Degree Name

Doctor of Philosophy


School of Mechanical, Materials and Mechatronic Engineering


TWinning Induced Plasticity (TWIP) steels with low stacking fault energy and stable face centred cubic austenitic microstructures have been developed as a promising material for automotive applications due to their characteristic work hardening behaviour. The present work investigates the recrystallisation and deformation behaviours of Fe-24Mn-3Al-2Si-1Ni-0.06C TWIP steel using various diffraction techniques and polycrystal plasticity modelling.

After 42% cold rolling reduction, transmission electron microscopy and electron backscattering diffraction (EBSD) found that a majority of grains contain a large fraction of primary twin densities and a smaller fraction of intersecting arrangements of primary and secondary twins. Bulk texture measurements using X-ray diffraction (XRD) returned the distinctive α-fibre for low stacking fault energy materials; with higher intensities for Goss ( {110}) compared to Brass ( {110}) orientations. Micro-texture suggests a significant contribution of slip and a possible role for micro shear banding in the development of the Brass orientation.

The cold-rolled material was subjected to isochronal annealing at temperatures between 600 and 850 °C for 300 s. Annealing twins contribute to recrystallisation from the early nucleation stage as twin bulges and twin-related nuclei were observed after annealing at 600 °C. A novel EBSD deconstruction technique was applied on partially recrystallised samples to separate deformed, recovered, newly nucleated and growing grains fractions. Whereas the orientations of the newly nucleated grains are similar to the deformation texture, their subsequent growth was found to be affected by orientationdependent, stored energy considerations as well as second order twinning.

The evolution of lattice strains in fully annealed sample was tracked via in-situ neutron diffraction during cyclic loading between strain limits of ±1%. The pronounced Bauschinger effect observed upon load reversal is accounted for by a combination of the intergranular residual stresses and the intragranular sources of back stress such as dislocation pile-ups at the intersection of stacking faults. A modified Elasto-Plastic Self- Consistent (EPSC) model, which accounts for both intergranular and intragranular back stresses, was applied to elucidate the contribution of various deformation mechanisms such as perfect and/or partial slip and twinning to the deformation accommodation. The Bauschinger effect was well captured when the reversibility of partial slip in the direction is accounted for. This result indicates an important contribution of the stress-induced separation of partial dislocations to the Bauschinger effect at the low strain levels employed in this study. It also raises the possibility that de-twinning events, associated with the reversibility of slip along the direction, could be occurring upon load reversal. On the other hand, the reversibility of partial slip fails to reproduce the experimentally observed tension-compression asymmetry as the directionality of twinning is not accounted for. For the low strain levels investigated in the present work, the EPSC model also predicts that the permanent softening frequently reported with the Bauschinger effect is a geometrical or loading path effect.

A combination of EBSD and XRD was used to track the evolution of the microstructure and texture of a fully recrystallised sample during interrupted uniaxial tensile testing. The interaction with the stableoriented grains results in preferential plastic flow in the unstableoriented grains. Consequently, the grains oriented along theandfibres record the highest and lowest values of intragranular local misorientation. Schmid factor analysis highlights the feasibility of twin nucleation at high strains via extrinsic stacking faults in the near--oriented grains that are nominally regarded as being unfavourably oriented for twinning. The Visco-Plastic Self-Consistent (VPSC) model was used to simulate the macroscopic stress-strain response as well as track the evolution of bulk crystallographic texture by detailing the contributions of perfect and/or partial slip, twinning and latent hardening. The simulations revealed the dominant role of perfect slip and the limited volume effect of twinning on the texture development. The stability of various orientation subjected to uniaxial tension was examined via the VPSC model. While thefibre remains stable and does not affect unstable orientations, the orientations along the stablefibre strongly affect the unstable orientations along thefibre.