the physical and magnetic microstructures of thinned sections of four different NdFeB-type alloys. Three of the materials were prepared by the solid-HDDR process and differed in that one was a simple ternary alloy, another contained Co & Ga additions and a third was appreciably richer in Nd than stoichiometric Nd2Fe14B1. For the fourth material HDDR powder had been hot-pressed into a fully dense compact prior to die-upsetting. Isotropic grains with a mean size ˜300 nm were found for the first three materials whilst the die-upset material had an average grain size of ˜300×700 nm2. All materials other than the simple ternary alloy showed strong alignment of the c-axis, the clustering of grains with similar alignment being most pronounced in the sample with Co & Ga additions. The sample with the largest region of alignment was the die-upset material, the c-axis orientation being approximately parallel to the pressing direction throughout. It was in this sample that magnetic alignment was found to be most extensive, the domains lying parallel to the mean c-axis across the whole of the visible area. Significant magnetic alignment was also found in the sample with Co & Ga whilst the Nd-rich material had relatively small areas of magnetic alignment. In the three samples with good crystallographic alignment local variations in domain wall orientation between neighbouring grains within an aligned region showed maximum variations that corresponded with the angular spread of spots in the selected area diffraction patterns.