Concurrent ordering and phase separation in the vicinity of the metastable critical point of order-disorder transition in Fe-Si alloys

Transmission electron microscopy was employed in study of microstructural change due to phase separation from B2 to (B2+DO₃) in Fe-base binary alloys with 12.4 and 13.8 at.% Si. which were annealed at temperatures around the critical line of metastable order-disorder transition between B2 and DO₃ expected within the miscibility gap. Dark-field images have shown that fine fluctuations of the degree of order appear in the early stage and then gradually fade away with development of phase separation in the following stage. These fluctuations are regarded as due to the thermal dissipation effect pronounced around the critical temperature of metastable order-disorder transition. It suggests that the critical point retains attributes of the second-order transition even inside the two-phase field, and it plays a role as the limit of metastability and instability for quenched B2 phase against phase separation into (B2+DO₃). Besides, a roughening transition of <100>/2 antiphase boundaries has been recognized in DO₃ phase in Fe-14.6 at.%Si at a temperature 1024 K slightly lower than the critical point of equilibrium B2-DO₃ order-disorder transition around 1030 K. The antiphase boundaries wander through the DO₃ matrix in the narrow temperature range between the two transitions, contrary to faceted orientation to {100} at lower temperature.