Evolution of Fe-Co rich particles in Alnico 8 alloy thermomagnetically treated at 800°C

The morphology and size of Fe-Co rich particles in Alnico 8 specimens thermomagnetically treated at 800°C for various times have been investigated by means of transmission electron microscope (TEM) observations and small angle X-ray scattering (SAXS). Phases rich in Fe-Co and Ni-Al were formed during spinodal decomposition. With longer treatment times, as Fe-Co rich particles coarsened to a critical size, a first splitting took place. When the resultant small particles originating from the first splitting also coarsened, and they reached critical size, a similar secondary splitting occurred. This is attributable to perfect maintenance of the coherent relationship between the Fe-Co and Ni-Al rich phases, as well as the negative interaction energy that arose from the large lattice misfit. The splitting resulted in a sharp decrease in the mean particle diameter and an increase in the mean aspect ratio of the larger particles studied, which is expected to increase the coercive force of the Alnico 8. In addition, the mean particle length always decreased with increasing treatment time.