Krystel Renard's PhD Thesis

High-manganese austenitic TWIP (TWinning Induced Plasticity) steels are currently under development for automotive applications where their excellent mechanical properties provide the potential for significant vehicle weight saving. However, the origin of their outstanding properties is still unclear due to multiple effects occurring in these steels at different scale levels. The main hypotheses available in the literature include deformation twinning and dynamic strain ageing. Twins are usually thought to have a huge impact on the outstanding properties of the materials, either by bringing about a dynamic Hall-Petch effect and/or a composite effect.
The aim of this study is to reach a better understanding of the work hardening mechanisms of TWIP steels, especially the Fe-20%Mn-1.2%C alloy. In order to do so, a reliable method to estimate the twinning rate based on point counting analysis on EBSD (Electron Back-Scattered Diffraction) micrographs is proposed. It is then shown that there exists a first order relationship between this twinning rate and the work hardening rate in tension. Depending on the loading mode (uniaxial tension, simple shear and rolling), differences in work hardening rate are observed. These discrepancies can be related to differences in texture, twinning rate, number of activated twinning systems in each grain, twin thickness and transmission of twins across grain boundaries. The influence of complex strain paths is also investigated, by means of Bauschinger tests. Internal stresses are found to increase with strain, without saturation, due to the formation of twins. In terms of texture, in cold rolling, the <110>//ND fibre appears during straining. In tension, the <111>//TA, favourably oriented for slip of partial dislocations and the <100>//TA fibre, favourably oriented for slip of perfect dislocations, are seen to develop. Finally, tensile tests at different strain rates with the help of digital image correlation were also performed to study the dynamic strain ageing effect occurring in this type of steel. It is suggested that supplementary hardening could come from reorientation of Mn-C pairs in the cores of the dislocations.

Jury :
Professeur Pascal JACQUES, promoteur (UCL)
Professeur Grégoire WINCKELMANS, président (UCL)
Professeur Francis DELANNAY, secrétaire (UCL)
Professeur Laurent DELANNAY (UCL)
Professeur Stéphane GODET (ULB)
Docteur Sébastien ALLAIN (ArcelorMittal Research, France)
Professeur Olivier BOUAZIZ (Ecole des Mines de Paris, France)