E-twinning is a common plastic deformation mechanism in calcite
deformed at low temperature. Strain rate, temperature and confining pressure
have negligible effects on twinning activation which is mainly dependent on
differential stress. The critical resolved shear stress (CRSS) required for
twinning activation is dependent on grain size and strain hardening. This
CRSS value may obey the Hall–Petch relation, but due to sparse experimental
data its actual evolution with grain size and strain still remains a matter
of debate.
In order to provide additional constraints on twinning activation and
development, new mechanical tests were carried out at room temperature on
unconfined single crystals of calcite, with different sizes and
crystallographic orientations. Uniaxial deformation was performed at a
controlled displacement rate, while the sample surface was monitored using
optical microscopy and a high-resolution CCD (charge-coupled device) camera. The retrieved macroscopic
stress–strain behavior of the crystals was correlated with the surface
observations of the deformation process.
Results show (1) the onset of crystal plasticity with the activation of the
first isolated mechanical twins during the strain hardening stage, and
(2) the densification and thickening of twin lamellae during the steady-state
flow stress stage. Such thickening of twin lamellae at room temperature
emphasizes that calcite twin morphology is not controlled solely by
temperature. The different values for the CRSS obtained for the activation of
isolated twins and for the onset of twin densification and thickening
raises questions regarding the appropriate value to be considered when using calcite twin data
for stress inversion purposes.</p
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