Effect of REF3 additions on the microstructure and on the magnetic properties of sintered Nd-Fe-B magnets

The grain growth of anisotropic Nd-Fe-B magnets with oxygen contents < 0.35 wt.% has been investigated at a sintering temperature of 1100 degrees C. Due to second-phase constituents the grain growth exponent of the fine-grained ((D) over bar approximate to 6.5 mu m) microstructure amounts to n approximate to 25, whereas for large grains ((D) over bar > 100 mu m) the grain growth exponent is comparable to pure metals: n approximate to 3.3. At sintering temperatures of 1070 degrees C additions of DyF3 improve the coercivity only by 1.3 kOe per wt.% DyF3 whereas at sintering temperatures of 1100 degrees C DyF3 additions result in an increase of the coercivity and of the squareness of the demagnetization curve J(H) by 6.6 kOe or 40 % per wt.% DyF3, respectively. Besides grain refinement due to the second-phase particles the partial solubility of DyF3 in the Nd-rich constituents contributes to the enhancement of the coercivity. Hence during sintering Dy2Fe14B crystallizes from the liquid phase at the surface of the grains and increases the coercivity and the squareness of the demagnetization curve.