David Barbier

EBSD for analysing the twinning microstructure in fine‐grained TWIP steels and its influence on work hardening

A 22 Mn–0.6 C twinning induced plasticity steel with an average grain size of 2.6 μm was deformed in tension at room temperature. The electron backscattered diffraction technique was used to characterize the twinning structure in relation with the local texture evolution. For nanoscale analysis, additional transmission electron microscopy analysis was performed. Nanotwins were activated in the largest grains from the beginning of the deformation. They interacted with a well‐developed dislocation structure that induced detectable intragranular orientation variations. With increasing deformation, dense bundles of nanotwins preferentially developed in grains oriented close to the <111>//tensile direction fibre (promoted by the deformation) as well as medium to high angle sub‐boundaries. These key features of the twinned microstructure were finally related to the remarkably high strain hardening, which evolved according to different stages.

Interactions between ferrite recrystallization and austenite formation in high-strength steels

Using both experimental and modeling approaches, we give some clarifications regarding the mechanisms of interaction between ferrite recrystallization and austenite formation in cold-rolled high-strength steels. Using different thermal paths, we show that ferrite recrystallization and austenite formation can be strongly interdependent. The nature of the interaction (weak or strong) affects significantly the austenite formation and the resulting microstructure. We show that the kinetics of austenite formation depends intrinsically on both heating rates and the extent of ferrite recrystallization. An unexpected behavior of austenite growth was also seen at high heating rates. A possible explanation is presented based on the nature of the local equilibrium at the ferrite–austenite interface. The microstructure is more heterogeneous and anisotropic when both austenite formation and ferrite recrystallization overlap. A mechanism of microstructural formation is proposed, and this is supported by 2D simulations’ images.

Characterization and quantification methods of complex BCC matrix microstructures in advanced high strength steels

The development of comprehensive and reliable microstructure characterization tools is very necessary for the understanding and modelling of both the formation of different phases during processing and the relationship between microstructure and mechanical properties. A series of samples containing different phases with a BCC structure have been characterised using different techniques including optical microscopy (OM), field emission gun scanning electron microscopy (FEG-SEM) with secondary electron (SE) and back scattered electron (BSE) modes, electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). It is shown that it is difficult to distinguish ferrite from bainite especially granular bainitic ferrite using conventional OM and FEG-SEM (SE) techniques, whereas FEG-SEM (BSE) and EBSD are the most suitable techniques to differentiate them. A new EBSD method has been developed to dissociate and quantify ferrite, bainite and martensite in multi-phase AHSS steels. This method has been proven to be pertinent and has been validated using reference specimens.

Restitution of the Shapes and Orientations of the Prior Austenitic Grains from Inherited Alpha' Orientation Maps in Steels

We propose to reconstruct the g orientation maps from the α maps inherited by the bainitic or martensitic γ→α phase transformation. Our approach comprises two main steps (1) identification of reliable fractions of parent grains – each γ fraction is determined from neighbouring variants related to a unique γ orientation with a low tolerance angle; (2) expansion of these fractions by collecting adjacent variants being in orientation relation (OR) with the calculated g orientation - using a higher tolerance around the strict OR. Moreover, this fully automated g map reconstruction is further improved by an additional semi-automated analysis of α/γ maps. This reconstruction procedure was applied to the α microtexture of bainitic steels. The reliability of the calculations was checked by comparing each γ grain orientation and their corresponding α variants. The results show that even with a large spread around OR, the shape and orientation of most of the γ grains are accurately calculated.