Thermal behaviour and magnetic properties of B-rich NdFeB nanocomposite hard magnetic alloys with partial substitution of Dy for Nd

B-rich NdFeB base alloys with partial substitutions of Dy for Nd ((Nd_1-xDy_x)_4.5Fe₇₇B_18.5 (x = 0-0.4)) were chosen to study the influence of Dy on thermal behaviour and magnetic properties. It was found that partial substitution of Dy for Nd results in a dramatic enhancement of coercivity _jH_c from 183 to 261 kA m^-1 and of maximum magnetic product (B H)_max from 110.6 to 133.7 kJ m^-3. However, the saturation magnetization M_s only decreases about 22.8 kA m^-1 for every 0.1 increment of Dy content x. Differential thermal analysis traces show that the crystallization process of the initial amorphous ribbons is a three-peak process. X-ray diffraction and differential thermal analysis indicate that the three peaks correspond to the formation of crystalline Fe₃B, Nd₂Fe₁₄B, and α-Fe, respectively. With increase of Dy concentration, _jH_c increases to a maximum value at about x = 0.1, and then reduces gradually. The enhancement of coercivity arises from the increase of the anisotropy field for the hard magnetic phase. The reduction of intrinsic coercivity _jH_c is attributed to the emergence of α-Fe after annealing at the third peak and the reduction of the fraction of hard magnetic phase due to the Dy addition. In order to obtain the optimum magnetic properties, a special annealing method was used.