Krystel Renard

TEM investigation of the formation mechanism of deformation twins in Fe-Mn-Si-Al TWIP steels

The microstructure of a Fe–Mn–Si–Al twinning-induced plasticity (TWIP) steel exhibiting remarkable work hardening rate under uniaxial tensile deformation was investigated using transmission electron microscopy to uncover the mechanism(s) controlling the nucleation and growth of the mechanically induced twins. The results show that the stair-rod cross-slip deviation mechanism is necessary for the formation of the twins, while large extrinsic stacking faults homogenously distributed within the grains could act as preferential sources for the activation of the deviation process. The influence of such features on the thickness and strength of the twins and the resulting mechanical behaviour is discussed and compared to similar works recently performed on Fe–Mn–C TWIP steels.

Automatic Crystallographic Characterization in a Transmission Electron Microscope: Applications to Twinning Induced Plasticity Steels and Al Thin Films

A new automated crystallographic orientation mapping tool in a transmission electron microscope technique, which is based on pattern matching between every acquired electron diffraction pattern and precalculated templates, has been used for the microstructural characterization of nondeformed and deformed aluminum thin films and twinning-induced plasticity steels. The increased spatial resolution and the use of electron diffraction patterns rather than Kikuchi lines make this tool very appropriate to characterize fine grained and deformed microstructures.

Strain heterogeneity and local anisotropy in TWIP steels

Deformation twinning is known to play a major role in the huge work hardening and formability of TWinning Induced Plasticity steels (TWIP steels). Twins carry a part of the plastic deformation and they act as barriers to dislocation motion. However, their interaction with their parent grain is already anisotropic and they strongly influence the development of texture and macroscopic anisotropy of the material. Twinning in TWIP steels is investigated in this study by means of experimental observations as well as an advanced crystal plasticity model. Individual twins, as they are formed, are treated as separate crystal entities co-deforming with the parent grain. The behavior of the polycrystalline aggregate is assessed by means of a multisite model of short-range grain interactions in comparison with the crystal plasticity finite element method (CPFEM) and the Taylor FC model. This approach allows validating the occurrence of twinning in grains with preferred orientations as well as the orientation relation between a twin and its parent grain with evolving deformation. Moreover, the results demonstrate that a combined prediction of macroscopic work hardening, texture and twin volume fraction requires both an appropriate description of strain heterogeneity in the polycrystal and a proper account of the anisotropic twin-grain interactions at the crystal level.

Multiscale characterisation of the plasticity of Fe-Mn-C TWIP steels

It is known for a long time that mechanical twinning occurring in FCC metal can bring about a very large work hardening rate. It is believed that deformation twins increase the work-hardening rate by acting as obstacles for gliding dislocations. This mode of plastic deformation is active, among others, in the Hadfield steel known for a very long time [1]. Fe-Mn-C grades exhibiting mechanical twinning present a renewed interest for some years since some applications are expected in the automotive industry. Several kinds of TWIP (Twinning Induced Plasticity) steels are now intensively studied.