| dc.description.abstract | The automotive industry has not been able to take full advantage of the lightweight of
magnesium, or its alloys, because of its reduced formability at room temperature. In order
to enhance the workability of magnesium and restore its ductility, elevated temperature
forming needs to be performed. Hot working of metallic alloys is often accompanied by
dynamic recrystallization (DRX), whereby the deforming grain structure is partially or
completely replaced by new defect free grains during deformation. Dynamic recrystallization
allows the nal microstructure, as well as the properties of the material (grain size,
texture strength), to be controlled. Therefore, DRX can be used as a tool to design a
materials microstructure.
Because it would be advantageous to be able to redesign the material properties of
magnesium, particularly for the automotive industry, this work takes a step towards such
an outcome by presenting a new model that predicts DRX in magnesium. The model
predicts DRX in magnesium alloys by using a crystal plasticity based nite element model
(CPFEM) coupled with a probabilistic cellular automata model (CA). The CPFEM employs
microstructural information obtained by electron backscatter di raction (EBSD) as
input and computes dislocation density evolution corresponding to the active deformation
modes. Because DRX proceeds via nucleation of new grains and their subsequent growth,
a nucleation criterion based on the local mismatch in dislocation density is employed. Subgrains
formed during deformation constitute the nuclei, and only those subgrains that have
a boundary with misorientation above a threshold value can grow. The probabilistic CA
is used to identify successful nucleation sites. The growth of viable nuclei depends on the
di erence in the stored energy of the nucleus and the stored energy of the surrounding
matrix. As such, the model is developed to predict the texture of magnesium alloys that
have experienced dynamic recrystallization solely from the initial texture and the applied
strain path.
The model is then extended to include deformation twinning and it can then be used
to study its e ect on the evolution of DRX. Deformation twinning is activated when the
c-axis of hexagonal close packed (HCP) crystal is under a tensile load and leads to the
reorientation of the crystal by speci c angle. However, rather than including both contraction
and extension twins, only extension twins are considered in the model due to their
important role during deformation at room temperature and above. Extension twins grow
during deformation at ambient temperatures and their in
uence on the texture formation
at elevated temperatures is not well studied. Contraction twins were not included in the
model because they are known to have negligible e ect on the nal texture of magnesium
alloys due to their relatively low thickness and inability to grow. In order to investigate the e ect of the extension twins on DRX evolution, a reorientation of the entire element
is performed to the dominant twin orientation before DRX is initiated. The approach is
similar to the PTR scheme (predominant twin reorientation).
To validate the capability of the developed model, rst, tensile simulations are performed
on rolled AZ31 commercial magnesium alloy at 300 C. The tensile test along the
rolling direction is a slip dominated deformation. The softening behaviour as well as the
nal texture are compared with available experimental data. Then, in order to activate
twinning, the compression test is simulated on the extruded AZ31 alloy along extrusion
direction (ED). Various temperatures are simulated to investigate the e ect of the twinning
at higher temperatures. Finally, the parametric study is accomplished to examine the
e ect of the di erent parameters in the model on the evolution of DRX. The simulations
with the new model show excellent agreement with experiments presented in the literature. | en |
