On some peculiarities of the curves sigma versus temperature on alni and alnico alloys

The curves σ =
f(T)
are taken at a very high heating rate (1000°C/min) on samples of Alni, Alnico 5, and Alnico 8 alloys, heat treated so that their equilibrium state should correspond to a temperature of about 600°C and the α' phase particles should be much bigger than after a usual heat treatment. The Curie point occurs respectively at 760, 905, and 920°C. As we think we have avoided the diffusion process between α' and α phases, the Curie points should be those of the α' phase in almost optimal permanent magnet conditions. From that it is possible to argue that in the Alni alloy the α' phase is not pure Fe, but also includes Al and Ni; in Alnico 5 and Alnico 8 alloys, on the other hand, the ferromagnetic phase is not only Fe-Co, but also includes Al or Ni or both. In other curves taken at lower rates (300°C/min) on samples with normal morphology we observed a hysteresis of about 20-30°C between the Curie points during heating and cooling, which may be eventually ascribed to the gap between the solution and spinodal decomposition temperatures.