Since, it is challenging to differentiate between martensite and DT
ferrite using XRD, optical microscopy, or secondary electron imaging,
the present study employed the Kernal Average Misorientation (KAM)
method [7]. In this method, internal misorientation between grains
is used to differentiate the martensite (body-centered tetragonal) from
the ferrite (body-centered cubic) by the method of EBSD images. Up to
the third nearest neighbor was considered for calculating KAM values,
and a threshold angle of 5° was used. To study the recovery behavior,
only the area fractions with less than 2° misorientation (i.e., KAM ≤
2°; green and blue regions) were considered. Using the above criteria,
the dynamic recovery process of the two phases was studied, and a
distinction was successfully made between ferrite and martensite. It is
known that ferrite has a higher stacking fault energy (SFE) [18]
than austenite, which would make dynamic recovery simpler when it is
further deformed to a higher strain (ε=0.8). Conversely, martensite
laths (from prior austenite), which are formed due to shape deformation
(displacive transformation), generate a higher amount of LAGBs,
resulting in higher misorientations within the laths.
Fig. 5 b shows the KAM map for the specimen deformed at 1150 °C and at a
strain rate of
0.25 s-1. Widmanstatten ferrite plates, as seen on the
Grain Boundary map of Fig. 5 a, have ˂2° misorientation (see Fig. 5 a),
thus proving that they are recovered ferrite grains. Conversely, grains
with KAM values between 3°-5° indicate the presence of martensite. The
volume fraction of dynamically transformed ferrite is 72% for 1150 °C
and 0.25 s-1.
Fig. 6, shows the phase map of specimen deformed at 1200 °C and a strain
rate of 0.25 s-1. This sample was strained till 0.5
strain and then held for 5 s interpass time at temperature of
1200 °C and water quenched to preserve the microstructure. The yellow
color represents ferrite (BCC phase) and red color represents austenite
(FCC phase). It can be seen that the austenite is present in trace
amounts in the ferrite phase. The phase fraction of austenite is 4.2 %
and that of ferrite is 95.8%. The austenite present in this specimen is
due to re-transformation from ferrite to austenite during interpass time
[1].This re-transformation happens by a diffusional transformation.