The magnetic and mechanical properties of NdFeB type permanent magnets and the effect of quenching

The fracture strength of Nd₁₅Fe₇₇B₈ and Nd_14.5Dy_1.5Nb₁Fe₇₆B₇ sintered magnets has been measured to be 285(40) MPa and 240(48) MPa, respectively. The fracture mechanism appears to be controlled by a critical level of tensile stress. In all the cases considered, Nd-Fe-B-type permanent magnets have been shown to fracture in a brittle manner along the grain boundaries. This is consistent with poor adhesion between Nd₂Fe₁₄B grains and could be due to the lack of the neodymium-rich phase along certain interfaces even in the fully sintered condition. The addition of dysprosium and niobium to the Nd-Fe-B alloy did not alter the fracture behavior of the material, whereas these additions have a marked effect on the magnetic properties. Quenching from 1370 K was found to greatly affect the fracture stress. This is possibly due to rapid cooling through the melting point of the neodymium-rich phase, which prevents sufficient wetting of the grains. Indeed, when the quench rate was increased the effect was so severe that some samples were prone to fail on pulse magnetization. However, where this did not occur, the effect was reversible on annealing at 903 K for 1 h. The intrinsic coercivity decreased progressively with increasing quench temperature above 900 K. This could be due to progressively less wetting with increasing quench temperature