ANNEALING EFFECT ON PHASE-TRANSITION OF EQUIATOMIC FERH ALLOY

The effect of thermal treatment on the behavior of the transition of an Fe-50.5 at%Rh alloy between the anti-ferromagnetic and ferromagnetic phase was carefully investigated with electric resistivity measurements and transmission electron microscopy (TEM). The specimens were annealed near 1000 K after solution treatment at 1370 K. In the as-quenched specimen from 1370 K the magnetic phase transition occurred abruptly near 300 K, and the extended transition temperature and temperature hysteresis loop resulted with annealing. Correspondingly, precipitates of the Rh-rich gamma phase appeared in the alpha(1) grains of B2 structure in the annealed specimens. This fact shows that the phase decomposition from alpha(1) to alpha(1) and gamma occurred in the alloy with annealing, and the change in the temperature of the magnetic phase transition is an effect of the change in the composition of the alpha(1) phase with the phase decomposition. These results reveal requirements for partial correction of the conventionally used phase diagram of the Fe-Rh alloy system. Neither anti-phase boundaries nor aggregation of point defects is observed in any specimen. The in-situ observation of the magnetic phase transition was also performed with Lorentz microscopy and TEM. Nucleation of transition phases occurred on the surfaces or the grain boundaries, and the phase transition progressed with rapid mobility of the phase boundaries between the anti-ferromagnetic and the ferromagnetic phases. No intermediate phase accompanying the transition was observed.