Subjecting
metals to thermomechanical treatments such as hot rolling improves
the properties of the metals by combining plastic deformation and
heat treatment on a macroscopic scale, causing deformation and active
recrystallization on a microscopic scale. The mechanisms for these
microscopic processes and the effect of external parameters, (such
as temperature, time, rate and degree of deformation), have in the
past proved to be difficult to study. Until recently, anticipating
the properties of a material after thermomechanical treatment has
remained largely an art.
Since
1993, the CNRS team at the "Laboratoire des propriétés
mecaniques et thermodynamiques des matériaux" (LPMTM,
Mechanical and Thermodynamic Properties of Materials Laboratory) has
been gathering experimental data on samples with diverse crystalline
structures with the aim of modeling recrystallization mechanisms.
It has developed two novel investigative methods to identify and model
the connections between the macroscopic behavior of materials and
their microstructure following thermomechanical treatment. The first,
static in situ annealing, uses a scanning electron microscope to observe
the changing microstructure of a deformed material as new states are
formed by grain nucleation and growth, and to measure grain orientation.
The second method uses LPMTM's unique high resolution X-ray diffraction
technique to study individual metal grains and measure stored deformation
energy, the driving force for new grain formation, in highly deformed
materials.
Great
progress has been made in collaboration with USINOR's "Institut
de recherches de la sidérurgie" (IRSID, the Iron and Steel
Industry Research Institute) in understanding recrystallization in
cold rolled mild steels. However, results regarding measurements of
stored energy have so far been less satisfactory, as the relationship
between grain orientation and stored energy is heavily dependent on
the model used. Research is continuing on aluminum, copper and zirconium
alloys.
