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

For the first time, forging at very high strain rates (epsilon (over dot) > 100 s(-1)) has been used successfully to produce magnets directly from bul k 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 2 00 kJ m(-3). Correlations between the rheological behaviour and the permane nt magnet properties of the alloy are demonstrated and four critical parame ters have been identified: (1) an increase in the strain fate improves coer civity but reduces extrinsic magnetic anisotropy; (2) an increase in the st rain promotes a microstructure with higher levels of coercivity and texture ; (3) when the overall viscosity of the sample decreases, the anisotropy in creases for a given deformation rate; and (4) high temperatures enhance the coercivity for very fast deformation rates. Through mechanical tests, it h as been shown that both the Level of anisotropy and the maximum stress at v arious temperatures can be closely correlated with the different microstruc tures obtained below and above 1273 K. A brittle to ductile transition in t he mechanical behaviour of the alloy has been observed. Grain refinement as well as alignment mechanisms are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.