Interphase boundary motion elucidated through in-situ high temperature electron back-scatter diffraction

Publication date: 15 October 2017
Source:Materials & Design, Volume 132
Author(s): G. Zijlstra, M.S.B. van Daalen, D.I. Vainchtein, V. Ocelík, J.Th.M. De Hosson
In this work we concentrate on the in situ dynamics of interphase boundary motion during transformations. In-situ high temperature electron-back scatter diffraction (HT EBSD) was employed to study the ferrite-austenite-ferrite transformation in low carbon steel. A novel method was designed to derive the velocity of the interphase boundaries from the EBSD phase maps. It is concluded that the motion of the transformation front occurs in a jerky-type motion, i.e. not continuous in time on a microscale, as the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation would predict on the macro-scale. It is shown that HT EBSD is capable of providing relevant supplementary insight in the ferrite-austenite phase transformations with adequate spatiotemporal resolution, which would remain hidden in volume averaging experimental techniques such as X-Ray diffraction. In particular, ferrite-austenite phase boundary velocities between 1.4±0.3 and 4.0±0.2nm/s were detected during isochronal heating with 0.5°C/min. The mean interphase boundary velocity was ranging between 5.0±0.2 and 6.4±0.2nm/s for austenite-ferrite transformation with a cooling rate of 1°C/min. Ledge growth at isothermal conditions, resulted in velocity of 23nm/s along phase boundary. A strong dependency of interphase boundary mobility and parent-daughter phase boundary misorientation angles was not found.

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