Discontinuous precipitation and coarsening of lamellar cellular precipitate in Fe-Zn alloys

The initiation of cellular reaction, formation of cells, and their development have been studied in two Fe-Zn alloys containing 20 and 25 wt.% zinc. Optical microscopy was utilized to study the cellular structure developed during aging in the temperature range 400–520°C. Deflection of the grain boundary was observed to be associated with the nucleation of soluterich phase. From the observation of grain boundary migration into puckered configuration and nucleation of new lamellae at such positions of the boundary, it is evident that the cellular phase transformation occurs in this alloy by the pucker mechanism proposed by Tu and Turnbull [1]. Multiplication of the lamellae occurred by nucleation of the new lamellae in the recess of the grain boundary and by branching.

Aging for longer periods resulted in a second cellular reaction or discontinuous coarsening, which decomposed the primary lamellar product into a coarse lamellar structure of the same two phases. The secondary cells were observed to nucleate at three different regions; namely, at the original position of the grain boundary, at the impinged region of two primary cells, and at the interface between the migrating cell boundary and original position of the grain boundaries.