A molecular dynamics simulation study of the velocities, mobility and activation energy of an austenite–ferrite interface in pure Fe

Molecular dynamics simulations have been used to obtain the mobility, in pure Fe, of a face-centered cubic (fcc)–body-centered cubic (bcc) interphase boundary with an orientation given by (1 1 0)bcc//(7 7 6)fcc and [0 0 1]bcc//[−1 1 0]fcc. The interface is best described by a 4.04° rotation, about an axis lying in the boundary plane, from the Nishiyama–Wasserman orientation and the boundary consists of a parallel array of steps (disconnections). An embedded atom method interatomic potential was employed to model Fe, and the free energy difference as a function of temperature between the fcc and bcc phases, which provided the driving force for boundary motion, was determined by a thermodynamic integration procedure. Although the boundary was found to be very mobile, the transformation did not proceed by a martensite mechanism. The boundary mobility was obtained for several temperatures in the range 600–1400 K and Arrhenius behavior was found with an activation energy of 16.5 ± 2.7 kJ mol−1 and a pre-exponential factor equal to 7.8( ± 0.9) × 10−3 mmol J−1 s−1. The activation energy is much lower than that extracted from experiments on the massive transformation in Fe alloys and possible reasons for the discrepancy are discussed.