The austenite grains in the 3rd generation of AHSS demonstrate the different microstructural features which are determined by the presence of varied defects, including the point defects (interstitial/substitutional atoms), line defects (dislocations), and area defects (stacking faults/twins/boundary)
[23][3]. Among these defects, the grain boundary is the most important one as it determines the domain, size, and morphology of the austenitic single crystal for the storage of other defects. In general, the defects in austenite grains are derived from prior phase transformation during the thermo-mechanical processing. The austenite grains in Q&P steel demonstrates blocky and film-like morphologies (
Figure 1a)
[44][4], which is related to the extent of the martensitic transformation. In particular, the extensive martensitic transformation leads to the severe partition of prior austenite grain into film-like retained austenite while the moderate martensitic transformation results in the co-existence of blocky and film-like residual austenite. The above observation is also applicable for the austenite grains in the CFB steel considering the displacive nature of bainitic transformation (
Figure 1b)
[45][5]. The number of defects in the retained austenite grains including dislocations, stacking faults, and deformation twins depends on the extent of phase transformation. The austenite grains retained after displacive shear transformation (martensitic/bainitic transformation) are deformed to accommodate the transformation strain of the adjacent transformed products
[46][6]. The defect density in austenite grains is heterogeneous owing to the localized deformation process, demonstrating increased defect density with the decrease of distance to the interface of austenite and product phase
[47][7]. Since the blocky austenite grains are subjected to less extensive martensitic transformation, they are expected to have lower amounts of internal defects than that of filmy-like counterparts. The retained austenite grains in medium Mn steel are generally obtained by reverse transformation from either quenched or deformed martensite, generating the lamellar or granular austenite grains, respectively (
Figure 1c)
[48][8]. In addition, the morphology of austenite grains in medium Mn steel also depends on the extent of reverse transformation. The reverse transformation from ferritic phase to austenite with different durations results in austenite grains with either lamellar or granular morphology
[49][9]. Although the mechanism of reverse transformation deviates from the displacive shear transformation, the lamellar austenite grains in medium Mn steel exhibit higher dislocation density than the granular counterpart, which can be rationalized from the concurrence of dislocations recovery or recrystallization with prolonged duration
[50][10].