The variations of the magnetic properties and microstructures of the sintered $Nd_{15}Fe_{75-x-y}B_8Si_xAl_y$ (x=0-5, Y=0-4) permanent magnets with the additions of Si and Al alloying elements were investigated.
Post-sintering heat treatments of the each magnet increased the intrinsic coercivity, and the 2at% Al containing alloys at each Si content, ($Nd_{15}Fe_{75-x}B_8Si_xAl_2$) showed peak in intrinsic coercivity values after post-sintering heat treatment at 600℃ for 1hr. The intrinsic coercivity (iHc) increased to 12.6kOe for the 1Si-2Al alloy after the sintering treatment at 1100℃ followed by post-sintering heat treatment at 600℃, compared to 7kOe for the $Nd_{15}Fe_{77}B_8$ base alloy and 10.5kOe for the 0Si-2Al alloy.
Remanence (Br) and maximum energy product ((BH)max) decreased with the additions of Al and Si due to the the lowered ferromagnetic interaction between Fe atoms by presences of Si and Al atoms in $Nd_2Fe_{14}B$ phase. X-ray diffraction patterns showed the incomplete magnetic alignments of the grains, resulting in the low remanence value. The Curie temperature (Tc) increased with the silicon addition at a rate of about 5℃/at%Si, while reduced with the aluminum addition at a rate of about -15℃/at%Al. For the Si-Al coexisting alloys, the Curie temperature was controlled by aluminum contents.
The phase identification through X-ray and TEM work for the 0Si-2Al and 1Si-2Al alloys showed the presence of $Nd_2Fe_{14}B$ tetragonal phase (main phase of the Nd-Fe-B permanent magnets), Nd-rich phases with different structure (fcc and hcp) and B-rich phase after post-sintering heat treatments of the magnets.
Through EDS analysis, al-rich phases, δ or μ, with (32-36)at%Nd-(53-58) at%Fe-(5-10)at%Al-(1-4)at%Si were found in the Nd-rich intergranular phase for the alloys containing aluminum. Those phases played a determining role for the increment of the intrinsic coercivity after the post-sintering heat treatment. The Nd-rich phases contained both silicon and aluminum for the Si-Al coexisting alloys. Because of the higher wettability of silicon containing Nd-rich phases, the intrinsic coercivity of 1Si-2Al alloy was higher than that of 0Si-2Al alloy. On the other hand, for the 5Si-2Al alloy, EDS analysis showed the presence of Nd-Si compound, $Nd_5Si_3$ with (60-67)at%Nd-4at\%Fe-(30-31)at%Si-(1-2)at%Al, which results in the reduction of intrinsic coercivity under the heat treated conditions.
