Transmission electron microscopy study of nanostructured Nd-Fe-B hard magnetic materials.

In the present study the effect of melt spinning roll speed on mean grain size and magnetic properties of near stoichiometric Nd-Fe-B alloy ribbons is investigated. A decrease in the ribbon thickness and mean Nd2Fe14B grain size and improvement in magnetic properties, including remanence enhancement, were observed on increasing the roll speed, prior to vitrification and consequent collapse of properties at higher speeds. The effect of roll speed and also annealing temperature on the microstructure and the magnetic properties and hysteresis loops of the stoichiometric alloy were studied and it was found that optimum nanostructure and magnetic properties could be achieved for a range of roll speeds and annealing temperatures which makes it an attractive route for commercial production of this alloy. The effect of 0-3 at.% Ga substitution for Fe in the stoichiometric alloy of composition Fe82.3Nd11.8B5.9, produced by both optimally quenching and over-quenching and annealing were studied and compared. Broadly, comparable results for both sets with some improvement in coercivity were observed for up to 2 at.% Ga addition. The effect of 0-3 at.% Ga substitution for Fe were also investigated in over-quenched and optimally annealed substoichiometric alloy of composition Fe85.1Nd9B5.9 and were compared with those achieved for ribbons prepared by optimally quenched route. Inferior magnetic properties especially for jHc and (BH)max, were observed for over-quenched and annealed alloys compared with those prepared by directly quenching. The Ga addition did however result in some improvement in jHc and (BH)max, especially for the directly quenched alloy containing 1.5 at.% Ga. The microstructure of ribbon samples were studied extensively by TEM and, in addition to normal polygonal Nd2Fe14B grains, other more unusual morphologies such as spherical and plate like grains, were found in a number of samples. Various grain size distributions were also observed. Texture and the orientation relationship between a-Fe precipitates and Nd2Fe14B matrix were studied in the near stoichiometric sample containing 3 at.% Ga, melt spun at low roll speeds, and an attempt was made to identify additional phases observed in this sample. Some dislocation-type features found in some samples were also studied.