Growth mode of austenite during reversion from martensite in Fe-2Mn-1.5Si-0.3C alloy: A transition in kinetics and morphology

The kinetics and element partitioning during austenite (γ) reversion from lath martensite in Fe-2Mn-1.5Si-0.3C (mass%) alloy have been investigated. Two different types of γ in terms of crystallography, with Kurdjumov-Sachs (K-S) and without K-S orientation relationships with respect to the surrounding tempered martensite matrix, are formed. A transition in kinetics and element partitioning is related to the differences in crystallography. At low temperature, the growth of both types of γ is accompanied with the partitioning of Mn and Si. Consequently, the growth rates and appearance of the K-S and non K-S γ are similar and the reverted γ structure consists of acicular γ dominantly. On the other hand, at high temperature, only the K-S γ grows with partitioning of alloying elements in contrast to the partitionless growth of the non K-S γ which leads to the formation of coarse globular γ. DICTRA simulation reveals that the variations in driving force for reversion and the interface mobility induce the transition of element partitioning. At large driving force and interface mobility, partitionless growth is possible and consequently coarse globular γ is formed by the fast growth of the non K-S γ at high temperature. On the other hand, reduction of the driving force or interface mobility induces the transition from partitionless to partitioning growth, which results in the dominant formation of thin acicular γ at low temperature.