Rheological study of hot-forged NdFeB and related permanent magnet properties

For the first time, forging at very high strain rates (ε̇>100 s−1) has been used successfully to produce magnets directly from bulk Nd–Fe–B–Cu alloy. This is achieved by reducing significantly the size of the Nd2Fe14B crystallites, which leads to a coercivity of 796 kA m−1 and by developing a Nd2Fe14B c-axis fibre texture along the forging direction, which leads to a remanence of 1 T, giving a final energy product close to 200 kJ m−3. Correlations between the rheological behaviour and the permanent magnet properties of the alloy are demonstrated and four critical parameters have been identified: (1) an increase in the strain rate improves coercivity but reduces extrinsic magnetic anisotropy; (2) an increase in the strain promotes a microstructure with higher levels of coercivity and texture; (3) when the overall viscosity of the sample decreases, the anisotropy increases for a given deformation rate; and (4) high temperatures enhance the coercivity for very fast deformation rates. Through mechanical tests, it has been shown that both the level of anisotropy and the maximum stress at various temperatures can be closely correlated with the different microstructures obtained below and above 1273 K. A brittle to ductile transition in the mechanical behaviour of the alloy has been observed. Grain refinement as well as alignment mechanisms are discussed.