Effects of W and Co additions on the phase transformation and magnetic properties of nanocomposite Nd2Fe14B/α-Fe magnets

The phase transformation and magnetic properties of nanocomposite Nd2Fe14B/alpha-Fe magnets with W and Co additions, according to the composition formula (1 - x wt%-y wt%) Nd(8.6)Fe(85)B(6.)4 + x wt% W + y wt% Co, prepared by mechanical alloying, have been systematically studied. It has been observed that separation and decomposition of the B-rich phase Nd1.1Fe4B4 occur during crystallization of the amorphous state of the alloys after annealing at different temperatures. The addition of W can promote separation of the B-rich phase, and some B-rich phase remains at a relatively high annealing temperature. The addition of Co increases the content of the B-rich phase. A small amount of W atoms can enter the lattice of the hard magnetic phase, substituting for Fe atoms, and cause the coercivity of the magnets to increase. However, an excessive addition of W degrades magnetic properties due to the increase of B-rich phase. Addition of Co decreases the crystallization temperature of the hard phase and refines the grain size of the soft phase at a relatively low annealing temperature. In addition, the addition of Co is favourable for improving the exchange coupling between nano-grains of the hard magnetic and the soft magnetic phases. The optimum grain size of the soft magnetic phase has been calculated. The calculated values of the optimum grain size of the soft phase fairly agree with experimental results only for nanocomposite magnets with a soft magnetic phase volume fraction chi close to 30%. Further, the B-rich phase dilutes the inter-grain exchange interaction resulting in a decrease of the coercivity for magnets with chi close to 30%. These observations imply that the exchange coupling between the nano-grains of the soft and the hard phases constitutes a predominant control mechanism of coercivity for nanocomposite magnets with chi close to 30%.