Considering that the double differentiation technique involves purely
mathematical findings, microstructural analysis is needed to validate
the presence of dynamically transformed ferrite, as presented below.
Fig. 5 a show the grain boundary map of the specimen after double hit
deformation at
1150 °C and a strain rate of 0.25 s-1. A detailed
observation of the microstructures shows a morphology composed of laths
and few quasi-polygonal grains (indicated with red arrows). The plates
observed in the microstructure appear to have Widmanstätten type
morphology, which originates from the interior of the prior austenite
grains. This observation is consistent with the results of numerous
researchers [1, 16, 17] who linked their formation to the applied
stress, which stimulates the displacive transformation of austenite to
Widmanstätten ferrite.
EBSD grain boundary maps, reported in Fig. 5 (a) shows that a
significant amount of low-angle grain boundaries is present in the
microstructure. This indicates that the microstructure is not fully
recovered after the second hit. However, the presence of any LAGBS was
not found in the laths and plates, which suggests that these grains
underwent recrystallization and phase
transformation.