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

Transmission electron microscopy was employed in study of microstructural c hange due to phase separation from B2 to (B2 + D0(3)) in Fe-base binary all oys 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 DO , expected within the miscibility gap. Dark-field images have shown that fi ne fluctuations of the degree of order appear in the early stage and then g radually fade away with development of phase separation in the following st age. These fluctuations are regarded as due to the thermal dissipation effe ct pronounced around the critical temperature of metastable order-disorder transition. It suggests that the critical point retains attributes of the s econd-order transition even inside the two-phase field, and it plays a role as the limit of metastability and instability for quenched B2 phase agains t phase separation into (B2 + D0(3)). Besides, a roughening transition of < 100 > /2 antiphase boundaries has been recognized in DO, phase in Fe-14.6 at.%Si at a temperature 1024 K slightly lower than the critical point of eq uilibrium B2-D0(3) order-disorder transition around 1030 K. The antiphase b oundaries wander through the DO, matrix in the narrow temperature range bet ween the two transitions, contrary to faceted orientation to {100} at lower temperature. (C) 2001 Elsevier Science B.V. All rights reserved.