The dynamical behavior of ordering with phase separation in Fe-13.8 and 14.6 at%Si alloys within the two-phase field of (B2+DO3) was studied by transmission electron microscopy and X-ray diffraction. We first compared two processes from single-phase states of DO3 and B2 in the Fe-14.6 at%Si alloy at a temperature of 953 K, and made clear the dependence of the behavior on the initial phase state. The phase separation from DO3 develops more promptly than that from B2. Many B2 plates precipitates along ⟨100⟩ directions inside the DO3 domains in the process from DO3. In the course from B2, the DO3-type order first prevails with numerous antiphase boundaries (APBs) in the alloy, and then B2 phase reappears mainly along the APBs. The formation of B2 on the APBs proceeds continuously without activation energy. The results of Fe-13.8 at%Si strongly suggested that the metastable extrapolation of the second-order transition line of B2−DO3 into the miscibility gap serves as the instability limit of B2 phase. The phase separation from B2 to (B2+DO3) proceeds by nucleation and growth of DO3 domains in the B2 matrix at a temperature above the metastable transition line. In contrast, the phase separation follows the continuous ordering to DO3, when the annealing temperature is lower than the line. We found that the critical point fluctuation in degree of order, which is characteristic of the second-order transition, appears in the early stage of decomposition near the metastable transition line.
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