Improved Microstructure and Magnetic Properties of Alnico 8 Alloys by B-Doping

The microstructure and magnetic properties of alnico 8 alloys have been investigated in this paper. The alloys with nominal composition of 32.5-xFe-7.5Al-1.0Nb-35.0Co-4.0Cu-14.0Ni-6.0Ti-xB (x=0-0.40 ) are prepared by arc melting and subsequent heat treatment cycles. The alloys are characterized by X-ray diffractometer, optical microscopy, scanning electron microscopy, and physical property measurement system. The results indicate that magnetic properties which originate from spinodal decomposition strongly depend on alloy chemistry, size of the spinodal phases, and processing parameters. Purifying α-phase and reducing the spatial dimensions of α
₁
- and α
₂
-phases are effective means to increase the coercivity H
_cj
of the alloys. Small concentration of B is beneficial to refine the microstructure and enhance the magnetic properties of alnico magnetic alloys. The alloys with 32.48Fe-7.5Al-1.0Nb-35.0Co-4.0Cu-14.0Ni-6.0Ti-0.02B exhibit best magnetic properties of H
_cj
=1.35 kOe, B
_r
=5.6 kGs, and (BH)
_max
= 2.35$ MGOe. The optimized alloy is further treated thermo-magnetically, and the properties are enhanced to H
_cj
=1.62, B
_r
=7 kGs, and (BH)
_max
=4.3 MGOe.